Micromethods for Determination of Zinc P. L. HIBBARD, Division of Plant Nutrition, University of California, Berkeley, Calif. Add a few drops of bromophenol blue indicator and ammonia until the solution becomes blue. Then add N hydrochloric acid until the solution is just yellow and the precipitate of iron, etc., is dissolved. Now add 3 to 5 cc. of sodium citrate buffer, according t o amount of iron present. This buffer contains about 12 grams of sodium citrate and 23 grams of citric acid in 100 cc. and is adjusted to pH 3 by adding one or the other as needed. Its function is to maintain the proper pH, and to hold iron compounds in solution. A larger amount of citrate buffer does no harm. Set the flask on the steam bath till hot, then add dilute acid or ammonia till the solution is gray (neither yellow nor blue), with a pH of 3.0 to 3.4, and pass in a rapid stream of hydrogen sulfide for a minute or two. (When the solution contains much Fe+++, it is difficult to adjust to pH 3 with bromophenol blue or other indicator. If the iron is pfeviously reduced in slightly acid solution by the use of sulfur dioxide, from sodium sulfite or compressed sulfur dioxide gas, it is much easier to see the color change of the indicator.) If there is much precipitate of lead or copper, no filter aid is needed; if not much precipitate, add about 0.05 gram of acid-washed powdered talc (1 cc. of a 5 per cent water suspension), set the flask in cold water, and continue passing hydrogen sulfide slowly till cold. Cork the flask and set aside till ready to filter. The talc collects the zinc sulfide so that a clear filtrate may be obtained a t once, but if desired, it may stand overnight before filtering. (Addition of copper to form cupric sulfide for flocculating the zinc sulfide has been recommended.) Filter on a close-textured filter, such as Whatman No. 1, 7 cm. Wash with water saturated with hydrogen sulfide plus 5 cc. of 90 per cent formic acid per liter (4). Washing must be very thorough, five to six times after the original solution has all passed through, in order to remove all the iron from the filter, The filter now contains the zinc as zinc sulfide, with possibly some arsenious, cupric, or lead sulfide. Small quantities of these impurities may be disregarded, but if there is more than 1 to 2 mg. of arsenic or 10 of lead, these ions should be removed before precipitating the zinc. It is best to start with another portion of the material to which no buffer has been added. INPRESENCE OF MUCHARSENIC OR LEAD. Add concentrated hydrochloric acid to make the solution 6 N . Saturate with hydrogen sulfide, cork the flask, and set it on the steam bath till hot. Remove by filtration the yellow arsenic sulfide that coagulates. Collect the filtrate in an evaporating dish and evaporate until nearly dry. No loss of zinc occurs by evaporation of hydrochloric acid solutions on the steam bath, and this is preferable to neutralizing the excess acid with an alkali. Return the solution to the small flask and make 0.25 N,, preferably by titration, since indicators are not accurate enough in the region of pH 0.52. Pass a rapid stream of hydrogen sulfide into the solution for a few minutes till it is saturated. Cork the flask and set on the steam bath till hot. The lead sulfide, at first flocculent, soon becomes granular and is easily filtered out. To obtain the zinc in the filtrate, neutralize the solution to pH 3, add citrate buffer, and precipitate the zinc with hydrogen sulfide as above described.
