A N A L Y T I C A L EDITION
May 15, 1941
requires less time and manipulation than does the melting point procedure.
Literature Cited (1) Carlsohn, H., Ber., 60, 473 (1927). (2) Fransden, Mikkel, Bur. Standards J . Research, 7 , 477-53 (1931).
367
(3) Patterson, Blackwood, and Stewart, J . Chem. SOC.,1933, 93-5. (4) Rast, K., Ber., 55, 1027, 1051 (1922). (5) Roth, H., and Daw, E. E., “Quantitative Organic Microanalysis of Pregl”, p. 239, Philadelphia, P. Blakiston’s Sons & Co., 1937. (6) Smith, James H. C., and Young, William G.. J . Biol. Chem., 75. 259-98 (1927).
Determination of Zinc by Precipitation as Zinc Anthranilate -
A Gravimetric Semimicromethod C. W. ANDERSON, 411 Roslyn Place, Chicago, Ill.
T
H E need for a more rapid and accurate method for the determination of zinc in tin-lead solder led to the investigation of the application of anthranilic acid, in view of the high sensitivity of the reaction involved. A method formerly in use required at least 4 hours for a determination on solder, while the recovery of zinc was always less than the total zinc present. Funk and Ditt (2) have shown t h a t zinc can be quantitatively precipitated by the sodium salt of anthranilic acid when i t is added to a neutral solution or one faintly acid with acetic acid. Precipitation takes place from a dilute solution at room temperature. Zinc anthranilate is crystalline and may be easily filtered. The sensitivity of the precipitation reaction is 1 in 1,000,000. Anthranilic acid, which may be used for analytical purposes, can be purchased from the Eastman Kodak Company, Rochester, X. Y. The literature contains no method describing the use of anthranilic acid for determining zinc in white metal alloys.
Interfering Elements Among the metals which may cause interference by coprecipitating to a marked extent are copper and iron. The concern is chiefly with copper, as iron causes no interference unless more than 10 mg. is present. The effect of iron upon the determination of zinc is shown in Table I. The data were obtained by taking various quantities of pure iron and pure zinc which were carried through as described below. Copper, which reacts with sodium anthranilate like zinc (8),can cause interference even when present in minute amount. Funk and Ditt (3) describe its accurate precipitation and determination. The sensitivity in the precipitation reaction between the copper and anthranilate ions is the same as with zinc. Of the splendid methods described in the literature for accurately separating copper from certain elements, including zinc, two are especially suitable. Copper can be completely separated from all the other elements present in all kinds of white metal alloys, in the form of a black, finely divided suspension of metallic copper from a sodium hydroxide solution, by addition of hydrazine hydrochloride (4). The copper is filtered off and the zinc determined in the filtrate. Microquantities of copper, in the range from 0.05 to about 50 micrograms of copper, can be determined accurately by a colorimetric method, using the very sensitive organic reagent dithizon (1) and the Keil colorimeter. The present method may be used for the determination of semimicroquantities of zinc, present as impurity in tin-lead solder in amount from 0.01 to 0.10 per cent, and for quantities of about 8 per cent in zinc-tin alloys.
The procedure, with certain modifications, can be used for determining zinc in other material of greater zinc contentfor example, the familiar fluxing agent consisting of a large quantity of zinc chloride with a small amount of ammonium chloride. It may also be used for the determination of zinc oxide impregnated in a certain type of enamel, coating the inside surface of tin cans (Table 11) in which various foods are packed, notably corn and meats. The time necessary to carry out a zinc-tin analysis is 1 hour, about 2 hours for solder, 1.5 hours for zinc chloride soldering salts, and 1.5 to 2 hours for the zinc oxide content of enamel coating. The accuracy of the method is shown in Table 111, giving data obtained by analyses of a sample of flux, and in Table IV, in which several of the analyses were carried out consecutively, in duplicate.
Solutions and Materials Required A saturated solution of pure bromine in concentrated hydrochloric acid is prepared by shaking vigorously in a glass-stoppered bottle 12 ml. of bromine in 100 ml. of acid. Sodium tartrate. Sodium anthranilate reagent, made by weighing 3 grams of anthranilic acid into a beaker and adding about 20 ml. of water and 22 ml. of N sodium hydroxide. Finally the solution is made neutral or faintly acid by adjustment with 2 per cent acetic acid, filtered, and made up t o volume in a 100-ml. flask. Gooch crucibles of about 10-ml. capacity.
