A N A L Y T I C A L CHEMISTRY
914 REAGENTS
Cupferron Solution. Dissolve 6 grams of cupferron in 100 ml. of cold water and add about 2 ml. of ammonium hydroxide. The freshly prepared reagent is preferred, but it may be stored in an icebox. The solution should not be used after 2 weeks. One milliliter of reagent is equivalent to 38 mg. of bismuth, but an excess of reagent is required. Diammonium Phosphate Reagent. Dissolve 31 grams of the salt in 1 liter of water. Acidify with nitric acid, heat to boiling, cool, and adjust to pH 0.6 with nitric acid. The solution is stable indefinitelv. Diammonium Phosphate Wash Solution. Dilute 50 ml. of the diammonium phosphate reagent to 300 ml. with water. Adjust to pH 0.6 with nitric acid. Hydrochloric Acid, c.P., specific gravity, 1.2. Nitric Acid, c.P., specific gravity, 1.42. Perchloric Acid, c.P., 72%. PROCEDURE
Sample Preparation. Weigh approximately 1 gram of bismuth-lead alloy, transfer the sample to a 600-ml. beaker, and dissolve the alloy in 10 ml. of 1 to 3 nitric acid. Add 8 ml. of perchloric acid 72% and evaporate to near dryness. Cool. Kash down the cover and walls of the beaker with 30 ml. of water. Separation of Lead Chloride. Add, dropwise and with stirring, 70 ml. of a 2 to 3 hydrochloric acid solution to precipitate lead chloride. Filter the solution through a No. 40, 15-cm. Whatman paper into another 600-ml. beaker and wash the paper and precipitate ivith 100 ml. of 1 to 9 hydrochloric acid solution. Discard the precipitate. Add 100 ml. of water to the filtrate. This solution is now 1 to 9 in hydrochloric acid. (If the sample contains less than 0.2 gram of lead, the separation of lead chloride is unnecessary.) Cupferron Precipitation. T o the bismuth solution add, dropwise and with stirring, 50 ml. of a cold 6 % aqueous cupferron solution. This DreciDitates all of the bismuth as cuuferride and the precipitate ‘is nearly free of lead. Without allowing the precipitate to stand more than a few minutes (since the necessary excess of cupferron is rapidly decomposing) filter through a No. 40, 15-em. Whatman paper and wash six times with a mixed cold 1 to 50 hydrochloric acid-1 % ’ cupferron solution followed by six washings with cold water. Wash six times with a cold 1 to 19 ammonium hydroxide solution to decompose the bismuth cupferride, then wash with water to remove ammonium hydroxide. Discard the filtrate. Return the precipitate and paper to the original beaker and add 25 ml. of nitric acid (specific gravity, 1.42) and 15 ml. of perchloric acid (72%). Heat gently to decompose the paper and
any organic residue, then evaporate rapidly to near dryness. Cool, add 50 to 100 ml. of water, and boil to expel free chlorine. Phosphate Determination. Dilute the solution to 300 ml. with water and adjust to pH 0.6 with nitric acid (specific gravity, 1.42). Heat to near boiling, and add dropwise 50 ml. of a specially prepared hot ammoniuni phosphate solution to the stirred (preferably mechanically) bismuth solution. Digest a t 80-” to 90” C. for 0.5 hour, then filter through a S o . 42, 15-cm. Whatman paper. Wash with 3. specially prepared hot ammonium phosphate wash solution to remove traces of lead, then wash six to 10 times with hot water to remove soluble phosphate. Transfer the paper and precipitate to a tared porcelain cru, cible, dry and char the paper on a hot plate, then ignite at 650 C. in a furnace. Weigh as bismuth phosphate (BiP04); the factor is 0.68i55. ACKNOWLEDGMENT
The authors wish to thank James Thomasson, who assisted in the preliminary work. LITERITURE CITED
(1) Am. Soc. Metals, Cleveland, Ohio, “lfetals Handbook,” pp.
