Colorimetric Microdetermination of Cadmium with Dithizone With Improved Separation of Interfering Metals BERNARD E. SALTZMMAN Division of Occupational Health, Public Health Service, Federal Security Agency, Cincinnati, Ohio -4 procedure for the microdetermination of cadmium by dithizone in chloroform solution is presented with innovations to give more stable colors and much greater tolerance to interfering metals. Improved separation of interfering substances is achieved by the use of small amounts of cyanide as a suppressing agent and tartaric acid as a stripping medium. Two extractions are made from strongly alkaline solution. Stable colors are obtained and losses due to decomposition are controlled by using hydroxylamine in the extractions and reducing the time of contact of the chloroform with the alkali. Purification of reagents is unnecessary. The simplified procedure determines cadmium spectrophotometrically in a volume of 15 ml. with a sensitivity of 0.05 microgram. Separation of thallium, previously disregarded, is made possible by the development of a special procedure which transposes the dithizonate of thallium with cobalt.
T
HE determination of cadmium a t the low concentration
which may be of toxicologic significance requires a highly sensitive method. The great variety of samples, such as dusts, fumes, spray, and biological specimens, requires that the method be specific. The dithizone method (8, 14) has the requisite sensitivity; however, nearly a score of other metals react with dithizone to produce intense colors, most of which resemble that of cadmium. Hence, the major problem has been that of increasing the specificity of the method. Efforts have also been directed toward the elimination of erratic lorn- results and the prevention of fading colors. Many procedures (1, d, 6,9-13) have becn used for the elimination of interfering metals based on separation by the differential extraction and stripping of metal dithizonates a t controlled pH, suppression of interferences with complexing agents, or separation by extraction with other reagents. The procedures nhich have previously been proposed are limited in their application because of the small amounts of most of the interfering metals (50 to 100 micrograms) which can be tolerated. In a number of procedures (f-5,8, 10-16, 14, 16) modifications or additional steps have been introduced in an effort to obtain a more stable color for the final reading. The procedure described herein is applicable to samples containing as much as 5 to 10 mg. of common interfering metals. The improved separation is accomplished by the use of cyanide as a suppressing agent in two extractions from strongly alkaline solution and by the use of tartaric acid as the stripping medium. By the use of reagent grade chloroform and simple precautions the author has been able to obtain consistently stable colors without resort to additional steps or reagent purification. The decrease in steps over the previous method4 permits a qaving in t i i n e and an increase in accuracy. APPAR ATUS
Separator\- Funnels, 125-ni1. rapacity, Squibh type. Stopcocks require regreasing, after each use because of the corFosive effect of the strong alkali. The funnel is then cleaned by rinsing with wash acid and the acid fumes are flushed out hy filling full m-ith tap water. Finally the funnel is thoroughly rinsed with tap and distilled water.
Spectrophotometer, Beckman Model DU. A set of matched test tubes, 22 X 175 mm., giving an optical light path of 2.02 em. was used in a special holder fitted to the spectrophotometer. Shaking Machine capable of shaking separatory funnels about 3 strokes a second, 1.5-inch amplitude. REAGENTS
A11 reagents are made from analytical grade chemicals. Yo special purification was found necessary. Sodium Potassium Tartrate. Dissolve 25 grams of sodium potassium tartrate tetrahydrate in distilled water and make up to 100 ml. Sodium Hydroxide. Dissolve 400 grains of sodium hydroxide and make up to 1 liter in distilled water. Store in a paraffinlined bottle with a rubber stopper. Sodium Hydroxide, 40%-Potassiuni Cyanide, 1 %. Dissolve 400 grams of sodium hydroxide and 10 grams of potassium cyanide and make to 1 liter with distilled water. The cyanide slowly deteriorates, but the solution may be used 1 to 2 months when stored in a paraffin-lined bottle with a rubber stopper. Sodium Hydroxide, 40~o-Potassium Cyanide, 0.05%. Dissolve 400 grams of sodium hydroxide and 0.5 gram of potassium cyanide and make to 1 liter with distilled water. The cyanide slowly deteriorates, but the solution may be used 1 to 2 months when stored in a paraffin-lined bottle with a rubber stopper. Hydroxylamine Hydrochloride. Dissolve 20 grams of hydroxylamine hydrochloride and make up to 100 ml. with distilled water. Chloroforni. To test the chloioforin for use in this procedure, add a minute amount of dithizone to a portion in a stoppered test tube so that a faint green tint is produced. If the chloroform is suitable, this color should be stable for a day. Extraction Dithizone Solution. Dissolve 80 mg. of diphenylthiocarbazone (dithizone) in 1 liter.of chloroform. Keep in a brown bottle in the refrigerator until required and use while still cold. Standard Dithizone Solution. Dissolve 8 mg. of diphenylthiocarbazone (dithizone) in 1 liter of chloroform. Keep in a brown bottle in the refrigerator and allow to warm to room temperature before using. The fresh solution should be aged a day or two, then standardized as given in the procedure. The