610
ANALYTICAL CHEMISTRY
of aluminum was carried through the standard extraction procedure, but not through the mercury cathode electrolysis, except that the final dilution was made to 100 ml. The solution was divided into four 25-m1. portions. The fluorescence of the solutions was measured a t temperatures ranging from 10’ to 40’ C. There was a slight decrease in fluorescence as the temperature increased but when a correction was applied for the volume change of the chloroform solution, the decrease was insignificant. Effect of Anions. Solutions containing 0.5 gram of anion as the ammonium salt and 10 micrograms of aluminum were carried through the standard extraction procedure. The anions studied were the acetate, chloride, citrate, nitrate, perchlorate, sulfate, and tartrate. Only the citrate and tartrate caused interference, the recovery of aluminum being only 25% in the presence of the citrate and 82% in the presence of tartrate. I n the presence of the other anions the recovery of aluminum varied from 93 to 100%. Effect of Cations. Iron, titanium, and vanadium were investigated since these cations are most likely to be involved in steel analysis. Iion is quantitatively removed from aluminum using a mercury cathode cell, but the amount that can be tolerated without serious error is of interest since iron is the main constituent in steel. Titanium and vanadium both remain with the aluminum and hence constitute a more serious problem. Three separate series containing 10 micrograms of aluminum and 0, 10, 20, 40, and 60 micrograms, respectively, of iron, titanium, or vanadium were carried through the standard extraction procedure but not through the mercury cathode electrolysis. The
results are shown in Figure 2. 111 of the cations mentioned quench the fluorescence. For a 1 to 1 weight ratio of titanium, vanadium, or iron t o aluminum, respectively, 100, 89, and 83% recovery of aluminum is obtained. For a 6 to 1 weight ratio of interfering element to aluminum, about 65% recovery of aluminum waa found in all three cases. Since the amounts of titanium and vanadium in steel often do not exceed the aluminurn concentration, the interference from these elements is lees serious than that encountered in the colorimetric method (IO). LITERATURE CITED
(1) Davydov, A. L., and Devekki, V. S., Zauodskaya Lab., 10, 134-8 (1941). (2) Fiegl, F., and Heisig, G. B., Anal. Chim. Acta, 3, 561-6 (1949). (3) Gentry, C. H. R., and Sherrington, L. G,, Analyst, 71, 432-8 (1946). (4) Grimaldi, F. S., and Levine, H., U. 8. Geol. Survey, Trace Elements Investigations, Rept. 60 (1950). ( 6 ) Sandell, E. B., IND.ENG.CHEM.,ANAL.ED.,13, 844-5 (1941). (6) Tullo, J. W., Stringer, W.J., and Harrison, G. A. F., Analyst, 74, 296 (1949). (7) TVeissler, A . , and White, C. E., IND.ENG.CHEM.,AN.AL.ED.,18, 530-4 (1946). (8) Welcher, F. J., “Organic .hialytical Reagents,” Vol. 1, p. 270, New York. D. Van Xostrand Co.. 1948. (9) White, C. E., and Lowe, C. S., IND.ENG.CHEW, AXAL.E D ,12, 229-31 (1940). (10) Wiberley, S. E., and Bassett, L. G , ANAL.CHEM.,21, 609-12 (1949). (11) Willard, H. H., and Horton, C. .1.,Ibid., 24, 862-5 (1952) RECEIVED for review February 26, 1951. -4ccepted J a n u a r y 15, 1953
Gravimetric Methods for Zinc and Its Separation from Certain Elements Use of a Radioisotope in Evaluation of Analytical Procedures JOHN E. VANCE
AND
RICHARD E. BORUP, R‘ew York University, iVew York, N. Y .
