QD 
 
 543 
 
 Mas. 
 
EXCHANGE 
 
A STUDY OF NICKEL FERROCYANIDE 
 AS A MEMBRANE IN THE 
 
 MEASUREMENT 
 OF OSMOTIC PRESSURE 
 
 Dissertation presented to the Board of University Studies 
 of the Johns Hopkins University in conformity with the 
 requirements for the Degree of Doctor of Philosophy* 
 
 JOSEPH LLEWELLYN MCGHEE 
 
 B. D. SMITH & BROS., PRINTERS, PULASKI, VA. 
 
A STUDY OF NICKEL FERROCYANIDE 
 AS A MEMBRANE IN THE 
 
 MEASUREMENT 
 OF OSMOTIC PRESSURE 
 
 Dissertation presented to the Board of University Studies 
 of the Johns Hopkins University in conformity with the 
 requirements for the Degree of Doctor of Philosophy* 
 
 JOSEPH LLEWELLYN MCGHEE 
 
 MAY, \ 9 \ \ 
 
ACKNOWLEDGMENT. 
 
 For the kind helpfulness of Pres. Bemsen, Prof. Morse, 
 Prof. Jones, Prof. Acree and Prof. Whitehead the writer wishes 
 to express hi appreciation, but because so much of inspiration, 
 information through personal instruction and a better apprecia- 
 tion of the highest ideals put to practice in research work came 
 to the author through Prof. Morse, it is to him chiefly that 
 credit is due for whatever merit or value may be found in these 
 pages. 
 
 He is also indebted to Prof. Eenouf for courtesies shown. 
 
 For all kindness received from his co-workers in the 
 laboratory full indebtedness is hereby acknowledged. 
 
 251816 
 
BIOGRAPHY. 
 
 J. L. MeGhee, bora in London County, Tennessee, in 1873. 
 Beared mostly in Virginia. High School work at Pearisburg 
 Academy and at Hiwassee. A. B. degree at Emory and Henry 
 College, Emory, Virginia, 1903. Graduate work at the Uni- 
 versity of Chicago. Head of the department of Chemistry at 
 Emory and Henry College. 
 
A STUDY OF NICKEL FERROCYANIDE AS A 
 
 MEMBRANE IN THE MEASUREMENT 
 
 OF OSMOTIC PRESSURE. 
 
 Dependence upon work previously done in this laboratory 
 for the development of the present investigation is fully recog- 
 nized. A brief statement of this pre-requisite work seems 
 necessary in order that the connection of the whole may be 
 properly set forth. 
 
 When the electrolytic process for the deposition of mem- 
 brane was first devised, about twenty-five colloidal substances 
 were tested to secure the one best adapted to osmotic pressure. 
 Among those showing most promise were copper ferrocyanide, 
 nickel ferrocyanide and several others. 
 
 These tests were largely qualitative owing to the scarcity 
 of suitable cells, manometers, thermostats and other devices for 
 quantitative work. Thus limited and the task being such a 
 stupendous one, the copper forrocyanide membrane was the only 
 one used extensively. 
 
 Although some work had been done to determine the 
 relative value of other colloids, it had not the advantage of 
 recent improvements, until the present year. 
 
 THE CELL. 
 
 Although the preparation of suitable cells preceded the 
 present work, it is in order to give more than a brief mention of 
 the troubles encountered and of the methods used to overcome 
 the difficulties. For obviously no membrane for measuring 
 osmotic pressure is of any use without a proper cell in which to 
 place it. Likewise numerous other requirements will be noted 
 more or less briefly. 
 
 The ordinary cell of the potter does not serve in measuring 
 osmotic pressure for the following reasons or causes: 
 
 1. They do not possess the strength necessary to withstand 
 the outward pressure; most of them crack under pressures below 
 20 atmospheres. 
 
 (5) 
 
2. They contain "air blisters/' which communicate with 
 various portions of the cell wall and permit the formation of 
 irregular, secondary membranes, situated somewhat in parallel 
 and at varying distances from the inner surface. Any consider- 
 able pressure brought upon a membrane deposited in such a cell 
 is sure to break through the colloid. 
 
 3. The potter's cell also lacks uniformity in its pores, an 
 indispensable property in suitable cells. This requirement 
 appears with more force when it is considered that the membrane 
 is deposited where the opposing ions meet, and that the part of 
 the cell wall in which they meet is determined largely by the 
 size of the pores in the cell. This will be presented more at 
 length under the discussion of the membrane. 
 
 To secure a cell combining the necessary strength and uni- 
 form porosity required several years of persistent work. Early 
 attempts to increase the strength of the material by adding 
 feldspar were not satisfactory because uniform porosity was not 
 secured at the same time. This was true even when every care 
 was used to properly wash, bolt, mix, dry and bake. 
 
 Finally a suitable combination of clays, one deficient in 
 binding material and another over-rich in such constituents, 
 was tried with success; producing a cell of uniform texture in 
 every respect, so far as known. 
 
 The upper half of the cell is glazed, inside and outside. 
 For this purpose a glaze is required whose coefficient of expan- 
 sion equals that of the cell itself. A suitable glaze was finally 
 obtained by mixing silica and feldspar with one of the better 
 grades of glaze used by the potter. 
 
 Before the cell is glazed it is carefully ground on the lathe 
 to fit the brass collar and the cone shaped stopper of the 
 manometer attachment. The general shape of the cell is that of 
 an inverted frustrum except at the top where a shoulder is left 
 to rest upon the collar that fastens together the cell and the 
 manometer. The upper and lower surfaces of the shoulder are 
 carefully turned on the lathe perpendicular to the longitudinal 
 axis of the cell so that the pressure from the collar may be 
 evenly distributed to prevent crushing when the cone of the 
 manometer is screwed into place. Equal care is used in cutting 
 all parts of the cell, which must not be too wet or too dry. 
 