standardization should be checked after a few weeks. Tartaric Acid. Dissolve 20 grams of tartaric acid in distilled water and make to 1 liter. Keep the solution in the refrigerator so that it is cold when used. Cobalt Wash Solution (for thallium separation) Dissolve 0.10 gram of cobalt sulfate heptahydrate and 5 grams of sodium potassium tartrate tetrahydrate in distilled water, add a solution of 40 grams of sodium bicarbonate in distilled water and make to 1 liter. Discard after a few weeks, as slow precipitation occurs. Standard Cadmium Solution. Prepare a stock solution containing 10 microgram of cadmium per milliliter in 1 to 99 nitric acid by dissolving a weighed amount of pure cadmium metal in nitric acid and diluting. The stock solution is diluted with 1 to 99 nitric acid to prepare a working solution containing 1 microgram of cadmium per milliliter. Wash Acid, 1 to 1 hydrochloric acid. Solution may be used 1cpe?tedly for rinsing glasslvare. PROCEDURE
Traiisf'er a &table portion (which is expected to contain less than 100 micrograms of cadmium) of the ashed sample to a separatory funnel. Ordinarily neutralization is not necessary, but if more than 0.5 ml. of nitric acid or equivalent is present in the funnel, add a small amount of thymol blue indicator and titrate to a yellow color with sodium hydroxide. Adjust the volume to 25 ml. and add reagents in the following order, mixing after each addition: 1 ml. of sodium potassium tartrate, 5 ml. of 40% sodium hydroxide-1% potassium cyanide, 1 nil. of hydroxylamine hydrochloride. Add 15 ml. of extraction dithizone, shake 1 minute, and drain the chloroform layer into a second funnel containing 25 ml. of cold tartaric acid. .Idd
493
494
ANALYTICAL CHEMISTRY
10 ml. of chloroform to the first funnel, shake 1 minute, and drain into the second funnel again. Do not permit the aqueous layer to enter the second funnel in these operations; the gas should be vented through the stopper rather than through the stopcock in these and all succeeding operations. The time of contact of the chloroform with the strong alkali must be kept to a minimum by performing the two extractions without delay after the addition of the dithizone. Lead and zinc are extracted in this step to a very slight extent, if the amounts present are large, giying the misleading impression that the extraction of the cadmium is not complete. Absence of the orange color of excess dithizone in the aqueous layer indicates that an excessively large portion of sample was taken. Shake the second funnel for 2 minutes. Discard the chloroform layer. If the amount of cadmium is thought to be larger than 10 micrograms, the aqueous layer should be aliquoted at this point and the aliquot made up to 25 ml. with additional tartaric acid. (The estimate is based on the difference in the intensity of the pink color of the first extract and the pink color of the chloroform wash extract. Most of the cadmium is present in the first extract, while the interfering metals give about the same color in both extracts. Upon shaking with the tartaric acid, the pink color of cadmium dithizonate immediately changes to green, while the colors of most interferences except lead do not.) Add 5 ml. of chloroform, shake 1 minute, and discard the chloroform layer again, making as close a separation as possible and evaporating the last drop floating on the surface by gentle blowing of air. Add 0.25 ml. of hydroxylamine hydrochloride and exactly 15 ml. of standard dithizone. Add 5 ml. of 4oy0sodium h y d r 0 x i d e 4 , 0 5 ~ potassium ~ cyanide and immediately shake for 1 minute. As in the first extraction, the time of coptact of the alkali with the chloroform should be kept to a mnimum. Insert a pledget of cotton in the stem of the funnel and filter the chloroform layer into a dry photometer tube. (If droplets of water are observed in the light path, remove them by carefully transferring the chloroform to a second tube.) Read the optical density of the pink color a t 518 mfi using a tube containing distilled water as a reference. The tubes should be stoppered to prevent evaporation and should be kept out of direct sunlight. A blank containing all reagents should be put through the complete procedure. Standardization. Prepare a series of separatory funnels containing graduated amounts of cadmium u p to 10 micrograms and add tartaric acid to make a.total volume of 25 ml. in each. Follow the regular procedure after the tartaric acid stripping, beginning with the wash with 5 ml. of chloroform. Preparation of Samples. The preliminary treatment and ashing of samples depend of course upon the nature of the substances involved. The methods commonly in use for various materials should be satisfactory. Wet-ashing methods are preferred to those of dry ashing because of the volatility of cadmium a t high temperatures. As the sample must be neutralized in the procedure, it is preferable to prepare an ash free from excessive amounts of acid. Nitric acid may be used conveniently to ash samples such as tap water, air and fume samples, or urine, in 250-ml. borosilicate Phillips beakers. Small amounts of perchloric acid may be added toward the end of the ashing process, if required. If the sample is very low in ash, the addition of 0.1 gram of sodium sulfate or bisulfate may aid the ashing and prevent loss of cadmium on the glass. I n the case of urine, 50-ml. portions are ashed satisfactorily by heating on a hot plate a t increasing temperatures up to 400’ C. using the technique previously described ( 7 ) , and the quantity of sodium potassium tartrate used in the procedure is increased to 5 ml. to prevent precipitation in the first funnel. Zinc alloys may be dissolved directly in nitric acid and ashing may be omitted. EXPERIMENTAL