745,1179 (1948). (2) Blasdale, W. C., and Psrle, W.C., ISD. ESG.CHZM.,SAL. ED., 8,352-3 (1936). (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” Sew York, John Wiley & Sons, 1929. (4) Kallmann, S., ;ISAL. CHEX 23, 1291-3 (1951). (5) Kallmann, S., ISD.ENG.CHEY.,ASAL. ED.,13, 897-900 (1941). (6) Lyon, R. S . ,ed., “Liquid Metals,” 2nd ed., p. 733, Washington, D. C., Government Printing Office, 1952. (7) Pinkus, A,, and Dernies, J., BulZ. soc. c h i ~ n .Belges, 37, 267-83 (1928). (8) Sandell, E . B., “Colorimetric Determination of Traces of Metals,” 2nd ed., Sew I-ork, Interscience Publishers, 1950. (9) Schoeller, TV. R., and Lainbie, D. A , , Analyst, 62, 533-7 (1937). (10) Schoeller, W. R., and Waterhouse, E. F.,Ibid., 45, 435 (1920). (11) Scott, W. W., “Standard Methods of Chemical Analysis,” 5th ed., New York. D. Van Xostrand Co., 1939. (12) Snell, F. D., and Snell. C.T., “Colorimetric ;\lethods of .Inalysis,” 3rd ed., Vo1. 11, S e F Tork, D. Van Sostrand Co., 1919. (13) Willard, H. H., and Goodspeed, E. W., ISD.ENG.CHEXI.. .\SAL. ED.,8,414-8 (1936). RECEIVED for
review M a y 4, 1933. Accepted January 20, 1954. Based on studies conducted for the S. .itoniic Energy Coinmission under contract AT-11-1-GEN-8. Presented a t the .inslytical Chemistry Information Meeting, Oak Ridge, Tenn.. J I a y 1953.
r.
Determination of Zinc by Dithizone in a Monophase Water-Glycol BERT L. VALLEE Massachusetts institute o f Technology, Harvard M e d i c a l School, and Peter Bent Brigham Hospital, Boston, Mass.
T
HE microchemical determination of metals with dithizone
(diphenylthiocarbazone) i3 generally performed as an extraction procedure. Because of the nonpolar characteristics of this compound, extractions of metal dithizonates from a water phase into chloroform or carbon tetrachloride have been standard practice (a). Such operations, however, are considered more tedious and time-consuming than the employment of monophasic procedures, which are preferred by most analysts. The method described is based on the solubility of dithizone in ethylene glycol monomethyl ether (methyl Cellosolve, Carbide and Carbon Chemicals Co.), which is readily miscible with water. Dithizone in 1 to 1 water-methyl Cellosolve mixtures remains in solution, as do inorganic salts in concentrations necessary to complex potentially interfering ions. While this system has been here adapted to the determination of zinc ions, it should be applicable to the determination of other metals with dithizone or other nonpolar agents used in colorimetry. Among the nonpolar solvents examined, diethylene glycol nionomethyl ether (methyl Carbitol), ethylene glycol monoethyl
ether (ethyl Cellosolve), acetone, and methyl formamide showed properties similarly favorable to the design of monophasic methods; however, these solvents were not investigated beyond exploratory studies. In the system reported, dithizone reacts promptly with zinc to form zinc dithizonate and is specific for this metal a t a critically adjusted pH value in the presence of acetate, thiosulfate, cyanide, and citrate ions. The technique is sensitive to 0.1 p.p.m. and reproducibility of 2.1% was achieved in replicates containing 3.00 y of zinc. Excess dithizone is not removed and the amount of zinc dithizonate present is determined colorimetrically. EXPERIMENTAL
Glassware was cleaned as previously described (1, 4 ) . All water used q-as obtained by passage of tap water through a mixed cation-anion exchange resin (IR-120 and IRA-410, Rohm and Haas Co.). The water obtained has a resistance of better than 2,000,000 ohms. Polyethylene containers were used for storage whenever possible.