 (6) 
 
The upper part of the cell is made larger to prevent the 
 outward pressure of the cone from splitting the cell, and also to 
 avoid crushing by the cylindrical collars. 
 
 The lower part of the brass collar upon which the cell rests 
 during a measurement is extended inward and covered with 
 lead in order to afford a cushion for the cell shoulder under 
 pressure, and a cylinder of the proper size is selected for each 
 cell so that the cell shoulder may not be chipped off by one that 
 is too large; or that the cell may not be crushed by one that is 
 too small. 
 
 The upper half of the brass collar is quite heavy. This is 
 required because a slot is left in it to admit the manometer tube, 
 as the collar cannot be slipped over the manometer from the top. 
 The middle portion of the upper cylinder is extended inward in 
 a carefully turned shoulder that rests on the top of the cell. In 
 this way pressure is applied without injury to the cell or to the 
 manometer. 
 
 The brass cone of the manometer is covered by rubber 
 tubing of the best quality, securely tied with several strands of 
 waxed shoemaker's thread at the top and bottom, more thread 
 being used at the top, where the last strands extend down to 
 the mouth of the cell, when the cone is screwed into place. 
 
 (For a full statement of the manufacture of cells employed 
 in osmotic pressure, the reader is referred to the American 
 Chem. Journal of Feb., 1911.) 
 
 THE REMOVAL OF AIR FROM THE CELL WALL. 
 
 All air must be removed from the pores of the cell wall 
 before membrane is deposited, in order to secure uniformity and 
 strength. This removal of air is accomplished by the electric 
 current passing from a platinum anode surrounding the lower 
 unglazed portion of the cell, through an electrolyte of lithium 
 sulphate to a platinum cathode placed inside the cell. The 
 large capacity of the lithium ion for water produces an ion of 
 such size that all air is removed in about six hours, from a cell 
 of average density. During this process lithium hydroxide 
 Accumulates at one electrode and sulphuric acid at the other. 
 
 (7) 
 
By mixing the two solutions repeatedly the lithium sulphate is 
 again ready for use. 
 
 Before membrane is deposited the lithium sulphate is 
 removed from the cell wall electrolytically, pure water being 
 frequently added to replace the base and acid as they accumulate 
 about the electrodes. When the salt is all removed the cell is 
 ready to receive the membrane. 
 
 THE DEPOSITION OF THE MEMBRANE. 
 
 A bath regulated to a definite temperature is now required. 
 This is secured by the use of both gas and electricity. Gas 
 supplies heat to raise the water nearly but not quite to the 
 desired temperature. Electric stoves with thermo-regulators 
 supply the margin of difference. 
 
 In the bath a stirring apparatus is run by a small motor. 
 The stirrer is placed in the upturned opening of an iron or brass 
 tube lying in the bottom of the bath and extending its entire 
 length, thus securing adequate mixing of the water and uniform 
 temperature in every part of the bath. For higher temperatures 
 the bath is fully covered to prevent evaporation and loss of heat. 
 
 Into glasses set in the bath water are placed cylindrical 
 nickel anodes. A solution of tenth normal nickel sulphate 
 nearly fills the glasses. A platinum cathode and a tenth normal 
 solution of potassium ferrocyanide are placed inside the cell, 
 which is provided with a funnel and exit tube held in place by 
 a two-hole rubber stopper. Through the funnel fresh potassium 
 ferrocyanide solution, kept at the bath temperature, is poured 
 every third minute during the deposition process. This pre- 
 vents the accumulation of alkali inside the cell which would 
 dissolve the membrane if not removed. 
 
 The position of the membrane in the cell is a matter of 
 great importance. With the very slow diffusion which would 
 exist in the wall between a membrane in the mid- wall position 
 and the solution within the cell, it would be impossible to 
 secure a proper determination of osmotic pressure. Another 
 objection to a mid-wall membrane is that great uncertainty 
 would exist about the state of such membrane in that section of 
 the cell between the outer and inner glaze, above the portion 
 
which is supposed to be the upper limit of the membrane as 
 now deposited. This will appear more clearly from what 
 follows. 
 
 While a mid- wall membrane is unsatisfactory, it is equally 
 so with one produced by surface precipitation. In the latter 
 case the membrane is not strong enough to withstand pressure 
 or to prevent flaking off. In order to secure the membrane to 
 the cell wall, the porosity of the cell wall and the voltage of the 
 electricity had to be so adjusted by experiment to the rate at 
 which the nickel ion and the ferrocyanogen ion penetrated the 
 cell wall, that they would meet just within the wall surface on 
 the inner side, where practically no hindrance to the diffusion 
 of entering- water, with the solution in the cell, is possible. 
 Thus deposited the membrane is satisfactory in every way so 
 far as experience has gone. A voltage of one hundred, and 
 ten to one hundred and fifteen seems best for this purpose. 
 
 During the deposition of the membrane in each cell a record 
 of amperage is kept and the risistance of the membrane in any 
 cell can be determined. After receiving membrane an hour or 
 so the cell is removed, washed and soaked for a day or two in a 
 water solution of thymol to remove all nickel sulphate and 
 potassium ferrocyanide and small amounts of any electrolytes 
 that might be present as impurities in the cell wall. The 
 temperature of the cell is permitted to change as little as pos- 
 sible. Thymol is used as a disinfectant to prevent the growth, 
 especially, of penicillium, which seems to feed upon the nitrogen 
 of the colloidal compound, thus destroying the membrane. The 
 cell is kept in this thymol solution until it is placed again 
 between the electrodes for more membrane, or till it is ready to 
 be set up for a measurement. Usually the first attempt to use a 
 membrane for measuring osmotic pressure is not successful. The 
 cause is supposed to lie in the need that the membrane be packed 
 or compressed by use until sufficiently firm; repeated additions 
 of the colloid being made in the meantime. Thus the weakest 
 places are broken through and are the ones first filled or mended 
 when membrane is again deposited, as the current passes through 
 the ruptured places most abundantly. 
 