T
HE purpose of this paper is to describe methods for
quantitatively estimating the small amounts of zinc which may be found in plant materials and other substances. These methods require ashing to remove organic material, then separation of the zinc from most other associated metallic ions. The ash is extracted with hydrochloric acid and the zinc separated by means of hydrogen sulfide or oxalic acid and ferrocyanide. The isolated zinc may be estimated by nephelometry, by iodometric titration, by a micromodification of the long-known ferrocyanide titration, or by the polarigraph (b). The amount of zinc to be measured may be 0.5 to 0.1 mg., quantities sometimes found in 5 grams of plant material. The polarigraph is capable of measuring very much smaller amounts. The methods described in this paper present the following improvements: use of a convenient citrate buffer adjusted to p H 3; more accurate adjustment to desired pH; use of powdered talc as a filter aid; several turbidity standards; and considerable saving of the analyst’s time. A description of the ferrocyanide-oxalate separation and details of the iodometric titration of minute amounts of zinc are also given. The time required to make a single determination by the hydrogen sulfide separation and ferrocyanide turbidity method may be 2 to 3 hours. One analyst can complete ten to twenty determinations in 8 hours, depending on the character of the samples to be analyzed and the adequacy of laboratory equipment. Before beginning an analysis, all the apparatus, chemicals, and water to be used should be tested for zinc. Jena and some other glassware easily give up zinc to dilute acid. Pyrex or Kavalier glassware seems satisfactory. Ordinary glass bottles and good porcelain seem to be free of soluble zinc. Most rubber stoppers and tubing give up zinc. I n preparation and grinding of plant material, all copper, brass, bronze, or galvanized apparatus should be excluded. A nickel wire sieve and ordinary iron grinding mill may be used. Plant material should be pulverized to pass a 1-mm. sieve. Woody material, such as twigs of trees, may contain much more zinc in fine than in coarser portions, and therefore should be ground to pass a I-mm. sieve and well mixed. ANALYTICAL PROCEDURE The procedures of several chemists (1, 3, 4,7 ) have been considered in working out this method. Most of the basic facts have long been known, but several new modifications of procedure have been introduced by the writer in order to simplify and shorten the work. INABSENCE OF MUCHARSENIC OR LEAD. In a 25-cc. porcelain evaporat,ing or other appropriate dish, burn 1 to 5 grams of materials in a muffle at low red heat in order to avoid fuqion of the ash. Apparently zinc is not lost at bright red heat, but more complete extraction of the zinc by dilute acid is secured and carbon is easier to burn off if the ash does not fuse. To the ash from 5 grams of material, add about 15 cc. of water and 7 cc. of 3 N hydrochloric acid and set on the steam bath till about half the liquid has evaporated. Further concentration may cause gelatinization of silica so that filtration becomes difficult. So long as the silica remains in solution, it does not interfere in subsequent operations. Filter the extract into a 50- to 100-cc. conical flask and wash till the volume is 20 to 25 cc. In case there is little insoluble residue, filtration may be omitted. If much unburned carbon remains on the filter, ignite again, extract with acid, and add the extract to the main portion. Usually the insoluble ash contains very little zinc. (In one case, the ash from four filters was found to contain 0.005 mg. of zinc; in another, ash from 5 filters contained 0.015 mg. of zinc.)
Attempts to remove the lead from the solution of the original ash by use of sulfuric acid instead of hydrochloric acid as solvent failed to retain the zinc in solution. Most of the lead was removed as lead sulfate, which seemed to retain some zinc. I n most cases, there is not enough lead to interfere or prevent complete separation of the zinc sulfide in such condition that it is easily soluble in N hydrochloric acid. Small amounts of lead, u p to 10 mg., may be present with the zinc sulfide, yet from this the zinc may be extracted by cold N hydrochloric acid, which also dissolves some of the lead. By applying the acid in portions of 2 to 3 cc. a t a time to the filter, the zinc is entirely dissolved by about 15 cc., nearly all in the first 5 cc. The solution of zinc, with perhaps some lead, is now ready for estimation of the amount of zinc, as described below. There is danger of occluding some zinc sulfide with the
423
ANALYTICAL EDITION
424
lead sulfide in separating the lead, unless the free acid is kept up t o 0.25 N , and from this strength of acid not all the lead is usually removed by hydrogen sulfide. A very little arsenic precipitated with the zinc sulfide may prevent solution of the latter in N hydrochloric acid, and therefore must be removed first. If the solution contains enough silica to interfere in filtration, it may hinder solution of the zinc sulfide and so should be removed by dehydration of the original acid extract of the ash. OXALATE-FERROCYANIDE METHOD.The success of this separation depends on t h e fact that the ferrocyanides of iron, lead, manganese, arsenic, and calcium are easily soluble in a 7 to 8 per cent solution of oxalic acid which also contains enough hydrochloric acid t o make i t about 0.5 N in free hydrochloric acid, while the ferrocyanides of zinc and copper are very insoluble in the same acid mixture. This unpublished method was devised by