V O L U M E 2 6 , N O . 5, M A Y 1 9 5 4
915
Reagents. DITHIZONE(Eastmaii Iiodak), 0.01% solution in methyl CelloTable I. Spectrophotometric Behavior of Dithizone and Zinc Dithizonate in solve. The solution is prepared fresh Carbon Tetrachloride, RIeth?l Cellosol\e, and hfethj 1 Cellosol\ e Aqiieous daily and stands in a dark polyethylene hlixtures container for 1 hour prior to use to allow Dithizone K a ter complete dissolution of the dye. I 31ax, Zinc Ditlrizonate p H of Absorption, - L- ~- .ibporption AIas., ~ I E T H YCELLOSOLVE. L The commerSolvent .inalysi.j ,\lax. I Max. I1 AIin. 31in Ilns. I1 LllU cially available solvent is heavily conCarbon tetrachloride 5 ,5 615 450 510 6 8 1 7 532 taminated with metals and is purified by hlethyl Cellosolve 600 444 502 3 9 1.P .. passage through a 4 X 5,'s inch cationic hlethyl Cellosolve, 60%4'0 $90 462 52: I 1 2 50; eschange resin (IR-120) column a t a rate complexing solution, 50% b of 12 drops per minut,e. Methyl Cellosolre, 50%4.0 ,592 437 4cl.5 3.5 1 tr :05 BCFFERAND COMPLEXING SOLUTIONS. acetate buffer, 50% Baker's sodium thiosulfate (1730 grams), reagent grade; 298 grams of sodium acea S o zinc dithizonate found witti 500 -, of zinc dissolved in 0.: r u l . of aqiienus s o h t i o n added fo C4.5 rill. of niethyl Cellosolve. tate, reagent grade, and 20 grams of pob l c e t a t e buffer, sodium thiosulfate, ~rotss?i~irii cyanide, and ritratr. ianium cyanide, reagent grade, are dissolved in about 2 liters of water. The pH is adjusted to 5.5 by the addition of glacial acetic acid (reagent grade) and brought to a total volume of 4.00 liters. The buffer is extracted with dithizone dissolved in carbon tetrachloride. .ill zinc present in the acid, salts, and water is thus removed just prior to use. The pH of aliquots is further adjusted to p H 4 n-it,h glacial acetic acid before use, since precipitation of salts occurs at pH 4 when the buffer is allowed to stand overnight. CITRICACID,reagent grade, 25% aqueous solution neut,ralized t o pH 4 with 6.V sodium hydroxide. A~INONIA.Concentrated ammonium hydroxide, reagent grade, is distilled from a borosilicate glass still and diluted 1 to 1 with water. It must be prepared a t weekly intervals and stored in polyethylene ware. THYMOL BLUEITDICATOR PAPER, LanIotte Chemical Co. HYDROCHLORIC ACID. Reagent grade hydrochloric acid is distilled in a borosilicate glass still and 6-V hydrochloric acid is stored WAVE LENGTH, M r in polyethylene containers. TRICHLOROACETIC ACID. Trichloroacetic acid crystals are Figure 2. Spectrophotonietric Absorption Curves redijtilled and stored in polyethylene bottles a t room temperain JIixtures of 1 to 1 Sfethyl Cellosolve-Acetic Acid ture: 200 nil. of a 407' solution in distilled water we made up as Buffer-Complexing Solution nretied. Cary spectrophotometer Procedure. A 1.5-ml. aliquot of sample is transferred to a 15A . Dithizone nil. graduated borosilicate glass centrifuge tube and titrated from B . Zinc dithizonate a 1-nil. microburet with either 1 to 1 ammonium hydroxide or 6.1.hydrochloric acid-depending upon the p H of t'he samplet o 3 color change of t.hymol blue paper a t about pH 4. Then, 0.2 nil. of the 25% citrate solution is added, folloxed by 2 ml. of and after 15 minutes 0.4 nil. of 40% trichloroacetic acid to 1 nil. the acetate buffer which has been adjusted t,o p H 4 just prior to of serum to remove zinc from proteins to which it is attached m e . The solutions are mixed thoroughly and brought to a tot,al ( 2 ). The supernatant fluid is removed by centrifugation. The volume of 4 ml. mi\ture is allowed to stand for 15 minutes at room tenipeiature, The subsequent addition of 3.5 ml. of methyl Cellosolve brings and J 1 S-ml. aliquot is removed for analysiq ahout a marked rise in temperature of the system, which is cooled to room temperature by immersion in water from a cold water SPECTROPHOTO>IETRIC RESULTS tap. Prior to cooling, the tube is covered tyith Parafilm (Marathon Corp.) and inverted three to four times to ensure thorough Spectrophotometric datn were obtained with a Cary recording niising of the water-methyl Cellosolve system. spectrophotometer. Colorimetric measurements were performed After complete cooling has taken place, 0.5 nil. of 0.01% with the Beckman Model DU spectrophotometer provided with ciithizone in methyl Cellosolve is added. The system is mixed by repeated inversion. An aliquot is transferred to 5-cm. absorpan adapter for cells of 5-cm. path length (3). When dithizone is tion cells ( 3 )and measured within 10 minutes. dissolved in carbon tetrachloride, one of the solvents conventionThe method was designed to facilitate the determination