 (9) 
 
Repetition of the above processes produces a suitable mem- 
 brane in about two months, on the average, if the whole of the 
 large number of necessary conditions is met. 
 
 It was found in the present work that commercial aluminium 
 which was used as racks for holding the cells while in soak left a 
 deposit in the thymol solution. Tests showed this to be in part 
 some compound of aluminium, probably the hydroxide, and was 
 doubtless the result of primary couples set up between the 
 aluminium and small particles of other metals present as im- 
 purities. Evidence of this was found in small pits that appeared 
 in the aluminium. These racks were discarded. The cells are 
 now placed in large desiccators which are kept in the baths, 
 and provided with woolen hoods over the tops to prevent radia- 
 tion of heat. 
 
 Great care is taken to avoid any dilution or concentration 
 of the cell contents. This may occur either in the failure to 
 have the whole cell and its attachment at the proper temperature 
 when set up, or it may be caused by the contraction or expan- 
 sion of the mercury in the manometer, due to a considerable 
 difference between the temperature of the bath in which it is 
 placed during the measurement, and the room temperature when 
 set up. It is impossible to have the whole manometer at the 
 temperature of the experiment when being set up, especially 
 while working at very high or very low temperatures. 
 
 If the cell and the solution are too cool when set up, expan- 
 sion occurs as the proper temperature is reached, water passes 
 out of the cell, leaving the solution concentrated. The resulting 
 pressure developed is higher than the intended conditions would 
 give. 
 
 On the other hand, if the cell and its contents are too warm 
 when set up, contraction takes place, water is forced through 
 the membrane as the cell is brought to the cooler temperature of 
 the experiment, resulting in dilution of the solution and a lower 
 pressure than the true one for the desired conditions. 
 
 Every precaution is necessary to prevent leakage of the 
 solution from the cell while under pressure. The various means 
 used for this are the following: The cell neck is so ground as to 
 receive the cone shaped stopper which has a definite taper. 
 
 (10) 
 
This cone is made of heavy brass. Through the cone are passed 
 the capillary manometer tube and a small metallic tube which 
 connects with the inside of the cell. After any air bubbles and 
 the excess of solution pass out through this tube, it is closed by a 
 metal screw plug which fits upon an oil-soaked washer. The 
 washer is kept from spreading by making the contact surface of 
 the plug slightly concave. For temperatures under 50 these 
 two tubes are fastened to the cone by u Wood's metal. 77 By use 
 of the plug referred to and the cylindrical screw collar already 
 described, it is possible to adjust the cell to any initial pressure 
 desired. In this way the dilution of the solution which would 
 occur by permitting the whole pressure to develop by osmosis is 
 prevented. Great caution is used in this, however, in order to 
 avoid too great an initial pressure. 
 
 After all the precautions mentioned above are taken, the 
 time required for the actual process of setting up the cell is only 
 a few minutes. This is essential for successful measurement of 
 osmotic pressure. The manometer cone is forced into the cell, 
 screwed as tightly as desired, the nut inserted into the top of 
 the small tube that connects with the inside of the cell, and the 
 cell is placed in hundredth normal solution of nickel sulphate. 
 In the sugar solution within the cell is placed an osmotically 
 equivalent amount of potassium ferrocyanide. The liquid inside 
 and outside the cell is a thousandth normal solution of thymol 
 in water, to which these other substances are added. 
 
 The whole apparatus is next placed inside of an enclosed 
 constant temperature bath described later of such construction 
 that the water below and the air above are both kept at the 
 same temperature. 
 
 THE MANOMETER. 
 
 Before the manometer is ready for use an incredible amount 
 of the most careful work must be done upon it. Adequate 
 description of this part of the work can not be given here. The 
 reader is referred to the American Chemical Journal of March, 
 1911, where the subject of manometers is not only presented but 
 illustrated in a most satisfactory manner by Prof. Morse, Dr. 
 Holland and Mr. Carpenter. The only part taken by the writer 
 
 (11) 
 
in the manometer problem was v the horizontal calibration of 
 manometers , , 0, and @ which suggested a slight improve- 
 ment in manipulation. 
 
 After the manometer tube has the " scratch 7 ' (a mark for 
 reference) etched upon it, a short column of purest mercury is 
 put into the thoroughly clean, dry tube. The latter is fixed 
 horizontally upon the dividing engine, and over a mirror scale 
 of millimeter divisions. Placing one end of the short mercury 
 column even with the " scratch," its length is carefully deter- 
 mined by use of the calibrating engine, provided with a micro- 
 scope which has a micrometer eye-piece. The column of 
 mercury is shifted exactly its length along the tube, the left 
 hand meniscus resting where the right hand meniscus was before. 
 By repeating the above- and weighing the mercury used, the 
 value of the meniscus correction between the limits observed 
 may be calculated. 
 
 To move the mercury thread from one position to the other 
 more quickly and to adjust the meniscus with greater ease and 
 precision than had been done before would facilitate a very 
 tedious work. It is believed that the apparatus described below 
 not only seives such a purpose but also provides some security 
 against the loss of the mercury thread when the calibration near 
 the upper end of the manoneter is being done. After several 
 days of hard work one may have it all lost in a moment by com- 
 pressing the air too much in moving the thread near the end of 
 the tube. 
 