Cadmium Recovery and Color Stability. I n the initial work on the procedure, erratic, low results were obtained similar to those reported for carbon tetrachloride (3). The reagents for the final extraction were added in the customary order, the strong alkali first, and little attention was paid to the time of contact before the addition of standard dithizone and final extraction. Investigation showed that the interaction of the strong alkali Kith chloroform dissolved in the aqueous phase from previous operations SIOWIY formed a water-soluble product which prevented the extraction of cadmium. Twts were made hg adding strong alkali to water which had
been saturated with chloroform. After 20 minutes of contact, when 5 micrograms of cadmium were added and immediately extracted with dithizone, the color obtained was only 70 to 80% of that obtained with minimal contact time. To overcome this, the addition of various organic compounds to the aqueous phase before the adding of alkali Tvas tried. Hydroxylamine hydrochloride, sodium azide, hydroquinone, aniline, pyrogallic acid, diphenylcarbazide, and diphenylcarbazone, as well as dithizone itself, were found to reduce the loss after 20 minutes’ contact to about 5%. Hence the initial procedure was modified by adding hydroxylamine hydrochloride to the aqueous phase before both alkaline extractions were made and by using cold solutions where convenient. I n the first funnel, as no chloroform is initially present, standing of the sample with the alkali does no harm. In the second funnel, as the aqueous phase is already saturated n i t h chloroform, the order of reagents was changed so that the alkali n-as added last. By these simple means and by minimizing the contact time betvieen the alkali and chloroform, complete recoveries mere uniformly obtained. Using the final procedure, no difficulty with color stability was experienced a t any time. Tests were made by exposing the colors in stoppered photometer tubes to the light of a 200-watt incandescent bulb a t a distance of 12 inches for 3 hours, and others were allowed to stand in the laboratory for several days, with no perceptible change being found. After the final extraction had been made, the contact time of the color in the chloroform phase and the strongly alkaline aqueous phase was found to be not critical. The brand of chloroform which was used contained a statement on the label that the ANCRICAN CHEMICAL SOCIETY test was passed for “Suitability for use in dithizone test.” This brand is readily available, and hence purification by the analyst is unnecessary. Separation from Interfering Metals. Cadmium is generally regarded as not being extractable in the presence of cyanide; hon-ever, Setterlind (IO) used a small amount in a single extraction procedure. Experimental work showed that small aniounts of cyanide did not prevent complete extraction of cadmium if a sufficient excess of dithizone xere used. The amount? of cyanide and dithizone used in the first extraction were established to give complete suppression of large amounts of most interfering metals and yet complete recovery of cadmium. The concentrations used in the final extraction n-ere selected to give good adherence to Beer’s law through well above the Tyorking range of optical density (up to density 1.0), with a single shaking operation. The small fraction of cadmium which is not extracted in this step is of no concern, since it is reproducible and is also included in the standardization procedure. The range through which Beer’s law is followed may be extended further if the use of smaller photometer cells is preferred, by increasing both concentrations in approximately the same proportion. Since most of the blank reading (using distilled water as a reference) is due to the slight decomposition of the dithizone reagent as it ages, the lowest practicable dithizone concentration is preferable. While stripping after the first extraction is generally performed with 1 to 99 or stronger mineral acid, 0.01 ;I’hydrochloric arid \vas used by Sandell (8) to improve the separation from interfering metals, and pH 2 Clark-Lubbs buffer v a s used by Church ( 2 ) . These media require the extra step of a \vater wash of the extract to remove the entrained alkali. Experiment showed that equally good separation could be obtained using 2% tartaric acid, which is well enough buffered to handle a leakage of 1 ml. of the strong alkali from the first funnel ~ i t h o u tserious effect. The procedure was thereby shortened by ehminating the waterwash step, without impairing the separation. In the first extraction, all the interfering metals are suppressed except thallium and mercury. Lead and zinc are extracted to a very slight extent. The stripping operation provides a separation from zinc and mercury. In the final extraction, the small
V O L U M E 25, NO. 3, M A R C H 1 9 5 3
495
remaining amounts of lead and other possible metals, except thallium, are suppressed. Thallium Separation Procedure. The fact that no separation from thallium is made by the regular procedure should ordinarily be of little concern, in view of the rarity of this metal. Good separation may be obtained, however, by the follonring extra step which is introduced b e h e e n the first and second extraction:
Table I.
