Improved Dithizone Method for Determination of Lead Mixed-Color Micromethod at High pH L. J. SNYDER, Ethyl Corporation, Baton Rouge, La. The colorimetric dithizone method for the determination of lead has been studied, and a new procedure deteloped based on extraction of the lead from a n aqueous solution a t pH 11.5. A saving i n time and ail improbement in the convenience and flexibility of the method are realized as a result of extending the w-orking range of the method to analyze samples from 1 to 450 micrograms in various cell sizes and with a single standard dithizone solution. The new method eliminates the need for titrimetric extraction to estimate the sample size within narrow limits and for selection of the proper cell size and standard dithizone solution prior to the final color debelopment.
T
HE introduction of dithizone (diphenylthiocarbazone) by
Fischer (8), and its later adoption as a special reagent for the determination of small quantities of heavy metal ions, have resulted in the development of t,hree principal methods applicable to the determination of lead. They may be classified as: onecolor, titrimetric-extraction, and mixed-color methods (4, 5 ) . The one-color methods (4,9, 13, 18) depend upon t'he extraction of lead with an excess of dithizone in chloroform or carbon tetrachloride solut.ion, followed by the removal of the excess dithizone from the extracts by washing with an ammoniacal 'potassium cyanide solution. The amount of lead dithizonate dissolved in the organic solvent or the free dithizonc obtained by treating the lead dithizonate with dilute acid is then estimated colorimetrically. The objection to the one-color method is that lead dithizonate is soluble in dilute ammonium hydroxide solution and the necessary removal of the excess dithizonc also removes lead dithizonate, producing low results (4,5 ) . Therefore, the accuracy of this method depends on making the same compensation for errors in the determination of the unknoivn as were made in the standard sample. The titrimetric-extraction methods (4,16, 1 7 ) involve the estraction of lead, from an aqueous solution adjusted to pH 7.5 to 8.0, with successive increments of standardized dithizone solution. -4lthough accurate to 0.001 mg. of lead, t,hese methods are tedious and considerable time is required to approach the end point carefully ( 5 ) . In the mixed-color methods (1-5, 10, ii, 12, 14, 15) the lead is extracted, from an aqueous solution adjusted to pH 9.5, with an excess of dithizone in chloroform solution, and the excess dithizone is permitted t,o partition b e b e e n the aqueous and organic phases. The lead dithizonate, in the presence of an excess of dithizone and dissolved in chloroform, is measured either visually, using Sessler tubes, or colorimetrically with a photometer. The mixed-color method avoids the objection t,o t'he one-color method by allowing the excess dithizone to remain in the chloroform solution at a pH such that the lead dithizonate is not soluble in the aqueous phase. However, dithizone also absorbs light in appreciable quantities in the same spectral region in which lead dithizonate is measured. Consequent,ly, when small amounts of lead are present, it is necessary to use correspondingly smaller amounts of dit,hizone. Therefore, to determine microquantities of lead it is necessary to use different, amounts of dithizone for analyzing various lead ranges (0 to 5, 0 to 10, 0 to 20, 0 to 50, 0 to 100, and 0 to 200 micrograms). Furthermore, the amount of lead present in the unknown sample must be carefully estimated in advance, so t,hat the proper standard dithizone solution, t,he volume of this solution, and the appropriate cell size may be selected before the final development of the color ( 5 ) . This objection to the mixed-color method can be overcome if
the determination of lead is carried out at a sufficiently high pH to concentrate the excess dithizone in the aqueous phase, presumably as the ammonium salt of dithizone. The purpose of this work is to present data shox-ing that the determination of lead by dithizone can be more advantageously carried out at' a high pH (11.5) than a t a pH of 9.5 (1-6, 10, 11, 18, 14, 15). The analytical method presented in this report has been successfully applied to the determination of lead in thousands of samples during the past two years. REAGENTS
All reagents must be checked for lead and if the tot,al blank amounts t,o more than 0.1 to 0.2 microgram they should be purified. Ammonium citrate, sodium sulfite, and potassium cyanide are purified according to Bambach (1). Sitric acid, ammoniuni hydroxide, and distilled water are purified as described by Clifford and Kichmann ( 5 ) . The functions of all reagents, escept sodium sulfite, have been described ( 5 ) . Sodium sulfite is added to reduce any possible oxidants which might decompose the dithizone. Dithizone Solution. Dissolve 60 mg. of purified dithizone 116, in 1000 ml. of C . P . chloroform and store in the absence of direct sunlight (preferably in a refrigerator). Eastman Kodak (white label) dithizone. was found sufficiently pure for use without further treatment. Ammonium Citrate, 5yc, Dissolve 5 grams of ammonium citrate in 100 ml. of water. Ammonium Hydroxide, 1 4 5 . Mix equal quantities of concentrated ammonium hydroxide (specific gravity 0.900) and distilled r-iater. Nitric Acid, 1% (free from oxides of nitrogen). Dilute 1 part of concentrated nitric acid with 99 parts of n-ater. Potassium Cyanide, 2Tc. Dissolve 2 grams of potassium cyanide in 100 ml. of water. Sodium Sulfite (anhvdrous 1 7 . Dissolve 1 gram of sodium sulfite in 100 ml. o f k a t i r . Solution A. Prepare Solution h by mixing 5 parte of animoi.iuni hydroxide (14c;O), 1 part of sodium sulfite (l?,, and 1 part of potassium cyanide (2%). Indicator, 0 . 1 5 . Dissolve 0.1 gram of thymol blue in 4 . 3 nil. of 0.05 -Ysodium hydroxide and dilute to 100 ml. Chloroform. Recover the used chloroform by distilling in the presence of an aqueous thiosulfate solution as described by Hibbard ( I O ) , and stabilize according to Hubbard (11). -1pi-ecaut,ion must be taken against decomposlng the chlorofoh during distillation. The chloroform should be distilled s l o ~ l yand at a temperature not over 57" C., because a t the temperatures corresponding to rapid distillation sufficient decomposition occurs to cause interference in the analysis. Standard Lead Solution. Dissolve 0,1599 gram of lead nit,raie,twice recrystallized from water and dried to constant weight a t 120" C., in 1 liter of 1% nitric acid. This solution', containing 0.1 gram of lead per liter, may be diluted further as needed. 1