of ally used for this dye, the absorption spectrum presents a maxizinc in human plasma or serum. Such samples were prepared mum ( I ) a t 615 mp and a second one (11) with markedly lower for analysis by the addition of 0.6 ml. of 6.Y hydrochloric acid intensity of absorption a t 450 mp. A niinimum of absorption is observed a t 510 mp. The solution is green to visual inspection. The formation of zinc dithizonate results in the appearance of an absorption peak at 532 mp, the approximate location of the dithizone minimum (Figure 1). I n a 1 to 1 methyl Cellosolve-acetic acid buffer-complexing solution mixture maximum I shifts to 590 m p , maximum I1 to 462 mp, and the minimum is found a t 525 mp. The intensity of absorption a t 590 mp (malimum I ) is markedly decreased. Consequently. the solution appears blue-green to visual inspection. The maximum of zinc dithizonate is W A V E LENGTH, Mp found a t 507 m p (Figure 2 ) . Figure 1. Spectrophotometric Absorption Curves in Carbon Tetrachloride Zinc dithizonate in niethyl CellosolveCars spectrophotometer buffer-complexing solution mixtures was deA . Dithizone termined a t 525 mp. The reference solution. B. Zinc dithizonate
ANALYTICAL CHEMISTRY
916 identical in composition with the sample except for the abPence of the metal solution and its replacement b>- n-ater, compensates for the absorption due t o dithizone a t 525 nip. Determinations were also made and calculated according to the method of measurement a t two rvave lengths previously suggested (1, 4 ) , but no advantage accrued over the conditions proposed. The absorption maxima and minima for dithizone in methyl Cellosolve alone and in 1 to 1 acetic acid buffer-methyl Cellosolve mixtures are s h o r n in Table I, as is the absorption maximum of zinc dithizonate in the latter. The formation of zinc dithizonate in methyl Cellosolre alone did not take place when as much as 500 y of zinc in 0.5 nil. of aqueous solution was added to 9.5 ml. of methyl Cellosolve. Table I summarizes the significant spectrophotometric findings.
Table 11. Recover) of Zinc b? \Ionophasic RZeth)l Cellosohe Technique and Carbon Tetrachloride Extraction" Sa>iii>le 1 7
3
4 5 6 7 8 9 10
11
hleth\l Cellosol\ e 3 02 2.93 2 86 2.94 3.00 3.01 2.98 2.86 3.03 2.96 2.95
.\lean 2.96 Standard deviation *O.OG Coefficient of variation 2,l 3.00
Zinc Found, y Carbon tetrachloride extraction 2 76 2.46 2.60 2.86 2.99 2.78 2.88 2.91 2.93 2.93 2 78 2.81 3z0.22 7.8%
-,of zinc present in 8.00nil. of solution
QUANTIT.ATlVE RESULTS
Calibration curves were prepared from analyses on a solution obtained by dissolving weighed quantities of zinc chloride and dilutions prepared from this stock. The calibration factor, K , was calculated from them. Calibration curves must be repeated whenever a new lot of buffer is prepared. Calibration curves obey the Beer-Lambert law, as shown in Figure 3. Zinc concentrations n-ere calculated according to the equation: y
% zinc
=
EiJ3 X IC
where Es: = absorbance a t 525 mM K = calibration constant Interferences. Table I1 lists the elements and their valence dates which were tested for t,heir effect on the reaction. The concentrations given in parts per million were the maximum values which could be present together with a total of 2.00 y of zinc without altering the absorbances obtained by niore than the standard deviation on replicate8. Repeatability. Table I11 shows 23 consecutive arialyses of zinc chloride standards containing 5.00 y of zinc. The results ranged from 4.88 to 5.45 y of zinc with a mean of 5.13 -/ and a
standard deviation of 0.14 y, the coefficient of variation being 2.6%. Accuracy and Recovery. The methyl Cellosolve method was compared with a technique described previously ( 1 , 4 ) employing the conventional extraction of zinc with dithizone in carbon tetrachloride. The data are shown in Table IV. Eleven aliquots, each containing 3.00 y of zinc, were measured with both techniques. The methyl Cellosolve technique resulted in a mean of 2.96 f 0.06 y, a coefficient of variation of 2.1,and a mean recovery of 99%. The extraction technique gave a mean of 2.8 f 0.22 y, a coefficient of variation of i.S%, and a mean recovery of 94%.
Table 11. Interference of Metal Ions w-ith Z i n c Determination on Al"+ Fe'++ Cu++
Maximum riniount S o t Interfering with Determination of 2 0 0 y of Z n + + , P P 31. 30 50 4
g+++++
3
Ag Hg++
3
+
Hg+
Sn++ Mn+* 31g++++ +; ; +
E :: co++
ZINC, MICROGRAMS PER 8 ML. OF SOLUTION
Figure 3. Calibration Curve for Zinc in Mixtures of 1 to 1 Methyl Cellosolve-Acetic Acid Complex Solution at 525 Mp
4 50 10
5-em. absorption cells, Beckman
40
spectrophotometer
1000 1800 1000 100