 A bulb is blown in one end of the manometer tube to be 
 used as a mercury reservoir later. To the other end of the 
 manometer a short piece of tube is sealed which is also provided 
 with a bulb. Rubber tubes are then used to connect as closely 
 as practicable a piece of glass tubing to each end of the manom- 
 eter tube. These connected tubes are bent at two right angles 
 and return along the front of the calibrating engine till within 
 about ten centimeters apart. Here the tubes are bent at right 
 angles toward the operator. The projecting end of each tube is 
 inserted into one end of a rubber tube which connects them. 
 The rubber tube is longer than the distance between the glass 
 tubes to allow for adj ustment of the glass tube to manometer 
 
 (12) 
 
tubes of varying length; and also to allow the operator full use 
 of the rubber tube without in any way displacing the manometer. 
 
 At the middle of the connecting rubber tube a small hole is 
 made. The operator places his thumb and finger on the tube over 
 this hole and by properly compressing the air on one side or the 
 other the column of mercury is forced along the tube in either 
 direction. If too much pressure should force the thread out of 
 the capillary, the bulb at either end would prevent the loss of 
 the mercury and save the work already done. 
 
 Another advantage which is obvious, is that the whole 
 space within the connected tubes is protected from dust and 
 moisture during the several days usually required for calibration. 
 
 The hole over which the thumb is placed permits immediate 
 equilibrium of pressure as soon as the thumb is removed and 
 prevents any tendency of the mercury to return to aay former 
 position by unequal compression of air on the two ends, as 
 might occur if the opening were not provided. 
 
 The whole apparatus can be made in a few minutes. It is 
 simple and inexpensive. 
 
 A perfectly clean tube is required for the succcessful 
 manipulation of the mercury thread; otherwise the mercury 
 u crawls 77 back toward the position from which it is moved for 
 a setting, resulting in serious errors. This " crawling 7 ' process 
 is so slow that it may occur unobserved. Thus the worker may 
 exercise the greatest care and still include serious errors, which 
 are not detected until the duplicate settings are made. Even 
 then the same l i crawling 7 7 of the mercury may produce similar 
 errors which would enable one to get almost the exact duplicate 
 checking data and still be far from the true measurement of the 
 mercury thread lengths in the various positions in the tubes. 
 
 The same trouble may arise from any roughening of the 
 capillary by the cleaning fluid used. This should not remain 
 in the tube a great while, in order to avoid such trouble. When 
 clean, the tubes are dried for several hours by passing air 
 through them over calcium chloride by use of the suction pump. 
 
 In the calibration above referred to, the shorter the mercury 
 thread used the more accurately is the volume of the capillary 
 determined, provided the weighing of the thread can be very 
 
 (13) 
 
accurately done. In one case, during the calibration above 
 referred to, an error of three milligrams in the weight of the 
 short thread of mercury resulted in a meniscus correction that 
 was one hundred per cent too large. Evidently, even with the 
 most accurate weighing, it is possible to use a mercury column 
 that is too short. In such case any error made is a large per 
 cent of the quantity weighed. On the other hand, the longer 
 the thread used the greater doubt exists as to the size of the 
 capilJary between the thread ends. Threads about 15 milli- 
 meters in length were used in this work. Three sets of readings 
 are made for each tube, which check within the limits of experi- 
 mental error. 
 
 Another small aid in this phase of the calibration process 
 consists in a movable light-regulator that is attached to the 
 standard arm of the engine. It, too, is quite simple, but saves 
 time in securing a suitable light on the meniscus of the mercury 
 column. This is often quite difficult during cloudy weather, 
 and at certain positions the proper light for a clear view of the 
 meniscus outline is hard to obtain in good weather. This little 
 attachment consists of a thin board or card-board that is so 
 clamped to the arm of the engine standard that it may be freely 
 rotated by a light touch. This enables the operator to shut off 
 or admit light at any angle which is found best by trial for a 
 clear meniscus. The trouble and time thus saved far outweigh 
 the outlay in making it, because neither the hand nor a card 
 can be so conveniently used. The rotating screen will stand at 
 any angle desired, and can be put on or removed in an instant. 
 
 BATHS FOR CONSTANT TEMPERATURE. 
 
 Three types of baths have been used in the osmotic pressure 
 work. The first was used for lower temperatures and has been 
 fully described in previous reports. 
 
 The second has been briefly described in these pages, and 
 perhaps a more full description of the third kind will embody 
 the essential features of the second so that it may be better 
 understood. 
 
 Since the work from C. to 25 C. has been completed the 
 recently constructed baths are for higher temperatures. 
 
 (14) 
 
r : ' i 
 
 f UN! 
 
 k or J 
 
 X^ft^pyjiHifrX 
 
 A well made box of the size desired is lined with copper^ 
 tin-coated. Inside the box is placed a motor-driven stirring 
 apparatus, which lifts the water from the upturned opening of a 
 pipe extending from one end of the bath to the other. By 
 regulating the speed at which the stirring is done, the water in 
 all parts of the bath may be kept at a uniform temperature. 
 For maintaining cooler temperatures a system of pipes conveys 
 a constant stream of cold water through the bath. This is 
 regulated by an overflow stand pipe which exerts constant pres- 
 sure upon the water passing through the bath. 
 
 A loop of the pipe through which the stirring apparatus 
 draws the water is extended outside the end of the bath. This 
 loop is heated by gas burners up nearly to the fc desired tempera- 
 ture. The few degrees more required are supplied by electric 
 stoves, placed in cylinders of copper that extend into the bath, 
 or entirely from one side to the other, and are closed with brass 
 caps. To prevent sparking at the contact point of the mercury 
 and platinum wire in the ther mo- regulator and also on the relays 
 which are used to control the current, condensers are used. 
 These are made of tinfoil placed between paraffined paper. 
 In order to operate the bath at different temperatures more or 
 less gas is used. The small difference of heat above referred to 
 is gotten from the electric current. 
 