Separation of Small Amounts of Cadmium from Interfering Metals Cd Found, y NO 5 7 Cd added Cd added
Interfering Metal3 Added 10 nig. A g -
+ + +
The chloroform layers from the first extraction are combined in a funnel containing 25 ml. of cobalt wash solution with which they are shaken for 2 minutes. They are then drained into the funnel containing 25 ml. of tartaric acid. Then 5 ml. of dithizone (8 mg. per liter) are added to the cobalt wash solution. After shaking for 2 minutes, the chloroform layer is drained into the funnel with the extract and tart.aric acid. The analysis is then continued as given in t,he second paragraph of the procedureLe., shake the second funnel for 2 minutes, etc. In this step, t,hallium dithizonate is transposed into cobalt dithizonate without the cadmium dithizonate being affected. Regulation of the pH with sodium bicarbonate and washing with a m a k dithizone solution prevent loss of cadmium. In the final funnel, any trace of cobalt I\-hich may be stripped by the tartaric acid is suppressed by the cyanide which is later added. RESULTS
In Figure 1 a density-concentrat'ion plot is shon-n for the standardization procedure as well as for cadmium through the complete procedure. The two lines coincide, proving that no loss of cadmium occurs in the first extraction. Extremely lor\- blanks were obtained in both cases when fresh dithizone was used. Beer's la17 is followed through the useful range of optical densities. Tests showed only a 3y0 deviation from Beer's l a v for a 20microgram cadmium standard, rrhich is double the amount of the indicated working range. In Table I, the effects of large amounts of interfering metals on the determination of small amounts of cadmium are given. Five- to 10-mg. amounts of nickel, cobalt, iron, copper, silver, bismuth, t,in, manganese, lead, and zinc gave no trouble. The small interference shown for lead and zinc may be due to a cad-
+
0 1 0 0
+
0 0
0 1
-
+
4.9 5.0 5.1
0 2 0 0 1 2
+
+ +
... ...
3 6 0 la 0 2 0 3a
+
__
5.1
0 0
4.9" 5.1 5.1"
___
niiuni impurity in these salts. In order to retain the required t'\ceQs of dithizone in the first eltraction, the amount of mercury nhich may be present is limited to 100 micrograms. A like '(mount of thallium can be tolerated and removed by the special vparation step. The results of other tests given in Table I1 show that amounts of cadmium up to 100 micrograms are well 1 ecovered in the presence of large amounts of interfering metals. The application of the procedure to 50-ml. portions of urine to which known amounts of cadmium xyere added before ashing gave the results shon-n in Table 111. Incomplete ashmg should lw avoided or off-colors may be obtained. More sodium potas-1um tartrate is used to prevent precipitation of calcium and magn r s u m phosphate.
