Determination of Hydrocyanic Acid in Air and in Air-Carbon Dioxide Mixtures H. L. CUPPLES,Bureau of Chemistry and Soils, Department of Agriculture, Whittier, Calif.
I
N FUMIGATlNG a tree with hydrocyanic acid, it is covered with a canvas tent, and the desired amount of liquid acid introduced beneath the tent. The canvas tents are by no means gas-tight, and consequently there is a rapid decrease in the concentration of hydrocyanic acid. The rapidity of decrease depends upon numerous factors, including the ratio of tent surface to tent volume, the “tightness” of the canvas under the prevailing conditions of humidity, the density of the foliage, and the velocity of the air currents. I n a typical fumigation, an initial concentration of about 15 mg. per liter was reduced to 1 mg. per liter in about 17 minutes (see Figure 1).
%
2 0
m (D
t ol
0
5
IO
IS
20
26
SO
35
40
45
MINUTES
FIGURE 1
I n order to determine satisfactorily the time-concentration relationship during such a field fumigation, it is necessary to obtain gas samples in rapid succession, and each sample must be of sufficient volume to make possible a reasonably accurate analysis. The apparatus here described has proved admirably adapted for this work, and the author believes that it may be used to advantage in other cases where the requirements are similar.
needle valve serves to adjust the rate of flow accurately to a definite value, as measured by the calibrated flowmeter. By properly manipulating the four glass stopcocks, the gas stream may be quickly diverted to either of the two absorbers, and by following a definite time schedule, the stream may thus be divided into separate portions of known volume. The apparatus is illustrated in Figure 3. The wooden stand, A , supports and protects the capillary-tube flowmeter. The absorbing bottles, C and D,are supported by separate, removable wooden stands, E, and are selected to fit easily the two rubber stoppers which carry the gas delivery and exit tubes, together with the glass stopcocks, B, which direct the flow of gas to either of the absorbing bottles. While the gas stream is passing through one absorbing bottle, the other one may be replaced by a bottle containing a fresh portion of absorbing solution. I n this manner a continuous series of samples may be taken without interrupting the flow of gas through the sampling tube. It is quite easy for one operator to take samples a t % h e rate of one per minute, and greater speed may be attained if desired. The flowmeter is of the capillary-tube type, and as the hydrocyanic acid-air mixture passes through the flowmeter before entering the absorbers it is necessary to guard against possible loss of hydrocyanic acid by absorption in the manometer liquid or in rubber connectors. The capillary tube is held in place by rubber connectors, but the glass parts are kept close together in order to minimize contact between the gas and the rubber. A light petroleum oil is used in the manometer, as hydrocyanic acid is said to be practically insoluble in such an oil. One leg of the manometer is connected through a glass stopcock to a wider side tube which is open to the atmosphere, With the use of this side tube, the level of oil in the manometer tubes may quickly and easily be adjusted to a fixed zero mark. For ordinary use it is necessary only to calibrate the flowmeter a t one fixed mark, and then adjust the rate of %ow during each experiment to give this
TUBE
81’
DESCRIPTION OF APPARATUS
A simple type of apparatus has been developed for making analyses during field fumigations with hydrocyanic acid. The principle of operation is to absorb the hydrocyanic acid from a continuous current of the gas mixture, the volume of which is proportional to the duration of the sampling period and to the rate of flow, as measured by a capillary-tube flowmeter (I). The chief point of interest in the present apparatus lies in the facility with which samples may be taken in rapid succession without interrupting the flow of the mixed gases through the sampling tube, together with the relathe simplicity of construction and manipulation. A simple flow diagram of this apparatus is shown in Figure 2. For field work the air exhaust is obtained by connection to the intake manifold of a motor car engine, which is kept in operation during a series of analyses. I n the laboratory, a water exhaust pump or a laboratory “oil” pump may be used. The surge tank eliminates sudden fluctuations which might be caused by irregular operation of the exhaust pump. The
FIGURE 2. FLOWDIAGRAM OF ANALYsIs APPARATUS
same manometer reading. The temperature of the oil in the manometer may be determined and the appropriate temperature correction applied. A small correction may also be applied for the temporary decrease in the rate of gas flow which occurs when the gas space above the absorbing liquid, in a fresh bottle of absorbent, is first opened to the exhaust line. I n order to allow for the rather slow drainage of oil from the walls of the manometer tubes, the flow of gas should be started about 10 minutes before beginning a series of analyses. In addition to its usefulness in making analyses during field