 In order to prevent evaporation of the bath water and the 
 formation of mist on the glass door through which the readings of 
 the manometer are taken, it is necessary to separate the upper 
 portion of the bath from the lower. For this purpose a brass 
 plate covers the lower part containing water. Heavy weighted 
 cans, open at the top, extend through the brass plate into the 
 water. The cans are large enough and deep enough to enclose 
 the bottle containing the cell and all the portions of the manom- 
 eter where the mercury reservoirs are. The can is then care- 
 fully covered by a thick heat-insulating pad, so that only very 
 slight change of temperature takes place when it becomes neces- 
 sary to open the bath. 
 
 The upper section of the bath is so constructed that it may 
 be removed, being clamped to the lower part by sash locks. 
 Rubber strips lie between the two sections and insure close con- 
 
 (15) 
 
tact. A therino regulator controls electric lamps in the upper 
 portion, as below, and uniform temperature in the air space is 
 secured by use of an electric fan. For higher temperatures the 
 air space is heated nearly to the desired temperature by a system 
 of water pipes through which hot water is circulated by a 
 stirring apparatus similar to the one described for the main bath. 
 The pipes are situated at one end of the bath just behind the 
 electric fan, but are connected with the stirring apparatus and 
 the gas flames on the outside of the bath. The whole bath is 
 placed upon a support as free from vibration as possible. Beams 
 pass through openings in the floor to the wall below and upon 
 these the bath is put. Three or four such supports are now 
 in use. 
 
 Another feature which avoids vibration is the method of 
 gearing the motor to the water pump or stirrer. On the motor 
 shaft is placed a contact pulley. A belt transmits power to the 
 water pump. All of this has a separate support from the bath, 
 but very little vibration is produced. 
 
 The cathetometer used in observations together with the 
 brass meter scale is also supported on the beams above referred 
 to. All manometer observations are referred to the " scratch" 
 or mark etched on the glass, and the distance between the upper 
 and lower limits of the nitrogen in the manometer is determined 
 by the use of the meter scale viewed through the telescope of the 
 cathetometer. In these observations it is necessary to bring the 
 telescope to a focus always from the same direction because of 
 the " back lash " of the adjustment screw. 
 
 Thus arranged the bath keeps constant temperature for 
 months. 
 
 THE NICKEL FERROCYANIDE CELLS. 
 
 A more abundant supply of suitable cells recently made 
 practicable further investigation of other new, electrolytically 
 precipitated membranes. For purposes of record and identifi- 
 cation the cells used in this investigation are marked Nil, Nia, 
 Nis, and so on. Their behavior during the removal of air from 
 the cell walls and upon the deposition of membrane will be noted 
 first. The cells used during this work were Mi, Nia, Ni, 
 They will be taken up in the order given. 
 (16) 
 
lumber one and number two were started together at the 
 beginning of the work. 
 
 The pores of Nil were somewhat less open than those of Ni2, 
 as shown by the current that passed in each case when the air 
 was being removed and also when the membrane was deposited. 
 Further comparison of these two cells is impossible because Nil 
 was burst by the manometer cone in being set up before any 
 osmotic record for this cell was established. 
 
 This was doubtless due to a baking crack which was not 
 visible. 
 
 M'iz, however, has the best record of any one of the series. 
 It was prepared as indicated in the preliminary statements. 
 From the amount of current which still passes through it when 
 membrane is deposited, it seems to be the least dense of the pix 
 cells. It was first set up for a trial measurement on Nov. 23, 
 1910, with a normal cane sugar solution ac 25. Within an 
 hour it had developed an osmotic pressure of approximately 
 twenty-five atmospheres. From eight minutes past twelve to 
 one o'clock the mercury rose steadily in a closed manometer 
 without any oscillation through two hundred and twenty-six 
 millimeters. 
 
 For six days the pressure remained fairly constant but a 
 loss of 1.7 occurred in the rotation of the solution from the cell. 
 This was probably due to the dilution caused by the develop- 
 ment of the entire pressure by osmosis; resulting in the entrance 
 of a relatively large amount of water and dilution. As a quan- 
 titative test this was not of much value, but the behavior was 
 considered promising. 
 
 Several subsequent attempts to secure a measurement failed 
 on account of a leak at the washer of the needle tube or other 
 assignable cause all of which were in no way dependent on the 
 membrane. For the measurements later developed with this 
 cell, see experiments 1, 2, and 5, on the following pages. 
 
 Nis met the same fate that befell Kii and hence it has no 
 record. 
 
 Ni* of all the cells showed the most irregular passage of the 
 electric current in the removal of air from the cell and in the 
 membrane deposition. A few times the resistance has reached 
 
 (17) 
 
abotit 6ne-half million ohms, but only after several months, 
 with repeated deposition of membrane; and this resistance 
 varies. So far no successful use has been made of Ni4, though 
 recently it gave some promise of more normal activity. 
 
 Nie and Nis are the only nickel ferrocyanide cells besides 
 Ni2 that we have so far used successfully to measure osmotic 
 pressure. This does not mean, however, that any one of those 
 that are still intact is unfit for such measurement. In this con- 
 nection it should be kept in mind that if any one of a large 
 number of conditions is abnormal, no true measure of osmosis is 
 possible. This accounts for the relatively large number of 
 experiments which must be performed in order to produce and 
 maintain proper conditions that fail in no point. 
 
 Nis showed a dense texture throughout. After membrane 
 had been deposited on Feb. 2 for an hour and ten minutes, on 
 Feb. 3 for an hour and ten minutes, and on Feb. 6 for an hour 
 and thirty minutes, the cell showed a resistance recorded during 
 these three periods of twelve thousand ohms. Comparing Nis 
 with NJ2 for similar conditions, we find the highest resistance in 
 Ni2 to be fifty-seven thousand ohms. The least resistance 
 offered by Ni2 in this time was ten thousand ohms. 
 
 Nis was used in experiment three herein recorded. So far 
 as known no membrane has ever been developed in a shorter 
 time than was required for this one to produce a measurement. 
 This point is more fully presented in the general discussion of 
 the nickel ferrocyanide membrane later on. 
 
