The douhle distillation procedure was found to be the simplest method of eliminating sulfuric acid from the hydrobromic acid distillate. Since the sulfuric acid was carried into the distillate as a fincly dispersed aerosol, various types of fractionating heads were either ineffective in its removal or they resulted in a loss of selenium because of their relatively large holdup volume. Complete elimination of sulfate was considered preferable to using the precautions necessary (3) when sulfate is present. Termination of the oxidation mas clearly defined by the appearance of the bright orange of the dichromate ion. The addition of water to the oxidation residue and reheating to white fumes served as an effective means of rinsing adhering particles and droplets from the sides of the flask and ascending portion of the condenser back into the flask. The prescribed minimum 24-hour heating period in the presence of nitric acid is somewhat arbitrary. For 10gram samples of yeast, the oxidation could usually be terminated 30 to 60 minutes after the addition of sodium dichromate if the mixture had been previously boiled for 24 hours. Shorter heating periods with nitric acid made it necessary to increase the subsequent heating period to 1 to 2 hours. Since
the higher temperatures employed in this latter period increased the possibility of loss of selenium, preference was given to the procedure that reduced its duration to a minimum value. The absence of erratic readings in the results presented in Table I shows that no difficulty was encountered in the positioning and alignment of the capillary absorption cells. The results of the analyses reported in Table I1 show that this method is satisfactory for determining selenium down to lrvels of a few hundredths of a part per million. A number of samples were prepared by adding selenium, in amounts ranging from 0.05 to 0.09 pg., to 10-gram portions of dried torula yeast, U.S.P. These additions of selenium, both as inorganic sodium selenite and as selenium-containing yeast, brought the levels of selenium in the samples (including the selenium contained in the reagents) up to a p proximately 0.015 to 1.0 p.p.m. The recoveries obtained on the added selenium were between 95.3 and 103’%. The selenium content of dried torula yeast, U.S.P., and of feed grade torula yeast was too low to be determined accurately. Approximately one half of the total selenium found in these samples arose from the reagents and
the errors were such as to allow the results to be expressed only as orders of magnitude. REFERENCES
(1) Assoc. Offic. Agr. Chemists, Washing-
ton, D. C., “hlethoch of Analysis,” 6th ed., p. 473, 1945. (2) Cheng, K. L., ANAL.CHEM.28, 1738
( 1956). (3) Danzuka, T., Ueno, K., Ibid., 30, 1370 (1958). (4) Gorsuch, T. T., Analyst 84,135 (1959). (5) Kelleher, W. J., Gitler, C., Sunde,
M. L., Johnson, M. J., Bsumann, C. A., J . Nutrilion 67, 433 (1950). (6) Kirk, P. L., Rosenfels,R. S., Hanahan, D. J., aid.,19 355 (1947). (7) Leddicotte, b. W., Reynolds, S. A.,
Nucleonics 8, 62 (1051). (8) Patterson, E. L., hlilstrey, R., Stok-
stad, E. L. Ii., Proc. SOC.Expfl. Biol.
M e d . 95, 617 (1857). (9) Schultze, M.O., Ann. Rev. Biochem. 29, 391 (1960). (10) Schwarz, K., Nutrition Revs. 18, 193 (1960). (11) Schwarz, K., Bieri, J. G., Briggs, G. M., Scott, bZ. L., Proc. SOC.Exptl. Biol. Med. 95, 621 (1957). (12) Schwarz, K., Foltz, C. hl., J . A m . Chem. SOC.79, 3292 (1957). (13) Smith, G. F., Anal. Chim. Acta 17, 175 ( 1957). (14) Watkinson, J. H., ANAL. CHEM.32, 981 (1960).
RECEIVEDfor review March 20, 1961. Accepted May 11, 1961. Published with the approval of the Director of the Wit+ consin Agricultural Experiment Station.