January 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
fumigations, this apparatus has proved to be very convenient for laboratory work. It has been used in the determination of hydrocyanic acid in admixture with carbon dioxide and air,
ANALYTICAL METHODS A well-known method for determining hydrocyanic acid in air consists in absorbing it in dilute sodium hydroxide solution and titrating with standard silver nitrate solution. This method seems to be convenient and reliable, and the end point is sharp if the titration is made with falorable conditions of illumination, such as are described below. For such an analysis the hydrocyanic acid is absorbed in 100 cc. of 2 per cent sodium hydroxide solution, 5 cc. of 2 per cent potassium iodide solution are added, and the titration is made with standard silver nitrate solution containing approximately 3 grams of silver nitrate per liter. The volume of the gas sample is adjusted to provide enough hydrocyanic acid for a satisfactory titration, and it is bubbled through the absorbing solution a t the rate of about one liter per minute. In this method of analysis the addition of potassium iodide increases the sharpness and reliability of the end point, which is indicated by the first permanent turbidity of the solution. However, the end point is somewhat difficult to see unless the illumination is favorable, Daylight is much more satisfactory than ordinary artificial illumination. The author has found that the end point is quite sharp when the titration is performed in semi-darkness, with a condensed beam of light passing through the solution in a generally horizontal direction. Satisfactory results may be obtained by the use of a
51
amount of reaction between the iodine and the sodium carbonate solution itself. Subsequent experiments confirmed this expectation. If the iodine solution is added without stirring, and considerable opportunity is allowed for it to react with the sodium carbonate solution, the titration may consume a 30 per cent excess of iodine. If the iodine solution is added slowly, with very efficient stirring, the results seem to be consistent and in agreement with those of silver nitrate titrations. However, any intermediate result can be obtained, and unless precautions are taken it is comparatively easy to incur errors of 5 or 10 per cent. Satisfactory analyses may be made by absorbing the hydrocyanic acid in about 100 cc. of 2 per cent sodium carbonate solution, adding 10 cc, of
..
PER CENT CO,
FIGURE4 10 per cent potassium iodide solution and 5 cc. of 2 per cent starch solution, then titrating with standard iodine solution, with efficient stirring. The reaction may be expressed by the equation:
NaCN
+ 1%= NaI f
CSI
According to another well-known method of analysis, the hydrocyanic acid-air mixture is bubbled through sodium bicarbonate solution which contains a measured amount of standard iodine solution, together with potassium iodide and starch. The analysis is made by determining the volume of the gas mixture which is required just to decolorize the absorbing solution. A few comparative tests show that this method yields results which are in agreement with those of analyses by the two methods described above. Although a sodium bicarbonate solution is not, of itself, a satisfactory absorbent for hydrocyanic acid, the hydrocyanic acid will be removed from air in an efficient manner so long as free iodine is present in the solution.
DETERMINATION OF HYDROCYANIC ACIDIN CARBON DIOXIDEAIR MIXTURES FIGURE3. ANALYSIS APPARATUS
focusing flashlight, or the more powerful beam from a microscope illuminator. With such illumination a slight excess of silver nitrate produces a distinct Tyndall effect. The reaction may be expressed by the equation: 2NaCN AgNOa = NaAg(CN)2 NaN08
+
+
A second method of analysis, which has been used extensively, involves the absorption of the hydrocyanic acid in a 2 per cent sodium carbonate solution, with subsequent titration with standard iodine solution (8). The aut,hor’s results were somewhat irregular when this method of analysis was used, a.nd it was concluded that this might be due to a variable
In performing a series of experiments to determine the toxicity to red scale of hydrocyanic acid-carbon dioxide-air mixtures, the problem arose of determining hydrocyanic acid in such mixtures. The presence of carbon dioxide makes necessary certain precautions in the measurement of the gas volume. If the gas is measured over water in an aspirator bottle, it must be kept in mind that carbon dioxide is moderately soluble in water. If the gas volume is determined by its rate of flow through a capillary-tube flowmeter, it is necessary to take into account the effect of the carbon dioxide on the calibration of the flowmeter. Moreover, carbon dioxide may be too readily absorbed by the solution which is used to absorb hydrocyanic acid, or it may interfere with the subsequent titration of cyanide.