 Nie in its density and resistance to the electric current is 
 very similar to Nis, perhaps as nearly alike as two cells could 
 be found. In both of these, the membrane, as shown by a 
 steadily decreasing current, has been deposited with regularity 
 and uniformity. Each one offers about a half million ohms 
 resistance at the end of a brief deposition of fresh membrane. 
 
 Below are presented several results with the use of the cells 
 just described: 
 
 EXPERIMENT I. 
 
 Concentration, 0.1 weight normal cane sugar solution; rota- 
 tion of original solution, 12. 7; rotation at the end of the 
 experiment, 12. 7; loss in rotation, Oj volume of nitrogen in the 
 
 (18) 
 
manometer, 454.14;' time setup, 2 P. M., Jan. 6, 1911; manom- 
 eter, 9; cell used ^12; resistance of membrane, 64,705 ohms; 
 initial pressure a little below the theoretical for these conditions; 
 pressure of mercury, 409; pressure of the solution, .01; capillary 
 depression, .02. Below is table I. with corrections applied. 
 
 Table I. 
 ( Corrected Pressure 
 
 Time Temp. Atmos. Pres. (Vol. of ETa Osmotic Gas Eatio 
 
 Jan. 7 25C. .999 155.69 2.624 2.431 1.080 
 
 u 8 " .993 155.51 2.635 " 1.084 
 
 Average 1.082 
 
 The ratio formerly established by the Cu2Fe(Cn)e cells 
 
 for these conditions is . . . . . 1.084 
 
 EXPERIMENT II. 
 
 Concentration, 0.1 weight normal; rotation of solution 
 before and after the experiment, 12.70; manometer 9; cell used, 
 Nia; set up 1 p. M., Jan. 13, 1911; volume of N2 in manometer, 
 454.14 calibration units; resistance of membrane, 70,000 ohms; 
 corrections: liquids in manometer, 0.418; capillary depression, 
 0.02. 
 
 Table II. 
 
 ( Corrected Pressure 
 
 Time Temp. Atmos. Pres. ( Vol. of N 2 Osmotic Gas Eatio 
 
 Jan. 15 15C. 0.991 147.08 2.533 2.349 1.078 
 
 " 16 " 1.010 146.72 2.523 " 1.074 
 
 " 17 " 1.011 146.68 2.523 " 1.074 
 
 u 18 " 1.014 146.61 2.517 " 1.072 
 
 Average 1.006 Average 1.074 
 
 Average total pressure, 3.531; corrected, 2.525. 
 When corrected by normal atmospheric pressure, the 
 
 ratio is 1.078 
 
 The average ratio established with the copper ferrocy- 
 
 anide for the same conditions is . 1.082 
 
 (19) 
 
EXPERIMENT in. 
 
 Concentration, 0.1 weight normal; rotation of solution before 
 and after the experiment, 12.70; manometer, 1; cell used, Nis; 
 set up at noon Feb. 18, 1911; volume of N 2 in manometer is 
 445.43; membrane resistance, 104,000 ohms; corrections: liquids 
 in manometer, .438; capillary depression, 0.02. 
 
 lable III. 
 C Corrected 
 
 Time Temp. Atmos. Pres. 
 
 ( Vol. of ff 
 
 Feb. 19 15C. 
 
 1.001 
 
 144.31 
 
 " 20 " 
 
 0.989 
 
 144.73 
 
 a 21 " 
 
 1.006 
 
 144.47 
 
 " 22 " 
 
 1.001 
 
 144.62 
 
 Pressure 
 
 Osmotic Gas Eatio 
 2.544 2.349 1.083 
 2.547 " 
 
 2.536 " 
 
 2.537 " 
 
 Average 0.999 Average 1.082 
 
 Average total pressure is 3.540; corrected, 2.541. 
 Average ratio for copper ferrocyanide is . 1.082 
 
 EXPERIMENT IV. 
 
 Concentration, 0.6; rotation before and after the experiment, 
 69.2; manometer, ; cell used, Ni 6 ; set up Mar. 29, 1911, 4 p. 
 M. ; volume of Kz in manometer, 445.43; resistence of membrane, 
 570,000 ohms; corrections: liquids in manometer, 0.642; capil- 
 lary depression, 0.02. 
 
 Table IV. 
 
 ( Corrected 
 
 Pressure 
 
 Time Temp. Atinos. Pres. ( Vol. of N 2 Osmotic Gas Eatio 
 
 Mar. 30 30 
 
 " 31 " 
 
 Apr. 1 " 
 
 2 ' " 
 
 " 3 " 
 
 u n 
 
 0.975 
 0.988 
 0.997 
 1.005 
 1.009 
 1.009 
 
 27.642 
 
 15.802 
 
 14.830 
 
 1.066 
 
 27.578 
 
 15.826 
 
 n 
 
 1.067 
 
 27.508 
 
 15.859 
 
 " 
 
 1.069 
 
 27.556 
 
 15.822 
 
 it 
 
 1.067 
 
 27.561 
 
 15.814 
 
 11 
 
 1.066 
 
 27.574 
 
 15.807 
 
 u 
 
 1.066 
 
 Average ratio is 1.067 
 
 Average for copper ferrocyanide of this concentration at 30 
 yet to be determined. 
 
 (20) 
 
EXPERIMENT V. 
 
 Concentration, 0.1 weight normal; rotation of the solution 
 before and after the experiment, 12.75; manometer, 1; cell used, 
 Ni2j set up 4 P. M., Apr. 6, 1911; volume of ^2 in manometer, 
 445.43 ; resistance of membrane, 142,500 ohms ; corrections : 
 liquids in manometer, .435 ; capillary depression, 0.02. 
 