Determination of Sulfide in Cyanide by the Lead Sulfide Turbidimetric and Cadmium Sulfide Iod o met ric Methods K. K. GEORGIEFF Research laboratories, Shawinigan Chemicals, lid., Shawinigan Falls, Quebec, Canada
b Concentrations of sulfide in cyanide as low as 1 p.p.m. (calculated as anhydrous Na2S) can be determined turbidimetrically. The sulflde is precipitated with 125 to 300% of the theoretical amount of lead acetate, and the absorbance of the colloid determined with a spectrophotometer. The concentration of NazS is then read from a calibration curve. Total time required is 1 to 2 minutes. Methods for minimizing errors due to haze, colored impurities in the sample, instability of the PbS colloid, oxidation of Na2S, and variations in the concentration of NaCN are described. When concentrations of NazS are in excess of 0.01%, the sulfide may be precipitated as CdS and determined iodometrically after being washed free of cyanide.
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ANALYTICAL CHEMISTRY
C
ONCENTRATIONS of sulfide in cya-
nide greater than 0.01% can be determined with an error not esceeding 5 to 20% by precipitation of the sulfide as cadmium sulfide, followed by titration with iodine. However, some samples of commercial sodium cyanide contain only a few parts per million of sulfide, and the cadmium sulfide procedure is not applicable. For such low concentrations the lead sulfide turbidimetric method has been used ( I , 6). Visual comparison of the lead sulfide colloid with a series of known standards (1) introduces human error in judgment, especially when the solutions are colored. Furthermore, since the standards change after a few minutes, some error due to this factor can also be expected. In any case this is rather tedious and hence a rapid spectrophotometric method
seemed desirable However, before reliable and reproducible results could be obtained it was necessary to minimize errors due to haze caused by precipitation of insoluble lead salts, instability of the lead sulfide colloid, color of the sample, and variations in cyanide concentration. REAGENTS AND PROCEDURES
Lead Sulfide Turbidimetric Method.
1% filtered solution of reagent grade Pb(0Ac)Z. 3Hz0in water. 5% stock solution of NaCN similar in color to that to be analyzed and containing less than 1 p.p.m. of NazS calculated on 100% NaCN. 5% aqueous solution of Morningstar gum arabic, which has been filtered through Johns-Manville Celite No. 535 until haze-free.
1
37 t
OS
t
36
w
05
V
Z
0.4
Q
m a
03
0 Ln
m
3 2
6 01
300 5
10
I5
20
25
C O N C E N T R A T I O N O F S O D i U M S U L F 1 D E . P P.M.(WtjVol.)
Figure 1 . Absorbance of lead sulfide at 500 and 600 mp v5. concentration of sodium sulfide in sodium cyanide Coleman Universal spectrophotometer No. 1 4 fltted with 1 3 X 1 3 X 100 mm. flat-walled cuvettes 0 Acetic acid added before lead acetate
0 l e a d acetate added before acetic acid
Standard aqueous sodium sulfide solution assaying about 370 p.p.m. All work should be carried out in a ventilated area. All water to be used for diluting should be deionized, freshly boiled, and cooled to room temperature. For very low conccntrations of sodium sulfide-Le., 1 to 10 p.p.in.-use a 10-ml. volumctric flask. For higher concentrations, use larger flasks, so that the final concentration after diluting is preferably between 5 and 15 p.p.m. All the following opcrations should be carried out moderately rapidly. The total time should bc less than 2 minutes. To the volumetric flask, add enough stock 5% sulfide-free cyanide and/or water so that the final concentration of sodium cyanide will be a t least 4%. Add with shaking 0.5 ml. of 5% gum arabic (more if the volume is much greater), then a solution of the sample to be analyzed, and finally 125 to 300% of the theoretical amount of 1% lead acetate solution t o precipitate the sulfide. The PbS colloid appears to the eye as a brown solution, not a suspension. If there is the slightest indication of hazinrss, add just enough 5001, C.P. acetic acid to remove it (only high punty NaCN does not require any acetic acid). Make up to the mark with 5% sulfide-frce NaCN solution or water. Stopper the flask, turn it upside down 8 to 10 times, and determine a t once the per cent transmittance or absorbance, using the same mixture in the reference cell as in the sample cell, but omit the lead acetate. For colorless solutions, make the measurements a t 500 mp and for yellow ones at 600 mp, if a Coleman Universal spectrophotometer No. 14 or similar instrument fitted with matched 13 x 13 x 100 mm. flat-walled cuvettes is available. With a Beckman DK-2, 400 to 450 mp can be used for white cyanide. If the absorbance obtained for any sample does not lie on a satisfactory portion of the calibration curve, throw away the re-