Procedure SOLDER AND ZINC-TINALLOYS. Dissolve the sample in bromine-hydrochloric acid, 0.5 gram for solder and 0.05 gram for zinc-tin alloys. When solution of the sample is complete add 2 TABLEI. EFFECTOF IRONUPON ZINC DETERMINATIONS Zinc Added
Iron Added
Weight of Precipitate
Zinc Found
Mg.
Mg.
Mg.
Mo.
TABLE11. ZINC DETERMINATION IN ENAMEL COATINGS FROM TIN CANS Film Weight of Sample
(Area of sample, 6.452 sq. om., 1 square inch) Weight Total of Zinc Zinc Zinc Precipitate Added Present Found
MQ.
Mg.
5.5 5.5
5.60 6.40
Me. 0.15
Recovery
Me.
Mg.
%
1.23
1.08 1.24
99.0
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 13, No. 5
cent acetic acid. Add the sodium acetate to the ferric chloride solution, dilute to 400 ml., heat just to boiling, and hold at this T~~~~111. D~~~~~~~~~~~~~ ON A sAMPLE zINCcHLoRIDE temperature for 1 minute. The basic acetate soon precipitates. SOLDERING SALTS Filter the solution while hot, and wash the precipitate with hot (Zinc added, 0.30 mg. Total zinc present, 18.26 mg.) 1 per cent ammonium acetate solution. Continue the washing Aliquot Taken Weight of until the volume is GOO ml. Transfer the filtrate to an 800-ml. from 2OO-Ml. Weight of PrecipiZinc Volume Sample tate Found Recovery beaker, add 5 ml. of concentrated sulfuric acid, and evaporate M1. Gram MQ. MQ. r /C the solution down to fumes. Just before fumes of sulfuric acid appear add 5 ml. of concentrated nitric acid to destroy inter20 0,08209 185.2 35 82 10 0.041045 91.8 17.975 fering ammonium salts. Continue evaporation to dense fumes 10 0.C41045 92.7 17.96 of sulfuric acid. Cool, wash into a 150-ml. beaker, and evaporate 10 0.041046 92 7 17.96 to a volume of 15 rnl. Neutralize, fint 10 0.041045 94.1 18.23 99.8 adding a saturated sodium hydroxide solution, then powdered sodium carbonate until alkaline, and finally make IN WHITEMETALALLOYS TABLE IV. ZINC DETERMINATION slightly acid by adding a few drops of 2 per cent acetic acid. Add 0.3 gram Zinc Total Zinc Weight of Weight of Zinc Recovery Found Present Precipitate Added Sample Sample of sodium tartrate, following the regular .MQ. MQ. Grain IMQ. grocedure, and finally precipitate zinc 7% MQ. y adding 25 ml. of reagent. 1 45 0.28 0.50 Solder
Solder sample containing 5.23 per cent of iron Zinc-tin alloy Synthetic mixtures of pure tin, lead, and zinc
0.47
1 85 0 90 2 70 1 30 2 10 3 30 3 30 3 85 10 80 10 90 11 90 1.50 2.40
0.36 0.17 0.52 0.25 0.41 0.64 0.64 0.75 2.09 2.11 2.30 0.29 0.47
4.70
22.20 22,20 24.20
4.30 4.30 4.69
1.55 2.60 20.80 3.40 31.10 29.00 33.60
0.30 0.504 4.03 0.669 6.02 5.62 6.50
0.08
0.36
0.35
0.52
0.50 0.50 0.50 0.50 0.50 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.50
0.175
2 29
0.175
0.05 0.05 0.05
0.40
O,l5
0.40
1.05
0 74
0.30 0.50 4.00 0.675
6.00
0.JJ
6.50
to 3 ml. of concentrated sulfuric acid and evaporate to fumes. Lead, which usually is not present in zinc-tin alloys, is filtered after suitable dilution, and washed uith 2 per cent sulfuric a-id wash solution. Dissolve in the filtrate 0.3 gram of sodium tartrate to hold tin in solution. Keutralize the solution to litmus, first adding a saturated solution of sodium hydroxide drop by drop and concludin the neutralization with powdered sodium carbonate, adding a sfight excess. Add 2 er cent acetic acid until the solution is neutral or faintly acid? Add t o the cold solution 25 ml. of the reagent slowly with constant stirring. Allow the solution to stand for 30 minutes or longer for complete precipitation of zinc. Filter through a weighed Gooch crucible, washing the precipitiite with a solution containing one part of reagent to 15 parts of water by volume. Wash n u t the excess reagent by two or three washings with alcohol. Dry the crucible in the oven a t 100" to 105' C. for 30 minutes, cool, weigh, and calculate the zinc content. Zinc anthranilate contains 19.37 per cent of zinc.