APPARATUS
All glassware (preferably Pyrex) must be thoroughly cleaned with warm dilute nitric acid, followed by rinsings with distilled water. The last traces of lead are removed from the separatory
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SEPTEMBER 1947 funnels by shaking with a mixture of 10 ml. of Solution h and 10 ml. of dithizone solution until the chloroform layer no longer becomes red. A single rinse with distilled water is then sufficient to remove the ammoniacal cyanide solution. Clear Vaseline is used as a stopcock grease for the funnels and pipets. A Beckman Model G pH meter with the special S o . 1190E electrode was used to establish the pH values. A Beckman quartz spectrophotometer x i t h 0.35, 1.0-, 2.0-, and 5.0-em. cells was used for transmittancy measurements at a wave length of 510 millimicrons. The 0.35-cm. cell was prepared by cutting and polishing a rectangular glass plug (4.0 X 0.95 X 0.65 em.) from a piece of 0.6-cm. (0.25-inch) plate glass and iflserting this plug into the standard rectangular (4.5 X 1.0 X 1.0 em.) Beckman 1-em. cell. The difference between th(1thicl;nclss of the gk3.q~plug and the 1-cm. re11 is 0.35 em. 3IETHOD IN BRIEF
The aqueous lrad solution containing ammonium citrate and potassium cyanide is adjusted to pH 9.5 to 10.0 with dilute aniinoniurii hydroside. The lead is estracted with a dithizone solution and a preliminary estimation of the total quantity of lead to the nearest 450 microgranis is obtained, so that the maximum range is not exceeded. After separation of the organic and aqueous phases, the lead is estracted from the organic phase with dilut,e nitric acid. The resulting aqueous solution is adjusted to pH 11.5 with an aninioniacal cyanide solution and the lead is again estracted x i t h dithizone. The quantity of lead present in the dithizone bolution is measured with a spectrophotometer. PROCEDURE
Place the sample containing the lead dissolved in nitric acid and free from interfering ions ( 5 ) in a 250-ml. glass-stoppered (Squibb-type) separatorp funnel. AXdcl4 drops of thymol blue, 10 ml. of ammonium citrate, and ammonium hydroside until the indicat.or turns pale blue (pH 9.5 to 10.0). -4dd 10 ml. of potassium cyanide solution and estract the lead with dithizone solution in the naj- discupwd htrenith.
Vigorously shake the sample with successive 20-ml. portions of dit,hizone solution until the green color of the dithizone remains unchanged. Draw off the chloroform layer after each estraction and save in a second separatory funnel containing 25.0 ml. of 1% nitric acid. Estimate the amount of lead present by recording the number of extractions necessary to remove all the lead from the aqueous to the chloroform layer (20 ml. of dithizone are equivalent to approximately 450 micrograms of lead). Wash the aqueous solution x i t h an additional 5 ml. of dithizone solution and add the washings to the combined extracts. Shake the separatory funnel containing the nitric acid and lead dithizonate for 1 minute and discard the chloroform phase, which is noir lead-free. Remove by evaporation the droplets of chloroform remaining on the surface of the acid. This acid solution is ready for the fimal color development unless it contains more than 480 micrograms of lead, in which case an aliquot of it must be taken. If aliquoting is necessary, the pH is maintained constant by using only 1% nitric acid for whatever dilution is needed, so that the 25-m1. aliquot required in the next step contains less than 450 micrograms of lead. Prepare a comparison blank by placing 25 nil. of 1 acid in a separator) funnel and carry this blank along unknown through all the subsequent steps. .4dd 75 ml. of Solution A and 25.0 ml. of dithizone solution to both the unknoim and the blank samples and shake each for 1 minute. L4110wthe chloroform phase to separate and filter it through a Whatnian 41-H filter paper directly from the separatory funnel into a clean, dry 30-ml. test, tube. Filter only as much as is needed to rinse and fill the photometer cell. Pour the rhloroform solution into a clean, dry colorimeter cell of appropriate size, after first rinsing the cell twice n-ith small portions of the same solution. Use a short, cell for denqe red solutions and a lone cell for faint solutions. For comparison blanks, use thc same size cell as the unknown. Make the colorimetric determination of lead immediately by comparing, at 510 millimicrons, the light transmittancy of the blank sample and the unknon-n in a spectrophotometer. AXdjust the instrument to 100% transmittancy with the blank in the light path. Read the per cent transmittancy with the unknon-n in the light path. From the per cent transmittancy of t,he unknown, read its lead content from a standard curve prepared in esactly the same manner, using known lead solutions. To obtain the highest accuracy, a blank sample should alprags be carried along with the unknown. The correction for the blank may be made by either (1) adjusting the instrument to 100% transniittancy with the blank in t,he light path as described above, or (2) measuring the optical density of both the unknown and blank, and making a corresponding correction. In the latter case, the transmittancy measurement,s should be made on the blank and on the unknown at, about, the same time, A standardization curve for both 1.00- and 0.35-em. cells is shown in Figure 1. AV4LYSIS O F KYOWIV SOLUTIOKS
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