I
T IS useful in the evaluation of a gravimetric procedure to
have information on the amount of an element remaining in solution in addition to the usual comparison between the weight of a precipitate obtained and the known amount of an element taken in any experiment. In the past, the amount of an element escaping precipitation has been estimated directly only infrequently because of the tedium involved. The use of radioisotopes makes the determination of the unprecipitated element a relatively simple problem. Apart from the additional information which may be obtained through their use, the simplification which radioisotopes bring to a systematic study of analytical methods makes more attractive than formerly the critical comparison of gravimetric procedures on which the choice of standard or referee procedures can be based. Similarly, useful studies can be made on separations and on coprecipitation phenomena. Zinc was chosen for an initial study of this type because of its general importance. The Zne6 isotope has decay characteristics which make its determination straightforward since it has a half life of about 250 days and since gamma rays of 1.11m.e.v. energy are a part of its decav scheme (15). The classical phosphate and sulfide precipitations, three precipitations with organic reagents and several separations of zinc from elements with which it is commonly encountered, were examined. MATERIALS AND METHODS
The method of studying each procedure was the same in outline. -4stork solution of zinc sulfate, containing a quantity of
zinc-65 together with stable zinc, was prepared and analyzed gravimetrically in several ways and the activity of the solution was determined; thus there \\-as obtained a relation between counts per minute and total weight of zinc. I n the course of the experiments, two such standard solutions were used; the initial activity of each corresponded to about 1000 counts per minute per milligram. The activity of the solution in use was measured repeatedly and the current value of the specific activity was applied to the experiments in progress a t that particular time. The stable zinc was a distilled product of the Xew Jersey Zinc Co. reported to have a purity of 99.999%. Two sources of zinc-65 were used, one obtained from the Oak Ridge National Laboratory in the form of dust (from which possible traces of members of the hydrogen sulfide group n-ere removed) and the other, a sample of the distilled zinc which was irradiated a t Oak Ridge. Samples of the standardized stock solution were measured from a weight buret and treated according to the procedure being investigated. It was estimated that less than 5 microcuries of zinc-65 were present in all but a few of the experiments. I n most cases, the precipitates were filtered on porcelain (Selas) filter crucibles of about 25-1111. rapacity and were heated to constant weight. The filtrates and washings were collected directly in volumetric flasks and after dilution to volume were set aside for the counting measurements. A conventional scaler was used with an RCL Mark 1, Model 30.4 gamma counter tube to measure the zinc-65, and therefore the total zinc, in the filtrates. Several counter tubes were used
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 The primary objective of this investigation is to evaluate certain gravimetric methods for zinc. The use of zinc-65 allows the determination of the amount escaping precipitation; this information, taken with gravimetric results, indicates whether or not a procedure will remove an element from solution quantitatively and also give a precipitate of the expected composition. A secondary objective is to present quantitative data on the separation of zinc from various elements by the sulfide precipitation. The phosphate procedure can be modified to advantage by the use of a much smaller excess of reagent than previously suggested, adjustment of the pH of the wash liquid, and use of a lower ignition temperature. The precipitation of zinc oxalate is an excellent gravimetric method with zinc oxide as the weighing form. Either anthranilic acid or 8quinolinol will remove zinc quantitatively from solution, but empirical factors are needed because the precipitates do not have the predicted compositions. The precipitation of zinc sulfide from a cold sulfate-bisulfate buffer provides a good separation when the ratio of zinc to the foreign ion is at least as low as 1 to 4 iron, 1 to 2 nickel, 1 to 8 manganese, and 1 to 4 aluminum. The modified phosphate procedure, the zinc oxalate precipitation, and the sulfide method all will provide average results within 0.1 mg. of the truth as determined both by gravimetric data and by counting unprecipitated zinc-65 in the filtrates.
611 minations arc reported since within the range of conditions giving quantitative recovery of the zinc, as measured by both gravimetric and counting results, the second determination agreed with the first within the sensitivity of the balance (&0.05 mg.) and the expected uncertainty of the counting procedure. 4MMONIUM PHOSPH4TE PROCEDURE
-4lthough the phosphate procedure for zinc is 80 years old a t least, the procedure in use today is that of Dakin (6) who studied the weighing forms and introduced ammonium phosphate as the precipitating reagent, and of Ball and -4gruss ( 1 ) whose principal interest was the effect of pH. In the present investigation, a survey was made of the effects of the following factors: pH, concentrations of ammonium salts, the excess of the precipitating reagent and its concentration, and the choice of xash solution; in addition, two weighing forms and necessary ignition temperatures were studied.