P. R. Stout in the attempt t o find a way to estimate the zinc in such a complex mixture by means of the polarigraph. Ash 2 grams of plant material as described above and extract the soluble material with about 3 cc. of 3 N hydrochloric acid and 10 cc. of water. After half the liquid has evaporated on the steam bath, add 10 to 11 cc. of a 10 per cent solution of oxalic acid. When further evaporation has reduced the total volume to about 20 cc., remove from the heat, and if copper is present, pass in hydro en sulfide for a few minutes and let stand till cold, best overnigb,. Filter out the insoluble residue and the cupric sulfide, together with the oxalates of calcium and lead, and wash with a little 5 per cent oxalic acid. To the clear filtrate add 10 cc. of 10 per cent oxalic acid and 1 cc. of 4 per cent potassium ferrocyanide and mix. A white cloud indicates the presence of zinc. If no cloud appears, appreciable amounts of zinc are absent. After 10 to 15 minutes, filter out the zinc ferrocyanide. Addition of 0.02 gram of powdered talc aids in obtaining a clear filtrate. It may be necessary to return the filtrate to the filter once or twice in order to retain all the precipitate. Wash the filter twice with 5 per cent solution of oxalic acid, decompose the precipitate on the filter with 6 to 10 cc. of N sodium sulfide, and wash three times with some of the same solution diluted with three volumes of water. Lastly, wash once with water. If zinc was present in the solution, zinc sulfide now remains on the filter. Dissolve the zinc off with N hydrochloric acid, returning the filtrate to the filter until it comes clear. In the clear filtrate, the zinc is a ain precipitated by potassium ferrocyanide and estimated neptelometrically, or it may be precipitated by hydrogen sulfide and estimated as described below.
If desired, centrifugalizing may be substituted for filtration in making oxalate separations. In making these separations, the concentration of hydrochloric acid must be about 0.5 N , sufficient to prevent precipitation of calcium and lead oxalates, and the solution must be strong enough in oxalic acid, about two-thirds saturated, to prevent appearance of much blue due to ferric ion, If the mixture on t h e filter becomes blue, iron is likely to be present with the zinc in the final filtrate from which the iron may be removed by repeating the separation. Although one inexperienced with this method may not succeed well at first, i t is preferable to the hydrogen sulfide separation when there is much lead or arsenic in the solution.
ESTIMATION OF ZINC NEPHELOMETRIC METHOD, The zinc solution dissolved off the filter by N hydrochloric acid is collected in a 20-cc. flatbottomed specimen tube suitable for comparisons in the manner in which a Nessler tube is used. To the zinc solution add 2 cc. of 5 M sodium hydroxide and fill to the mark with N hydrochloric acid, Add 2 drops of 2 er cent potassium ferrocyanide; mix quickly. After standing aiout 15 minutes to develop its greatest turbidity, it is compared with similar tubes containing known amounts of zinc precipitated in the same manner. For comparison, set the tubes on ti dead black surface where the light strikes the tube perpendicular to its length. Tubes containing 0.02, 0.04, 0.06, 0.08, and 0.10 mg. of zinc will usually be sufficient and permit a better estimation of the amount than if a single tube is used to contain the standard which is adjusted
Vol. 6, No. 6
to match the unknown by pouring out part of the mixture. The standards are good for an hour or so, but should be made fresh for each set of samples to be examined. The cloudy suspension of zinc ferrocyanide should be pure white, not colored blue by iron, and the precipitate should not settle perceptibly in 1 hour, Use of a nephelometer to measure the turbidity may promote increased accuracy. Greater precision is attainable by use of a photoelectric cell in making the comparisons. If some lead sulfide was mixed with the zinc sulfide on the filter, some of the lead goes with the zinc dissolved by the N hydrochloric acid, but this does no harm since lead ferrocyanide is easily soluble in 0.5 N hydrochloric and seems not to influence the turbidity produced by the zinc. In order to secure greatest accuracy, concentrations of reagents, manner of mixing, and all other conditions in standards and in unknowns should be as nearly alike as possible. Iron, copper, and manganese must not be present. Not more than 0.1 mg. of zinc in 20 cc. of solution is well measured by this method. MEASUREMENT BY POLARIGRAPH. This instrument permits measuring to ~5 to 10 per cent accuracy much smaller amounts of zinc than can be measured by chemical methods. However, the apparatus is expensive and not readily available, and the necessary special preparation of the solution is not generally understood. IODOMETRIC TITRATION METHOD.This method, a modification of that proposed by Lang (8), is adapted to measuring with a good degree of accuracy 0.05 to 1.0 mg. of zinc or more. The reagents needed are: 1. A phosphate buffer made by mixing 25 grams of acid potassium phosphate with 5 cc. of sirupy phosphoric acid and diluting to 100 cc. The pH is adjusted to 3 by addition of acid or alkali as needed. For use, it is diluted 10 times. 2. Potassium iodide, 20 per cent solution in water. 3. Clear starch for indicator, 0.5 per cent in water. 4. Potassium ferricyanide,. 1 per - cent in water, made fresh every few days. 5. Sodium thiosulfate, 0.001 N in cold, boiled water, made fresh every day by dilution of a 0.01 N solution. It loses strength perceptibly in a few hours.