400
W A V E
500
6 00
700
800
LENGTH,mp
Figure 2. Absorbance of lead sulfide in absence of sodium cyanide vs. wave length Beckman DK-2 spectrophotometer with 1 -cm. flat-wolled cells Water in reference cell. Gum arabic used as protective colloid
many samples the addition of thiosulfate caused the forination of haze and could not be used. Since sodium sulfide in solution is action mixture and repeat the analysis after adjuvting the size of the sample. rapidly oxidized by osygcn, only freshly boiled-out, cool, deioiiizcd water must be used for making dilutions. All adA straight-line calibration curve (cf. ditions of reagents must be inade reasonFigure 1) is obtained by plotting several ably quickly into the same volumetric absorbance values obtained by adding flask. Pipetting the lead sulfide colloid various known amounts of sodium sulfide into another flask to adjust the conto a solution containing a t least 4% centration invariably rcaults in lower of sulfide-free sodium cyanide having absorbance values, prob:ibly due to color similar t o the unknown. While agglomeration and precipitation of the the same curve is obtained whether colloidal lead sulfide particles. Baker reagent, Shawinigan Chemicals, INSTRUMENTA E ~ D OPTIMUMWAVE Ltd., pilot plant, or commercial white LENGTH. Almost idcntiral results were cyanide briquets are used, it is preferable obtained with a Coleman U n i v e r d to use the sample that most closely respectrophotometer Nodel 14 and a semblcs the unknown in impurity conBeckman DK-2. Hence, the former tent. The esperimental procedure dewas adopted for routine analysis, even scribed in the previous paragraph is though it was adequately stable only used. Satisfactory absorbance values as low as 500 mp. Furtheirnore, values were obtaincd conveniently by adding were found to be in good agreement 0.5 to 3 ml. of sodium sulfide solution with Beer’s law. assaying about 370 p.p.m. by the iodoWhen 4 to 6 p.p.m. of sodium sulfide nirtric method (6) to a 5% solution of was treated with an escess of lead acetate sodium cyanide in a 50-ml. volumetric in the presence of gum ar:ibic, the curve flask. The stock sodium sulfide solugiven in Figure 2 was ubtained. The tion must be made up, assayed, and absorbance, and hence the sensitivity of uscd on the same day. STABILITY OF LEADSULFIDE COLLOID. the analysis, increase rapidly with decrease in wave length. However, beLead sulfide usually precipitates too cause of interference by haze and color, rapidly even in the presence of cyanide as described below, wave lengths below to permit an accurate reading to be 400 to 450 mp were not used. taken on the spectrophotometer. Hence INTERFERENCE BY HAZE. Some Sama protective colloid is necessary. Amples of commercial sodium cyanide monium alginate was found ineffective form a haze on addition of enough lead even in viscous solutions but gum acetate to precipitate all the sulfide. arabic was sufficiently effective a t conThe haze from three samples mas excentrations from 0.05 to 0.25%. Since amined by x-ray diffrartioii and infrared commcrc.ia1 gum arabic solutions are absorption. In two samples, the main hazy, they should be purified and the constituent appcarcd to be basic lead same concentration used in both the carbonate, 2PbC03. I%( OH),, and in sample and reference cells. I n some the third lead thiocyanate. Since comsamples the further addition of 1 ml. mercial cyanides vary grcatly in imof 0.1 N sodium thiosulfate was found purity content, the p o s d d i t y should not to aid stabilization, probably by acting be ruled out that under certain circumas an oxygen scavenger. However, in VOL. 33, NO. 10, SEPTEMBER 1961
1433