SEMIMICROQUANTITIES OF ZINC IN PRESENCE OF LARGE AMOUNTSOF IRON.I n the presence of large amounts of iron semimicroquantities of zinc can be accurately determined by a procedure described in the literature (6),which is a modification of the basic acetate separation of iron. It can be used to separate very large quantiltes of iron, as much as 0.5 gram and higher, quantitatively from elements present as impurities in alloy steels and minerals. WHITE METALALLOYS. Dissolve a 0.5-gram sample in bromine-hydrochloric acid. When solution is complete add 1 gram of potassium chloride to the acid solution of the metallic chlorides. Evaporate to apparent dryness, break up the residue, and heat at 100" C. until the odor of hydrochloric acid is very faint (5 to 7 minutes). In the presence of potassium chloride, it is possible to evaporate the solution to dryness without decomposition of ferric chloride. Enough hydrochloric acid remains to give a clear solution when the residue is treated with 20 ml. of water and boiled gently. Dissolve 3 grams of sodium acetate in 100 ml. of water and make the solution neutral to litmus with the addition of 1 per
102.5 101 3
100.4
99.8 103.7 100.7 98.6 100 4 101 2
This is an excellent method for separating large quantities of iron from small amounts of zinc, as there is neither adsorption nor coprecipitation of zinc. The iron content of the solder sample shown in Table IV was found to be 5.23 per cent, a quantity much too large to enable determination of zinc according to the regular procedure without removal of iron. The zinc content of the sample is 0.058 per cent. The recoveries are proof of sufficient precision and accuracy.
ZINC CHLORIDE FLUXES. Take anv convenient weight of sample,dissolve i6 water containing a few dro 9 of hvdrochloric acid, and make up to volume in a volumetric &k. Measure off aliquots accurately in a 150-ml. beaker, using a buret of IO-ml. capacity, neutralize the solution to litmus with powdered sodium carbonate, add the anthranilic acid reagent, and continue as in the procedure for white metal alloys. Calxlation of the amount of zinc chloride from the value obtained for zinc, carried out in duplicate, shown in Table I11 gives 91.20 per cent. These experiments also prove precision and accuracy to be entirely sufficient. ENAMEL COATING OF TIN CANS. Cut a disk with an area of 12.9 sq. cm. (2 sq. inches), and remove the enamel film from the whole piece, or cut the disk into two (3.452-sq.cm. (I-sq. inch) pieces, using one of them for the determination. Tlie film weight is obtained by weighing the section before and after removal of the a m . Immerse the sample in a few milliliterd of chloroform in a 50-ml. beaker for rapid removal of the enamel coating. Evaporate the chloroform cautiously to avoid loss of small portions of the film. Add 1 to 2 ml. of concentrated sulfuric acid, heat over a small flame, and add a few drops of Super0x01 to destroy organic matter. Cool and add 10 ml. of water and 0.10 to 0.20 gram of sodium tartrate. Allow to dissolve and continue ns in the other procedures. SYNTHETICMIXTURESOF Trx, LEAD, A N D ZINC. Suitable standard samples of known zinc content were not available m a means for determining accuracy. Therefore pure tin and lead were taken in the same proportion as in the type of solder most widely used-i. e., 40 per cent of tin and GO per cent leadtogether with various added quantities of pure zinc, carefully measured from a solution of a zinc salt. The data obtained by these analyses, shown in Table
IV, give proof of the accuracy. Literature Cited (1) (2) (3) (4) (5)
Fischer and Leopoldi, 2. anal. Chem., 47, 90 (1934). Funk and Ditt, Ibid., 91, 332 (1933). Ibid., 93, 271 (1933:. Ibbotson and Aitchison, Chem. News, 107, 121 (1913). Willard and Furman, "Textbook of Elementary Quantitative Analysis", New York, D. Van Nostrand Co., 1933.