TITRATION PROCEDURE The zinc chloride from the zinc sulfide dissolved off the filter by N hydrochloric acid is collected in a 50-cc. porcelain casserole and evaporated to dryness on the steam bath. To the dry residue, add, in the order given, 0.5 cc. of dilute phosphate buffer, 1 to 2 drops of potassium iodide, 3 drops of starch, and 3 drops of potassium ferricyanide. If zinc is present, the mixture becomes blue and is titrated with sodium thiosulfate slowly till the blue color changes to pure yellow. After a few minutes, the blue returns slowly, but this is disre arded. If no zinc is present some blue a pears, but this may %e discharged by 1 or 2 drops of sodium thosulfate, 1 cc. of which usually represents about 0.1 mg. of zinc. It should always be checked against a known amount of zinc at the time the unknown is titrated. It is hardly worth while to prepare for making this titration unless a number of samples are to be analyzed at one time. TNOreactions are apparently involved in this titration : KsFe(CN)e -k K I % &Fe(CN)s
+ 1/zIz
(1)
This is slow and reversible and soon reaches equilibrium, but if zinc is present the second reaction takes place, so that iodine is set free as long as any zinc remains. Zn++
+ KdFe(CN)s = ZnKnFe(CN)s 4- 2 K f
(2)
This is practically instantaneous. Lang indicates that the reaction is 2KsFe(CN)a
+ 2KI + 3ZnS04 = KzZns[Fe(CN)~lz3- 3K&01 4-
IP (3)
The ferricyanide and iodide of Reaction 1 also react slowly to liberate free iodine by action of the oxygen of the air, so that oxygen should be excluded as much as possible. To this end, the titration is performed in a very small volume and mixing is obtained by gentle rolling of the casserole from side to side
I N D U ST R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Kovember 15, 1934
without use of a stirring rod or other means of vigorous agitation. If the liquid is covered with a layer of gasoline or the work is done in an atmosphere of carbon dioxide, the return of the blue color after completion of the titration is very much slower, supposedly because oxygen is nearly excluded. However since a titration may be completed within less than a minute, there is very little error from the oxygen of the air. Copper, iron, and manganese must be absent.
425
was 0.16 mg. in each of six tests. With reagents alone plus 20 mg. of lead, the zinc found was 0.03 mg., indicating a plus error of 0.02 mg. of zinc, due to added lead, which is almost within the limits of error of reading the turbidities. To &gram portions of pine sawdust were added varying amounts of zinc, the material was burned, and zinc estimated: ZINC ADDED Me. 0. 0.02 0.05 0.10
ZINCFOUND Me. 0.06 0.08,0.08 0.12, 0.12 0.15. 0.16
Estimation of the separated zinc by the colorimetric phosphate method (7‘) was not successful because of difficulty in removing all phosphate not combined with zinc-i. e., a sufficient amount of phosphate to vitiate the determination was always found in running a blank on reagents. The well-known method for macrotitration of zinc with ferrocyanide and uranium indicator may be used when the amount of zinc is more than 0.3 mg., though the iodometric method above described is probabIy preferable, being quicker and better adapted to measurement of very small amounts. The Eegriwe test for zinc (9) with diethylaniline and potassium ferricyanide is not much more sensitive than the cloud test with potassium ferrocyanide and is not adapted to quantitative estimation of zinc.
