UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE BERKELEY AGRICULTURAL EXPERIMENT STATION BENJ. IDE WHEELER, PRESIDENT THOMAS FORSYTH HUNT, Dean and Director H. E. VAN NORMAN, Vice-Director and Dean University Farm School CIRCULAR No. 139. THE GENERATION OF HYDROCYANIC ACID GAS IN FUMIGATION BY PORTABLE MACHINES By H. D. Young. The latest important development in fumigation practice has been the use of portable machines for generating hydrocyanic acid gas outside of the fumigating tents. During the past two years, the commercial success of machine generation has become an assured fact. This has been accomplished chiefly by the Owl fumigating machine (Fig. 1). The new method has a number of advantages over the old one of using pots. In the machine the cyanide is used dissolved in water. This Figure I. Owl Fumigating Machine. (Season 1915.) solution can be measured more accurately than lump cyanide can be weighed at night under field conditions, so that the dosage is made more accurate. As the materials for a number of charges are all contained in the machine, very much less handling of the chemicals is required, *Paper No. 13, Citrus Experiment Station, College of Agriculture, University of California, Riverside, California. 18682 involving some economy of material and much greater cleanliness. The reaction also takes place with great rapidity so that the gas is produced almost instantaneously; while in the pots it takes place much more slowly, since the solid cyanide must first dissolve. The gas being delivered so quickly under the tent should be more effective than the slowly generated gas from pots, as in the latter case part of the gas will have leaked out of the tent before the generation is complete and the maximum possible concentration will never be obtained. In pot fumigation there is always a greater or less amount of injury done to the tents by the spilling of sulfuric acid, dragging the tents through the acid residue, etc. This not only greatly increases the IB, Figure II. Cyanofumer. (Season 1915.) expense of fumigation, but it impairs the effectiveness of fumiga- tion because of the greater tent leakage caused by small holes which are overlooked. Machines should almost entirely eliminate tent burn- ing, since the acid is not handled for each tree and since the residue can be disposed of in a safe place. Description of the Owl Fumigating Machine. The machine consists of a cylindrical drum within which there is a tray suspended. Mounted above the drum are two reservoirs for sul- furic acid and cyanide solution, respectively. These solutions are measured in the cylinders shown in Fig. I, and are then run within the drum. The gas is very quickly generated and by its own pressure forced through the outlet hose under the tent. — 3 — There is in this machine no radical departure from the dosages used in pot fumigation. The charges are measured separately for each tree, and in the same proportions that would be used in a pot. The chief difference in principle is that in the machine the cyanide is added in solution instead of in the solid form as in the pots, and the generation therefore takes place much more quickly. It has previously been reported that acid burns of the tents have been almost entirely elimi- nated by this machine.* Several cases of tent burning, however, have been recently reported. Description of the Cyanofumer. A new machine is being introduced this season, called the Cyanofumer (Fig. II). It consists essentially of two tanks, one above the other. In the lower, tank is placed the sulfuric acid and water, in the upper one, the cyanide solution. By the action of a suitable pump, measured quantities of the cyanide solution are forced into the tank containing the acid and water, and the gas is generated almost instantly and dis- charges through the delivery hose with considerable force. The pump is graduated on the basis of a standard solution obtained by dissolving 200 pounds of pure sodium cyanide in 50 gallons of water. With a solution of any different strength it would of course give other amounts than those marked on the pump cylinder. The basic principle involved is that small successive quantities of cyanide solution are added to a large amount of acid and water until the acid is nearly exhausted. This constitutes such a radical departure from the accepted methods of fumigation that its accuracy has been widely questioned. With the machine charged to its full capacity, 600 ounces of sodium cyanide dissolved in 1,440 ounces of water are placed in the upper reservoir b, Figure III, while 600 ounces of water and 600 ounces of concentrated sulfuric acid are placed in the lower reservoir, c, or enough acid to generate all of the cyanide solution at once, if the two were to be mixed. Assuming that the