Table 11. Determination of Large imounts of Cadmium in Presence of Interfering Metals Cd Added, Cd Found, Interfering Mctals 4dded Sone 0 . 1 nig. I I g + + 5 mg. Zn++.5 mg. I'h 0 . 1 mg. Tl+ 5 ing. P b ++, 1 nig. Zn * +, 0 . 1 ing. Ti
Y
Y
100 100
100 98 95 97" 49s
100
+
10-
4.8 4.9 4.9 5.0 4.9 4.8 5.0
0 1
0 0 0 0 0 0
10 nig. Bi 5 mg. Cu 5 ine. C o + + .5 m i . F e + - + 5 mg. Fe 0 . 1 rig. Hg++a 5 mg. M n 5 mg. N i + 10 ma. Pb 10 mg. Sn 0 , 0 1 mg. T1+ 0.03 mg. TI+ 0 . 1 0 mg. TI 5 ing. Z n T T j mg. Pb'+, 1 mg. Zn +,0 . 1 nig. T1+ Using thalliurn separation procedure.
100
50
Using thalliu~nseparation procedui E 09-
Table 111. Cadmium Recovery from Urine and Water
08-
Cd Added, Sample .jO nil. urine
-
0.7
a
- 0.6 u7
-
100 nil. Cincinnati tap water
>
Cd Found,
Y
Y
0.0
0.2
4.0 8.0 0.0 1 0 4 0 9 0
4.1 7.7 0.0 0.9 3.9 8 9
2.0
2.0
-I- 0.5-
v)
z
w
0
0.4-
J
a
0 k Q 3 -
-
0.2
0
2
4
I
I
4
6
8
MICROGRAMS OF Cd
I IO
I
12
(IN I 5 ML.)
Figure 1. Density-Concentration Plot X Standards 0 Cadmium through procedure
One hundred-milliliter portions of Cincinnati tap water (treated Ohio River water), similarly ashed a t a somewhat lower temperature, gave good recovery of cadmium as also indicated in Table 111. Stantiarti sample 110 of the Sational Bureau of Standards, consisting of turnings of slab zinc (spelter), \\-as taken for analysis. Only 1 mg., a small fraction of a single turning, was required for the analysis. Analyses of single turnings showed considerable variation in cadmium content; therefore, three %gram portions \yere used for the determinations. They mere dissolved in dilute nitric acid directly in volumetric flasks, a dilution was made with addition of enough nitric acid to keep the acidity 1 to 99, and an aliquot corresponding to 1 mg. was taken for the determination. The entire analysis required less than 1 hour and gave results in excpllent agreement v i t h the value given by the Bureau
496
ANALYTICAL CHEMISTRY
of Standards certificate of 0.56%. yielded 0.55, 0.55, and 0.55%.
The three determinations
permits the athinment of a sensitivity of 0.05 microgram of cadmium. The procedure is proposed for general use because of its great freedom from interferences and its simplicity.
DISCUSSION
Failure to obtain the orange color of excess dithizonein theaqueous phase after the first extraction ordinarily indicates that the 100microgram limit of the procedure for cadmium has been exceeded. In unusual cases, it may indicate the presence of more than 100 niicrograms of mercury or thallium. Mercury may be recognized by the orange color of the chloroform extract. The analysis may be completed in these cases, if required, by using additional dithizone and washing with chloroform after an excess has been attained. I n the case of unusually large amounts of other interfering metals exceeding the combining power of the 50 mg. of potassium cyanide used in this extraction, the best procedure is to repeat the analysis on a smaller portion of the ashed sample. 4 preliminary extraction with dithizone a t p H 2 map be used to remove excessive amounts of mercury, silver, and copper. In the final extraction, the absence of the yellow color of escess dithizone in the aqueous phase and the obtaining of a darker extract indicate that an excessively large amount of cadmium has been taken in the aliquot. If more of the sample is not available, all of the cadmium may be readily recovered by shaking with additional dithizone and chloroform. The conibined extracts are stripped with tartaric acid and the latter may then be aliquoted and the final extraction repeated in the usual mnnner. The proper estimation of the aliquot to be used requires some experience. A wide variety of types of samples may ordinarily be treated without complication. The colorimetric system as described
ACKNOWLEDGMENT
The author is grateful to D. €1. Byers and H. E. Stokinger, under whose direction the w-oil; was rarried out, for their review and criticism. LITERATURE CITED
(1) Cholak, J., and IIubbnrd, D. RI., ISD.Esn. CHEM.,.Is.AI.. ED., 16, 333 (1944). ( 2 ) Church, F. JJ’., J . Ind. Hug. Tu.rico?., 29, 34 (1947). ( 3 ) Klein, il. K., J . Assoc. Ofic.Agr. rhemists, 30, 455 ( t 9 4 7 ) . (4) Ibid., 32, 349 (1949). (5) I b i d . , 33, 592 (1950). (6) Klein, A. IC., and Wichman, H. J., Ibid., 28, 257 (1945). (7) Salteman, B. E., ANAL.CHEM.,24, 1016 (1952).