stances other insoluble lead salts such as the sulfate, thiosulfate, cyanate, and ferrocyanide might precipitate. On the other hand, the more soluble salts such as the chloride and formate should present no problem. Typical values for the absorbance by haze were determined by treating a 33% solution of Baker, sulfide-free, reagent grade sodium cyanide with lead acetate and measuring from 360 to 900 mp with a Beckman DK-2 spectrophotometer, using mater in the reference cell. The absorbance-Le., 0.06CLwas constant from 900 to 600 mp, increased slowly from 600 to 400-450 mp, and increased very rapidly a t still lower wave lengths. Hence, wave lengths lower than 400 t o 450 mp should not be used for the determination of lead sulfide unless the absorbance due to haze is determined. The error due to haze is often great enough to require the removal of all of it discernible to the eye. Various methods were tried. Increasing the concentration of gum arabic, diluting the solution, and adding sodium acetate were partially effective. In one sample, sodium acetate removed 35 to 60% of the haze, but this could not be further increased no matter how much salt was used. Acetic acid was adequately effective, while hydrochloric and citric acids were not. Acetic acid should be added dropwise, even though a moderate excess does not significantly affect the absorbance value. However, a large excess will liberate appreciable quantities of gaseous hydrogen cyanide. As a precaution, it is strongly recommended that all work be carried out close to a fume cupboard. In one example, the addition of acetic acid lowered the pH from 11.1 to 10.7. It is known that acetic acid is a solvent for basic lead carbonate (4) and that sodium acetate increases the solubility of certain lead salts such as the basic carbonate and the sulfate (9, 7'). However, the system is so complex that a rigorous explanation of their mode of action is beyond the scope of this investigation. INTERFERENCE BY COLOR. When the cyanide solution has considerable yellow color due to hydrogen cyanide polymers, etc., compensation for the
Effect of Pb(0Ac)Z Treatment on Color
Table 1.
Wave Length, Mil 600 550 500 450 I.
'6
Absorbance Untreated Treated NaCNe NaCN%* 0.027 0.086 0.268 0.561
0.001 0.008 0.026 0.105
Water in reference cell. PbS filtered off.
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ANALYTICAL CHEMISTRY
color must be made by using the same concentration of sample in the reference cell as in the sample cell. Unfortunately, some of the color is removed simultaneously with the precipitation of the lead sulfide, as will be seen in Table I. The experimental sample used was much more highly colored than most commercial material of salable quality but was chosen to illustrate the magnitude of the error that could be expected in extreme cases. Since the absorbance increases rapidly below 600 mp, it is recommended that a wave length of 600 mp be used for yellow solutions. Errors a t this wave length are small for most samples of commercial aqueous sodium cyanide, which are very slightly yellow in color. AMOUNT OF LEAD ACETATEREQUIRED. Amounts of lead acetate from 105 to 2600% of the theoretical to precipitate the sulfide were added to the cyanide, and the absorbance was determined a t 600 mp. The addition of 120 to 125Yc of the lead acetate equivalent to the sulfide was sufficient to ensure maximum precipitation. Absorbance values were const,ant from 120-125 to 1050% but rose rapidly a t 2600y0 because of precipitation of insoluble lead salts. Also the pH values were constant-i.e., 11.15-from 125 to 1050% of lead acetate but decreased to 10.6 a t 2600%EFFECTOF CONCENTRATION OF SoDIUM CYANIDE.The effect of the concentration of sodium cyanide on the absorbance values was determined by precipitating 10.6 p.p.m. of sodium sulfide with an excess of lead acetate in the presence of various concentrations of sodium cyanide from 0 to 19%. Absorbances were measured a t both 500 and 600 mp, Values were constant a t both wave lengths from 4 to 19% of sodium cyanide, but below 4% they fluctuated in a cyclic manner. Hence, concentrations below 4% should be avoided. ORDERAND RATE OF ADDXTION OF REAGENTS.The usual order was to add the gum arabic first to the cyanide solution, then the lead acetate, and finally the acetic acid. The lead sulfide colloid so produced is stable for only a short time, and hence the spectrophotometric reading must be taken within a minute or two. When the acid is added prior to the lead acetate, the absorbance is slightly higher and the colloid is 100% stable for many hours. This latter procedure requires two operations, since the amount of acetic acid must be determined beforehand by the first procedure. With some very impure cyanides only momentary clarity can be obtained by following the usual order, no matter how much acetic acid is wed. However, by adding the acid prior to the lead acetate, a clear stable colloid can often be obtained.