 Table V. 
 C Corrected 
 
 Pressure 
 
 Time Temp. Atmos. Pres. ( Vol. of ^2 Osmotic Gas Eatio 
 Apr. 22 30 
 
 23 
 24 
 25 
 26 
 
 28 
 29 
 30 
 
 0.995 
 
 146.99 
 
 2.480 
 
 2.472 
 
 1.004 
 
 1.006 
 
 146.19 
 
 2.486 
 
 u 
 
 1.006 
 
 1.006 
 
 146.44 
 
 2.482 
 
 it 
 
 1.004 
 
 1.006 
 
 146.88 
 
 2.471 
 
 it 
 
 1.000 
 
 1.009 
 
 146.72 
 
 2.472 
 
 u 
 
 1.000 
 
 1.010 
 
 146.52 
 
 2.474 
 
 it 
 
 1.001 
 
 1.000 
 
 147.37 
 
 2.467 
 
 u 
 
 0.998 
 
 0.990 
 
 147.48 
 
 2.473 
 
 it 
 
 0.999 
 
 Average 
 
 1.001 
 
 Average 1.003 
 Average total pressure is 3.478. 
 
 Average total pressure corrected by the average atmospheric 
 pressure is 2.475. 
 
 GENERAL DISCUSSION. 
 
 One of many requirements for a proper membrane to meas- 
 ure osmotic pressure is that it shall not affect the solution used. 
 To determine whether the nickel ferrocyanide causes any inver- 
 sion of cane sugar, two tests were made. 
 
 The first was by the use of Fehling Solution, which showed 
 no indication of inversion. 
 
 The second consisted in the addition of some precipitated 
 nickel ferrocyanide, which was entirely free from other electro- 
 lytes, to a sugar solution of known concentration and rotation. 
 
 A saccharimeter tube was used in order to take observations 
 from time to time of the rotation. On Jan. 16 the colloid and a 
 sugar solution, whose rotation and concentration had just been 
 determined, were mixed. The rotation was retaken at once to 
 
 (21) 
 
see the initial state produced. On Jan. 17 there was no change 
 in rotation. On Jan. 27 no change had occurred, none on Jan. 
 30, and on Feb. 13 the rotation was still the same. The whole 
 time was 28 days. The temperature was about 20 C. The 
 sugar solution was taken from some that had been prepared to 
 use in a measurement. Hence the potassium ferrocyanide and 
 the thousandth normal thymol solution were both present just 
 as in taking a measurement. So far as can be seen, there is no 
 inversion of the solution in the cell, caused by the action of the 
 membrane. 
 
 ACTIVITY. 
 
 When the copper ferrocyanide membrane is used for some 
 time two or three years, its activity in some cases falls off to 
 such an extent that much time is required for the development 
 of maximum pressure. Beca.use there are so many possibilities 
 of failure iu measuring osmosis, the chances for success are 
 greatly increased by shortening the time required for reaching 
 maximum pressure. It is quite possible that nickel ferrocyanide 
 as a membrane will also come to similar sluggish action with 
 equal age. So far, however, this tendency hns not appeared in 
 the oldest cell, which has been in use seven months. By refer- 
 ence to the record of Ni*2, some idea of the activity of the mem- 
 brane in the beginning is obtained. At the present time if a 
 cell is placed in the bath for a measurement only a few hours 
 elapse before a maximum pressure is exerted. 
 
 As already indicated, a large per cent of the efforts to 
 measure osmotic pressure fail because it is so difficult to have 
 all necessary conditions fulfilled. One of the chief difficulties, 
 prior to the work done in this laboratory, was the lack of a good 
 membrane in a suitable cell and at the proper place in the cell. 
 In nearly all cases of failure to obtain satisfactory results in this 
 work with the nickel ferrocyanide membrane, the trouble has 
 been traced to other causes than defects in the membrane or 
 in the cell. 
 
 No special duration tests have been made, but one experi- 
 ment extended over twenty-four days with unimpaired mem- 
 brane at the conclusion of the experiment. 
 
 (22) 
 
MEASURING Low CONCENTRATIONS. 
 
 There is nob much evidence to show that this membrane 
 responds promptly to barometric changes, although this property 
 may be expected to accompany a membrane which shows such 
 activity. In measuring the pressure of dilute solutions much 
 difficulty is found in obtaining concordant ratios, due largely to 
 the lag in the response of the cell to changes of atmospheric 
 pressure. The corrections made for barometric readings are not 
 true expressions of the air pressure then in effect through the 
 membrane, especially at times of sudden changes in the weight 
 of the atmosphere. Any error thus made is so large a per cent 
 of the quantity measured that the relative effect on the ratio of 
 osmotic to gas pressure is large. In order to equalize these 
 effects the average of all barometric readings is used to correct 
 the average of all the total pressures exerted. The effect of this 
 is seen in experiment 5. 
 
 This response is doubtless more prompt at higher tempera- 
 tures. Consequently less trouble would be expected from this 
 cause at forty degrees than at ten or fifteen degrees. Experience 
 bears this out. 
 
 The nickel ferrocyauide membrane was used some during 
 the present year to measure the pressure of one-tenth normal 
 solutions due to the greater lag of the older copper ferrocyanide 
 membranes in responding to atmospheric changes. And it may 
 be that with older nickel ferrocyanide cells the same trouble 
 would exist. This remains for time to test. Since there were also 
 new copper ferrocyauide cells in use in this work, the weight of 
 evidence is somewhat in favor of the view that the activity of the 
 nickel ferrocyanide membrane produces a more prompt response 
 to changes in atmospheric pressure. This may not prove to be 
 true of all such cells, because it is to be noted that most of this 
 work on one-tenth normal solutions was done with the most 
 active of all the nickel ferrocyanide cells. But here attention is 
 called to experiment three, in which Nis was used. This cell, 
 as already stated, is dense and any changes of atmospheric pres- 
 sure would be slower in producing an effect with Kis than with 
 the more porous Ni, if cells of different densities exert any 
 influence in the matter. But by comparing the ratios obtained 
 
 (23) 
 
in the two experiments, No. 2 and No. 3, those of the more 
 dense and supposedly less active cell are the more nearly con- 
 stant, although other conditions were nearly the same. 
 