(8) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp. 170-7, New Tork, Interscience Publishers, 1944.
(9) Sandell, E. B., IND. E x . CHEW,SAL. ED.,11, 364 (1939). (10) Setterlind, A. S . , and Krause, -1,H., “llicroanalysis of Cad-
mium by the Diphenylthiocarbazone (Dithizone) Rlethod,” Dept. of Public Health, State of Illinois, January 1943. (11) Shirley, R. L., Benne, E. +J,, and JIiller, E. J., ASAL.CHGY., 21, 300 (1949). (la) Stein, C., J . Assoc. Ofic. A g r . Chemists, 34, 417 (1951). (13) Ftrafford, S . , W y a t t , P. F., and Kershaw, F. G.. .4!ld&
70,
232 (1945). (14; (15)
JVelcher, F. J . , “Organic Analytical Reagents,” Vol. 3, pp. 536-9, New York, D. Van Nostrand Co., 1947. Wichman, H. J., .I. Assoc. O ~ C-4gr. . Chemists, 32, 343 (1949).
R E C E I ~ for E Dreview AIay 2 6 . 10.52. Accepted Ijoveniber 17, 1952.
Determination of lindane in Air IRWIN HORNSTEIN AND W . N. SULLIVAN Bureau of Entonzn1og.1. a n d Plant Qtrarantine, C‘nited Stares Departmpnt os Agricrcltrrre, Beltscille, .Md. I\DI\E
is being used to kill insects by its fumigating effect,
L and devices for dispersing lindane vapor have been placed in pulhc p l a c ~ sto control flying insects. KO adequate procedure has Iwen dvailable for determining small amounts of lindane in air.
In the method described in this paper lindane is collected either in acetic acid or preferably 011 an alumina column and determined colorimetrically. As little as 0.1 miciogram of lindane per liter of air can be determined. This method should prove useful in entomological and toxicological investigations. The gamma isomer is the insecticidal component in technical benzene hexachloride. Lindane, which contains a t least 99% of the gamma isomer, has a high enough vapor pressure, approximately 10-5 mm. of mercury a t 30” C. ( I ) , to kill insects by its fumigating action (3, 4, 6). Vapor-pressure measurements of lindane were obtained in a private communication (2). Various methods are used to disperse lindane vapors in air. Thermal generators heating lindane to just below its melting point are available. Burning lindane-impregnated papers and blowing air through lindane-coated screens have also proved effective. Determination of the amounts of lindane that are thus disseminated in the air is extremely important, because of the toxicity of this insecticide to man and animals. There is always the possibility of contaminating foodstuffs that may be exposed to the vapors in stores and warehouses. In addition, information of this nature is essential in entomological studies. The lindane concentration in air ha5 been previously estimated from total chlorine determinations ( 3 ) . In the procedure described in this paper the colorimetric method for determining benzene lieuachloride, recently described by Schechter and Horn-
stein (5) has been adapted to the determination of lindane in air. In brief, this procedure involves the dechlorination of benzene hexachloride or lindane to benzene and its subsequent nitration to ?n-dinitrobenzene,which after extraction is reacted with methyl ethyl ketone in the presence of strong alkali. The violet-red color that develops is measured photometrically. As little as 5 micrograms of lindane can be determined in this manner. Although in this procedure all the isomers of benzene hexachloride are reported as lindane, no error is involved, since insect control is achieved by vaporization of lindane alone. Two methods for obtaining the desired air sample have proved satisfactory. In one, the air sample is drawn through gaswashing bottles containing acetic acid, and the amount of lindane ahorbed in the acetic acid is determined. In the other, the air sample is pulled through an alumina adsorption column, and the adsorbed lindane is washed off the column with acetic acid. The second procedure requires a minimum of equipment and is considerably more rapid than the first. The results are precise and are in agreement with those obtained by the acetic method. For all practical purposes adsorption on alumina is the more useful method. In the dispersal of lindane vapor by heat it is to be expected that, when the vapors become cool, solid partisles of lindane may lie distributed in the air. Thus the amount of lindane reported in a given analysis would actually be the lindane in the vapor phase plus a random number of lindane particles that might be collected in the adsorption train. The average of several determinations in an enclosed space would give a good indication of the total amount of lindane in the air. To avoid partirle formation and thus learn how precise this