All the reagents are ordinarily added by pipet in slightly less than a minute, with slight shaking of the flask. The flask is then turned upside down 8 t o 10 times. No significant differences in absorbance values were noted because of the differences in the rate of pipetting and mixing which ordinarily occur. The maximum fluctuation in absorbance values due to all factors, including instrumental, was usually about &2.5% at the optimum part of the curve. LOWERLIMITOF SENSITIVITY.At a concentration of 1 p.p.m. of sodium sulfide, the percentage errors due to fluctuations in the absorbance values become substantial-Le., 10 t o 50%with a Coleman instrument a t 500 to 600 mp. If greater accuracy or a somewhat lower useful limit is required, it can be obtained by using a lower wave length, a longer sample cell, or a more sophisticated spectrophotometer. Cadmium Sulfide Method for Determining Sulfide. A large excess of cadmium chloride solution was added to the sample of cyanide containing sulfide and the mixture acidified with sulfuric acid. After standing for 15 to 30 minutes a t room temperature, the cadmium sulfide was filtered off, washed with water until practically cyanide-free by the benzidine-copper acetate test (8), and determined iodometrically. To determine the accuracy of the method, various amounts of reagent grade sodium sulfide, which were assayed iodometrically (6), were added to sulfide-free reagent grade sodium cyanide and the values determined. iill the results u-ere low, ranging from 94% of the theoretical for 0.45% XazS to 79% for 0.075%. The mechanism by which the loss occurred was not established, but it might have been due to increased solubility of the cadmium sulli.de caused by the presence of such large concentrations of sodium salts. When less extensive washing was used, agreement between the cadmium sulfide and the PbS turbidimetric method was better. Attempts to remove the hydrogen cyanide by boiling rather than washing resulted in much greater losses, probably due to hydrolysis to hydrogen sulfide (2) followed by volatilization. Cadmium ferrocyanide interfered, consuming 10 to 20% of the theoretical amount of iodine. COMPARISON OF RESULTS
To compare results under extreme conditions, two colored solutions of sodium cyanide containing total impurities of 5 to 10% on a dry basis, were analyzed by both the PbS spectrophotometric and the cadmium sulfide method a t a sodium sulfide level of 100 to 140 p.p.m. Two analyses agreed within 10% and two within 20%.
Since this is within the limit of accuracy of the cadmium sulfide method a t this concentration of sulfide, agreement was good as could be a further check, several samples to which 6.3 to P.P,m. of lead were added in the form of the acetate or carbonate were analyzed by both the spectrophotometric lead dithizonate and the lead sulfide methods. In the latter, an excess of sodium sulfide was used. Agreement was within 10 to 20%. Since some concentrations of lead were to ” little 2‘4 P*P’m*Of sodium perhaps better agreement should not be expected.
~
ACKNOWLEDGMENT
The author thanks Shawinigan Chemicals, Ltd.; for pWElkSiOn to publish this paperr D* Denomme for h i assistance, and A. Dupre for the x-ray diffraction and infrared absorption studies* LITERATURE CITED
( 1 ) E. 1. du pant de N~~~~~~ & c0.,
“Sodium Cyanide, Properties and Handling Procedures,” p. 6. onJinoghic Chemistry,” Vol. IV, p. 603, Longmans, Green, London, 1923.
(3) Ibid., Vol. VU,p. 836. (4) Ibid., p. 838. ( 5 ) Rossiter, E. C., J. SOC.Chem. I n d . 30, 583 (1911). (6) Scott, W. W., “Standard Methods of Chemical Analysis,” N. H. Furman, ed., 5th ed., V O ~ .LI, p. 2182, Van Nostrand, New Yo& 1939. ( 7 ) Seidell, Atherton, “Solubilities of Inorganic and Metal Organic Compounds,” 3rd ed., Vol. I, p. 1414, Van Nostrand, New York, 1940. (8) WilliaFs, H. E., “Cyanogen Compounds, 2nd ed. p. 330, Edward Arnold & Co., London, 1948.
‘ 2 ~ , h e I ~ ~ ~ ~ ~ f ~ m $ { ~ ~ ~R ~E$ C
~forI review ~ ~ September 6, 1960. Resubmitted November 21, 1960. Accepted June 12, 1961.