ZINC ESTIMATED BY IODOMETRIC TITRATION. To 5-gram samples of apple leaves containing much lead varying amounts of zinc were added. The mixtures were burned, the zinc was separated, and the amount measured by iodometric titration:
RESULTSOBTAINED To the solution of the ZINC ESTIMATEDBY TURBIDITY. ash of several 5-gram portions of peach leaves 20 mg. of lead as acetate were added, and the zinc was separated by hydrogen sulfide and estimated by ferrocyanide turbidity as above described. Without added lead, the zinc found on two tests was 0.15 mg. each time. With 20 mg. of lead added, the zinc found
(1) Bodansky, M., J. IND. ENO.CHBM.,13, 696 (1921). (2) Eegriwe, E.,2. anal. Chem., 74,225 (1928). (3) Fairhall and Richardson, J . Am. Chem. Soc., 52, 938 (1930). (4) Fales and Ware, Ibid., 41,487 (1919). (5) Heyrovsky, J., Mikrochemie, 12,26-64 (1932). (6) Lang, R., 2. anal. Chem., 93, 21 (1933); C. A., 27, 3419 (1933). (7) Todd and Elvohjem, J . Bid. Chem., 94,609 (1932).
ZINC ADDED
ZINCFOUND Mg 0.015, 0.015, 0.015 0.055 0.26, 0.22
.
Me. 0. 0.05 0.20
LITERATURE CITED
RECEIVED March 14. 1934.
Quantitative Determination of Lead as Periodate HOBART H. WILLARD AND J. J. THOMPSON, University of Michigan, Ann Arbor, Mich.
I
N ANOTHER paper (3) the proper conditions have been
given for the precipitation of lead as pure triplumbic pareperiodate. It was found that this salt was sufficiently insoluble in very dilute acid solutions to make possible its use in the volumetric and gravimetric determination of lead.
EXPERIMENTAL Weighed samples of Mallinckrodt’s reagent-quality lead or Kahlbaum’s sheet lead were dissolved in nitric acid and evaporated t o dryness. The dry salt was then dissolved in 200 cc. of 0.025 N nitric acid and the lead precipitated at 100’ C. by the slow addition of 2 grams of sodium periodate, NaIOa, dissolved in 50 cc. of water. When the amount of lead was very small. no precipitate formed until after 1 CC. of the periodate solution had been added. When this occurred, it was necessary to decrease the acidity t o 0.006 N ; otherwise the precipitate did not have the theoretical composition. After the periodate was added, the solution was cooled in ice water and the cold solution stirred for 0.5 hour, because supersaturation was very pronounced. The lead periodate was filtered on a porcelain filterin crucible, washed with ice water, and dried for 2 hours a t 110’ It was then weighed as PbsH4(IO&. Results of analyses are shown in Table 1. To determine the lead periodate volumetrically the method of Andrews (1, 9) was used, because the best solvent for the salt is concentrated hydrochloric acid. The equation for the reaction can be expressed as follows: P b 8 H ~ ( I O ~ ) ~8HC1 6HsAsOs = 3PbC1.2 2IC1 6HsAsOd 6Hz0
8.
+
+
+
+
+
If arsenious acid is present in excess, all the chlorine will react with i t to form arsenic acid. The excess arsenious acid can then be titrated with iodate, using chloroform as indicator. TABLEI. GRAVIMETRIC DETERMINATION OF LEADAS PbaH4(IOeh L ~ A TAKEN D Gram 0.7001 0.6953 0.6028 0.5596 0.5591 0.5553 0.3853 0.1145 0.1121 0.0735
LEADFOUND Gram 0.7001 0.6954 0.6025 0.5596 0.5588 0.5552 0.3852 0.1147 0.1122 0.0734
ERROR
Me .
*o.o
$0.1 -0.3 fO.0 -0.3 -0.1 -0.1 +0.2 10.1 -0.1
TABLE11. VOLTJMETRIC DETERMINATION OF LEADAS Pbs%(IOa)n LEADTAKEN Gram 0.5920 0.4896 0.3069 0.2920 0.2060 0.1554 0.1103 0.1037 0.1001 0.0653 0.0496
LEADFOUND Gram 0.5921 0.4895 0.3067 0.2920 0.2063 0.1557 0.1101 0.1035 0,0999 0.0655 0.0494
ERROR
Me. +0.1 -0.1 -0.2 0.0 +0.3 4-0.3 -0.2 -0.2 -0.2 +0.2
-0.2
Am03 ADDBID
Gram 0.7111 0.7447 0.4006 0.4135 0.3297 0.2320 0.1299 0.1271 0.1125 0.1003 0.2116