ANALYTICAL EDITION
52
voi. 5 , No. i
TABLEI. DETERMINATION OF HYDROCYANIC ACID IN CARBON termined, and each flowmeter which is to be used for this DIOXIDE-AIRMIXTURES purpose should be individually calibrated. HYDROCYANIC ACID A solution of hydrocyanic acid in water, of a definite conCARBONDIOXIDE SAMPLE VOLGMEI Total Per liter centration and a t a definite temperature, has a definite hydro% Liters MO Ms. cyanic acid vapor pressure. This means that it will produce 6.73 1.52 0.0 4.43 5.90 100.0 3.93 1.50 the same volume concentration of hydrocyanic acid in any 47.0 4.29 5.46 1.27 indifferent gas which is brought into intimate contact with 27.0 4.37 5.54 1.27 the solution. Therefore, if pure carbon dioxide is slowly 5.75 1.31 12.4 4.40 4.41 5.85 7.0 1.33 bubbled through such a solution, it will acquire the same vol0.0 4.42 5.70 1.29 5.76 1.31 ume concentration of hydrocyanic acid as will a current of 7.0 4.41 12.4 4.40 5.63 1.28 air which is passed through the same solution. By the use 8.0 4.41 4.23 0.96 of this device the same concentrations of hydrocyanic acid 4.20 0.0 4.42 0.95 in air and in various carbon dioxide-air mixtures have been 8.0 4.41 3.64 0.83 produced, and the method of analysis has been checked by 3.63 0.82 0.0 4.42 analyzing the mixtures. Results obtained in this manner are 2.92 0.66 0.0 4.42 given in Table I, grouped in such manner that the results in 8.0 4.41 2.92 0.66 column 4 would be identical within each group if there were no errors of any kind during the experiments. Considering It appeared that under these circumstances hydrocyanic the fact that there are probably other inaccuracies in addition acid might be determined satisfactorily by absorption in 2 to those of analysis, the results indicate that this method is per cent sodium carbonate solution, with subsequent titra- suitable for the determination of hydrocyanic acid in carbon tion with standard iodine. This plan contemplated that the dioxide-air mixtures, volume of gas mixture would be determined by measuring LITERATURE CITED its rate of flow through a flowmeter which had been calibrated previously for carbon dioxide-air mixtures. The calibration (1) Griffin and Skinner, IND.ENG.CHIM.,24,862 (1932). curve for this purpose is shown in Figure 4. This curve is (2) Pratt, Swain, and Eldred, J.Econ. Entorno!., 24,1041 (1931). strictly applicable only to the flowmeter for which it was de- RECEIVEDJuly 28, 1932.
Determination of Small Quantities of Antimony in Solder in Presence of Iron C. W.
ANDERSON,
Continental C a n Company, Inc., 4633 West Grand Ave., Chicago, Ill.
T
HIS laboratory is called upon to analyze numerous
samples of scrap solder which consist chiefly of tin and lead, but which are contaminated with as much as 5 per cent of iron. The presence of iron interferes with the titration of antimony by the usual bromate method, particularly if the amount of iron in the sample is more than 0.1 per cent. A method has therefore been worked out which eliminates this difficulty. I n reviewing the literature it was found that very little information had been recorded on the titration of antimony in the presence of iron. However, Rowell (1) in his method of direct estimation of antimony by the bromate method notes that the presence of 1 per cent iron increases the results by about 0.02 per cent. He gives no definite figures with larger quantities of iron. The following procedure has given satisfactory results when as much as 15 per cent of iron is present in the sample: Dissolve a 3-gram sample in 15 to 20 cc. of hot concentrated sulfuric acid in a 250-cc. Erlenmeyer Aask. Cool the solution and add carefully 50 cc. of concentrated hydrochloric acid. Add about 0.5 gram of potassium chlorate to oxidize the iron and expel the chlorine by boiling. Cool the solution and add 20 to 25 cc. of phosphoric acid, sp. gr. 1.37, and then 3 to 4 grams of sodium sulfite. Allow the solution to stand at a temperature of about 60" C. for 15 minutes and expel the excess sulfur dioxide by boiling in a current of carbon dioxide or air for 5 t o 10 minutes. Add about 50 cc. of water, and titrate at about 60' C. with 0.033 N potassium bromate solution which has been standardized against metallic antimony using methyl orange as indicator. About the same quantity of iron should be present in the antimony solution used for standardizing the bromate as is
present in the sample for analysis. The iron may be added as ferrous or ferric sulfate and the process of solution, reduction, and titration conducted in the same manner as in a regular determination. The methyl orange indicator should be added toward the end of the titration, having ascertained the approximate amount of bromate required by a preliminary test. The color change a t the end point is readily detected, since a dilute phosphoric acid solution is only faintly colored by the usual quantities of iron occurring in scrap solder. The amount of bromate required to destroy the methyl orange color is very slight, the correction usually being approximately 0.1 cc. which is deducted from the number of cubic centimeters used in titrating. ANALYSES TABLEI. RESULTSOF EXPERIMENTAL IRON TAXEN SOLDER TAKEN AS FeSOa.7HzO
a b
ANTIMONY TAKEN
Cram Grams Gram 3.0" 0.0300 0.1 0.0198 3.0a 0.16 0.0498 3.05 0.20 0.0104 3.0a 0.30 0,0400 3.0" 0.14 0.0250 0.10 3.0" 0.0050 2.26 0.16 0.0251 3.OC 0.4 0.0321 3.0C 0.6 0.0141 3.0d 0.4 0.0300 3.0d 0.5 Antimony content 0.03%. Antimony content 0.015%.
TOTAL ANTIANTIMONY PREEENT FOUND Cram Uram 0,0309 0.0310 0.0206 0.0207 0.0507 0.0600 0.0113 0.0106 0.0405 0.0394 0.0269 0.0254 0.0053 0.0058 0.0269 0.0268 0.0339 0.0344 0.0153 0.0148 0.0312 0.0302 0 Antimony content 0.06 d Antimony content 0.04g: MONY
LITERATURE CITED (1) Rowell, H. W., J. SOC.Chem. Ind., 25, 1181 (1906). RBIOEIVED Sugust 25, 1932.