Determination of Mercury in Mercurial and Organome rcu ria I Pesticides FRED VERNON’ Analytical Department, British Schering Mfg. Laboratories,’ lfd., Hazel Grove, Cheshire, England
b Mercury in mercurial-organomercurial preparations is dissolved by employing a suitable organic solvent together with sodium sulfide solution. The mercury is then electrodeposited quantitatively onto a silver-coated, platinum gauze cathode. The method outlined combines accuracy with simplicity and rapidity of determination and has been applied successfully to a large number of mercurial compounds, singly and in the presence of other pesticides. Indications are that this method has a wider application than any other in current use. The accuracy lies within i1%.
T
HE NEED for a rapid method of determining the mercury content of formulated pesticides prompted an investigation which led to the electrodeposition method presented here. A technique virtually independent of the other ingredients usually found in mercurial pesticides has long been required to replace the numerous limited methods and their more numerous modifications. It is felt that electrodeposition meets this requirement. The method of Sporek (4), while giving excellent results, was time consuming. Therefore, having dissolved the mercurial compound as the complex with sodium sulfide, it was decided that rather than try to obtain a solution of a mercuric salt which could be estimated by the thiocyanate method, the mercury should be obtained directly and in a 1 Present address, Pure Chemicals, Ltd., Kirkby, Liverpool, England.
measurable form from the complex. The answer, apparently, lay in electrodeposition, as Sand ( I ) refers to the deposition of mercury from sulfide ores via the complex with sodium sulfide. EXPERIMENTAL
Electrodes. The anode consists of a platinum wire in the form of a helix; height is 30 mm., diameter 10 mm., and weight 10 grams. The cathode is a silver-coated (2) platinum gauze cylinder; height is 45 mm., diameter 45 mm., weight 28 grams, and approximate surface area 120 sq. cm. The electrodeposition apparatus used was built here, with batteries of 2-volt cells in series supplying the power. Procedure. Take sufficient sample (powder or liquid) to yield 250 to 300 mg. of mercury in a 250-ml. flask. Add 25 ml. of ethyl alcohol, fit a reflux condenser t o the flask, and bring to boil (with powders, provide constant shaking to ensure thorough wetting). Add 75 ml. of a filtered 50% solution of crystalline sodium sulfide and allow to reflux for 15 minutes. Cool, and in the case of other organic compounds soluble in hot, dilute ethyl alcohol, chill below 20” C. Filter by suction, remove the filter cake, stir it into a paste with cold sodium sulfide solution, and refilter, finally washing the cake with cold water. Transfer the filtrate to the electrolytic cell (a tall lipless beaker of 250-ml. capacity), add 5 grams of sodium sulfite, heat the solution to 60” to 80” C. with agitation (a hot plate with built-in magnetic stirrer), and adjust the volume to approximately 200 ml. with water. Immerse the electrodes, having previously weighed the cathode [which must
be silver-plated to prevent alloying of mercury with platinum (I)] and carry out the deposition for 45 minutes a t the elevated temperature. The cell current should be 3 to 4 amperes (25 to 33 ma. per sq. cm. a t cathode) for mercurial, arylmercurial, and alkoxymercurial solutions, and 12 amperes (100 ma. per sq. cm. a t cathode) for alkylmercurial solutions, After 45 minutes, remove the cathode (do not switch off current until electrodes are out of the electrolyte); wash thoroughly with cold water, ethyl alcohol, and ether; allow it to normalize for 10 minutes; and reweigh. The mercury can be removed by heating the cathode for about 30 seconds in a Bunsen flame in a fume chamber fitted with a very efficient extraction system. RESULTS AND DISCUSSION
Considerable acuity in effecting complete solution in sodium sulfide was experienced at first because of formation of a protective coating around the organomercurial compound which prevented further solubilization. I t was therefore decided to effect prior solubilization in an organic solvent, then to subject this to attack by sulfide. After much investigation, ethyl alcohol was found to be best suited. Owing t o the nature of preparation of the electrolytic solution, most substances which interfere in other methods are removed by filtration and, a t present, no ingredients in use in organomercurial pesticides have been found which cause interference. The next dificulty encountered was effecting the quantitative breakdown VOL 33, NO. 10, SEPTEMBER 1961
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