trees to be fumigated each require 10 ounces of sodium cyanide, the pump, a, is set at "10," the lever raised to that point and as it is forced down 24 ounces of solution containing 10 ounces of cyanide, if the standard solution is used, are forced through the pipe, /, into the acid and water. Instead of the ratio being 1 ounce of cyanide, 1J ounces of sulfuric acid and 2 ounces of water, as is the case in pot fumigation, the ratio in the first charge delivered is 1 of sodium cyanide, 60 sulfuric acid, 62.4 water. As successive charges are added the acid becomes proportionately less and less, as it is used up by the sodium cyanide; while the amount of water is continually increased, due to that added, in which the cyanide is dissolved. The *Geo. P. Gray, Monthly Bulletin, Vol. IV, No. 2, "New Fumigating- Machines." Also Proc. 45, Fruit Growers Convention, pp. 199-208. — 4 — ratio for the sixtieth charge would, therefore, be 1, 27, 204. At no time would the proportions be those which have been formerly used. In view of the great importance which has been placed on the neces- a — Pump. b — Cyanide solution. c — Acid and water. d — Vent for discharging residue. e — Gas outlet. f — Pipe delivering cyanide solution from pump. g — Filling vent for cyanide tank. h — Filling vent for acid and water. sity of accurate proportions, prospective fumigators are thoroughly justified in requesting an investigation of such a radical departure from the accepted standards. Drip From the Delivery Hose. There sometimes occurs a slight amount of liquid discharge from the end of the delivery hose. This is rather disquieting to the fumigator, as it suggests acid tent burning. The drip from over three hundred charges was collected and analyzed without finding a trace of free sulfuric acid. — 5 — Gas Generation. The following tables present the results of an investigation of the accuracy of the new method. Samples of the solution in the generating machine were taken after each five charges, and the amount of hydro- cyanic acid determined. The difference is assumed to be the amount of hydrocyanic acid gas delivered through the hose. TABLE I. Charges Generated at One- Minute Intervals. Amount of HCN found in residue. Machine charged for fifty 10 -ounce charges. Sample taken after the Indicated number of charges Total amount NaCN added. Ounces Total amount HCN found in residue. Ounces Increase in HCN due to 5 charges. Ounces Per cent HCN in residue due to 5 charges Per cent gas evolved by difference 5 charges 10 charges 15 charges 20 charges 25 charges 30 charges 35 charges 40 charges 45 charges 50 charges 50 100 150 200 250 300 350 400 450 500 1.17 3.00 4.60 6.12 8.06 10,10 12.74 15.05 16.60 18.63 1.17 1.83 1.60 1.52 1.94 2.04 2.64 2.31 1.55 2.03 2.3 3.6 3.2 3.0 3.8 4.1 5.3 4.5 3.1 4.1 97.3 96.4 £6.8 97.0 96.2 95.9 94.7 95.5 96.9 95.9 The preceding table shows the extremely high and uniform per cent of gas evolved when the machine is working under the conditions to which it is best adapted, that is, fairly high charges at short intervals. Almost as satisfactory results are obtained with a longer interval (Table II), but toward the end of the run the amount of gas delivered begins to fall off slightly. A rate of thirty charges per hour is probably as slow as it is wise to run the machine, and with small charges, this is too slow. TABLE II. Charges Generated at Two- Minute Intervals. Amount of HCN found In residue. Machine charged for fifty 10 -ounce charges. Sample taken after the indicated number of charges Total amount NaCN added. Ounces Total amount HCN found in residue. Ounces Increase in HCN due to 5 charges. Ounces Per cent HCN in residue due to 5 charges Per cent gas evolved by difference 5 charges .__ 50 100 150 200 250 300 350 400 450 500 1.68 2.83 4.14 6.24 9.03 12.90 16.22 20.15 23.56 28.08 1.68 1.15 1.31 2.10 2.79 3.87 2.32 3.93 3.41 4.52 3.4 2.4 2.5 4.2 5.6 7.7 6.5 7.9 6.8 9.0 96.6 10 charges _. 97.6 15 charges __ __ _ _ 97.5 20 charges 95.8 25 charges __ 94.4 30 1 charges _ 92.3 35 charges 93.5 40 charges 92.1 45 charges 93.2 50' charges 91.0 — 6 — TABLE III. Charges Generated at One-Minute Intervals. Amount of HCN found in residue. Machine charged for fifty 4-ounce charges. Sample taken after the indicated number of charges Total amount NaCN added. Ounces Total amount HCN found in residue. Ounces Increase in HCN due to 5 charges. Ounces Per cent HCN in residue due to 5 charges Per cent gas evolved by difference 5 charges _ 28 • 48 68 88 108 128 148 168 188 208 1.92 2.90 4.02 5.06 6.44 7.71 9.36 • 10.57 12.01 13.66 1.92 .98 1.12 1.02 1.38 1.27 1.65 1.21 1.44 1.65 6.9 4.9 5.5 5.0 6.9 6,3 8.2 6.1 7.1 8.2 93,1 10' charges 95.1 15 charges 94.5 20 1 charges 95.0 25 charges __ 93.1 30 charges _ _ 93.7 35 charges __ _ 91.8 40 charges 93.9 45 charges — 92.9 50 charges 91.8 Table III shows the results obtained with four-ounce charges. A smaller, but still efficient, amount of gas is developed. With the smaller charges, the temperature of the acid solution is lower (because of the heat taken up by the machine from the smaller quantity of solution used), and the amount of gas evolved consequently somewhat less. The effectiveness of the machine depends on maintaining as high a temperature as possible. The gas is not held in solution to a great extent by a hot solution, but a cold one retains great amounts of it. As the temperature falls, therefore, the amount of loss increases. The necessary high temperature is obtained at the beginning by the mixing of sulfuric acid and water. These should not be placed in the machine until the last possible moment before starting the series of generation. By the rapid addition of the alkaline solution of sodium cyanide to the hot acid, enough heat is formed to maintain a satisfactory temperature. If climatic conditions should render it difficult to keep the temperature high enough, this might be done by further additions of sulfuric acid, or by jacketing the machine to lessen the heat loss. Comparison With Pot Generation. The amount of gas given off by the machine is, under such conditions as given in Table I, greater than the usual amount given off by pots. The latter give a very irregular yield of gas, varying probably from 85 per cent to 95 per cent. The average of a small number of analyses made in connection with this investigation was 92.5 per cent. The smaller yield from the pots probably is due to a lower temperature than that obtained in the machine. Using lump cyanide the reaction pro- ceeds only as the lumps dissolve so that ten or fifteen minutes may be required for complete generation. By this time enough heat has been given off or absorbed by the pot to lower the temperature of the solution to a point where a larger amount of the gas will be absorbed. This Figure IV. Gasometer for collecting hydrocyanic acid gas. (The delivery tube from a Cyanofumer may be seen.) ■ "if BiTm* 1 JHPr Figure V. Gasometer immediately after receiving the gas from a ten-ounce charge in the Cyanofumer. — 8 — factor would vary greatly with the size of the charge since small quan- tities of solution would naturally lose proportionately more heat and so retain more gas. Direct Determination of Gas Generated. The figures given above, on the percentage of gas generated, are based on the "indirect" method of determination, the one which has been the basis of practically all fumigation schedules. In this method the amount of hydrocyanic acid left in the residue is determined, and the assumption is made that the difference between the per cent so obtained and 100 per cent represented the gas evolved. The error involved in this comparison is, of course, greater or less depending on the amount of decomposition of the gas. To determine the extent of the error with the machine, the apparatus shown in Figures IV and V was constructed. It consists of a metal tank 24 inches in diameter by 36 inches deep, within which a similar tank is inverted. The delivery hose from the Cyanofumer was connected with a pipe which rises within the tank nearly to its top. The tank having been filled with an alkali solution, and the weight of the inner tank counterbalanced to prevent back pressure on the machine, the gas from a ten-ounce charge raised the inner tank as shown in Figure V. After closing the delivery pipe, the gas was slowly absorbed by the alkaline solution in the tank. Analysis of this solution showed an amount of sodium cyanide equiva- lent to 95.1 per cent of the theoretical yield of hydrocyanic acid gas. As this was obtained under the conditions which showed from 2 per cent to 4 per cent of gas in residue, the amount of decomposition must be very slight. SUMMARY. The principal advantages of machine fumigation in general are : Greater accuracy of dosage, cleanliness, rapid generation so that a greater concentration of gas under the tent is obtained, and a lessening of tent burning. (This probably constitutes the greatest saving of the new methods.) The Cyanofumer introduces an entirely different ratio in the dosage schedule, since successive quantities of sodium cyanide are added to a large amount of sulfuric acid. The amounts of sulfuric acid, water and cyanide recommended for use, give a high and uniform production of gas under the proper con- ditions. The best production of gas is obtained with a high temperature. It is extremely important to keep the cyanide solution and tank scrupulously clean. Any dirt or small bits of wood may interrupt the pump and so make the charges irregular. Under normal conditions with clean solutions, the pump works with great regularity.