 Manometer 9 and manometer have been compared and 
 found to agree. The variations of the barometer extended over 
 eleven- thousandth of an atmosphere in each case. The duration 
 was the same in both cases, both were at 15C. and tenth normal 
 concentration. Again the evidence seems to point to some 
 property of the membrane for prompt action, but more evidence 
 is necessary before any conclusion can be reached. 
 
 In general the resistance of the copper ferrocyanide mem- 
 brane to the current is greater than that of the nickel compound. 
 Whether longer use and age will produce a change in this 
 respect is a matter for future observation. Also, whether this 
 is an advantage or a disadvantage is not known. 
 
 By reference again to the tables it is seen that experiment 
 No. 3 shows an average ratio of osmotic to gas pressure of 1.082, 
 with .01 normal concentration at 15C. But No. 5 of the same 
 concentration with a temperature of 30 gives a ratio of unity. 
 
 The average ratio of all concentrations between 0C. and 
 and 25C. is 1.083. Up to 25 stability seems to exist. At 30 
 and at 40 the ratio is unity. If the same is found at higher 
 temperatures the Law of Boyle as well as that of Gay Lussac 
 will be demonstrated to apply to osmotic pressure, as to gas 
 pressure. It has already been demonstrated by the work be- 
 tween and 25 that the Law of Gay Lussac holds between 
 those limits for all concentrations up to the weight normal. 
 The final pressure of a 0.1 normal solution at 30 is the same as 
 that which the solution would be if its volume were confined to 
 the volume of the solvent. 
 
 What causes the change in osmotic pressure of dilute 
 solutions between 25 and 30 is not known. It seems possible 
 that hydration changes may produce dilation. Whatever the 
 cause, the process is one which requires some time. At first a 
 pressure is developed which nearly equals that at 25 and then 
 it gradually decreases to constant theoretical gas 1 pressure. In 
 several cases this has been the relative behavior, and about a 
 month is required for a measurement of this kind. Experiment 
 
 (24) 
 
5 was begun Apr. 6 and did not develop constant pressure or 
 equilibrium until Apr. 21. From Apr. 21 till May 1, there 
 was constant pressure as shown in the table (5). 
 
 The element of time required for the deposition of mem- 
 branes of sufficient strength to withstand maximum pressures is 
 of some interest. Eepeated subjection of the cell to the current 
 through the two solutions, as already described, is necessary in 
 order to secure sufficient stability or thickness of membrane to 
 withstand the higher osmotic pressure exerted upon it. Alter- 
 nate soaking and depositing produce the desired results. The 
 exact effect of soaking the cells is not definitely known, but it- 
 seems probable that practically all ionic or non-colloidal material 
 is thus removed, resulting in a more homogeneous membrane; 
 and also less subject to attack by any solutions coming against 
 it. Summing up all the time required for the quickest deposi- 
 tion of a copper ferrocyanide membrane and the shortest time 
 required for a nickel ferrocyanide membrane, each giving a 
 measurement, we have the following for comparison: the copper 
 ferrocyanide required 28 hours; the time required by the nickel 
 ferrocyanide was ten hours and forty-six minutes. Whether 
 any advantage besides the saving of time is to be derived from 
 this fact cannot be stated. 
 
 It will be recalled that in depositing membrane the potas- 
 sium ferrocyanide within the cell is frequently renewed, by 
 pouring fresh solution through the funnel set in the cell. The 
 funnel extends nearly to the bottom of the cell, and the end of 
 the exit tyibe is just below the rubber stopper, so that the fresh 
 solution enters at the bottom, forcing the solution already in the 
 cell upward and out the exit tube. In this way there is an 
 upward flow around the electrode. If the amperage is noted 
 while the replacement of the solution is taking place it is found 
 that quite an increase of current flows. In one case where the 
 resistance before pouring in fresh potassium ferrocyanide was 
 110,000 ohms, it became only 55,000 while the solution was 
 being changed. The current in this instance was doubled. 
 
 The explanation suggested for this is similar to that given 
 for the rotating anode in electrolytic work. But if the solution 
 in the cell is agitated by moving the funnel supporting the 
 
 (25) 
 
electrode, no such change is seen in the current flow. In fact, 
 no difference in the current is observed with this treatment. 
 
 When the liquid has ceased to flow from the cell, after a 
 pouring, the current invariably decreases till it becomes the 
 same that it was before the change of solution, so that the cause 
 can not be in the difference of the two solutions. It is possible 
 that the force of the flowing liquid produces a mechanical dis- 
 turbance which reduces the resistance for the time. 
 
 Whether any importance is to be attached to this phenome- 
 non is not known. The facts are recorded, but the explanations 
 suggested may not be correct. 
 
 OSMOTIC PRESSURE is NOT A FUNCTION OF THE MEMBRANE 
 
 USED. 
 
 Until the present year only the copper ferrocyanide mem- 
 brane was used in the quantitative determination of osmotic 
 pressure, and while it has been supposed that the nature of the 
 membrane was not a factor in osmosis, no comparison of quanti- 
 tative results obtained by different kinds of membrane has before 
 been possible. Some comparisons of the measurements obtained 
 with both membranes appear in the tables. It will be seen that 
 they agree quite as well as duplicate measurements with the 
 same membrane. Here we have ample experimental evidence 
 for the conclusion that osmotic pressure is not a function of the 
 membrane used. 
 
 (26) 
 
M 
 
 251816