1.0
GRAVIMETRIC ZN TAKEN ZN RECOVERED
-
e
E
PH COUNTING ZN IN FILTRATE
0
0
in the course of the investigation and in each case the performance was checked by means of a standard source of cobalt-60 which emits gamma-rays of nearly the same energy as those from zinc-65. Periodic checks on the tubes were made with the standard source and daily counts of the background were taken. Great accuracy in the measurement of the activity of the solutions was not required to provide satisfactory information on the efficiency of the analytical procedure. As the effectiveness of the precipitation method increased, the number of counts per minute from the zinc-65 in the filtrate approached background and the determination of the absolute amount of unprecipitated zinc became less certain; with the counting times used, an activity of twice background corresponded to an uncertainty of about 9% in the amount of zinc in the filtrate. When compared with the activity of the original solution, however, such a counting ratc corresponded to 0.1 mg. or less of total zinc in the filtrate, so the uncertainty in the determination of this quantity waa no greater than &0.01 mg. Since the total zinc in the original sample was 50 mg. or greater, the precision of the observation, so far as the quantitative removal of this amount was concerned, was quite good-Le., about 0.02%. Preliminary experiments showed that it was satisfactory and convenient to count liquid samples in small, open glass dishec with vertical sides; a volume of 25 ml. was chosen for convenience. Various volumes of the stock solution containing stable and radio-zinc were brought to a volume of 25 ml. and counted iii this fashion; a plot of counts per minute against the known concentration of total zinc was linear, a t least from background to I500 counts per minute. All samples which were counted i:i the subsqeuent experiments were diluted in known ratios, if necessary, to bring the observed activity down to this range. All experiments were performed in duplicate and separate gravimetric and counting measurements were made when the determination of zinc was the object. In the tables, single deter-
Figure 1.
Effect of pH and Ammonium Sulfate 0 Ammonium sulfate added 0 0.25 M ammonium sulfate
The basic analytical procedure x a s to use a sample of 50 t o 100 mg. of zinc in 140 ml. of solution and, except in those cases in which the effect of p H was being studied, or as othernise noted, the solution was made just red t o methyl orange by the addition of sulfuric acid. The solution was heated to about 60" C. and 10 ml. of a 10% solution of diammonium hydrogen phosphate were added, thus providing a tenfold excess over the theoretical requirement in a fina1,volume of 150 ml. The solution of the ammonium phosphate x a s barely pink to phenolphthalein. After the precipitates had become crystalline, they were allowed t o stand overnight a t room temperature. In most cases, the precipitates were filtered on porcelain crucibles and were mashed first ~ i t ha 1% solution of diamnionium hvdrogen phosphate nhose pH was adjusted to about 6.8 and finally with neutral 50% alcohol. The precipitates mere first dried for 1 hour a t 110' C. and were then heated in a larger solid porcelain crucible over a MPker burner. The heating was repeated, but only rarely did the second weight differ from the first.
pH of the Precipitation. The effect of pH was studied first by following the basic procedure and by adding either ammonium hydroxide or sulfuric acid to give the desired pH; no salts were added. A similar study was made in the presence of various ammonium salts. Figure 1 shows these results, which indicate that, in the absence of ammonium salts, other than the reagent, the permissible p H range is approximately 5.8 to 8.1 and, within this range, the error was less than 0.1 mg. as determined by counting the unprecipitated zinc in the filtrate and did not exceed 0.15 nig. in the weighrd zinc pyrophosphate; between p H 5.8 and 7 3, neither method showed an error greater than 0.1 mg. of zinc. The gravimetric results show, as did those of Ball and Agruss (I), that the composition of the ignited precipitate agrees most
612
ANALYTICAL CHEMISTRY Table I.
Zn Expt. Taken, KO. Rlg. 106 45.23 102 45.23 101 46.19 104 44.62 109 44.26 111 45.74 112 43.30 1 I3 44.44 118 47.57 136 48.97 126 49.72 127 47.01 47.34 146 47.93 148 149 47.17 140 46.95 137 49.06 141 46.48 142 47.05 143 46.77 144 47.36
Effect of pH and Ammonium Salts on Precipitation of ZnNH4PO4 A4dditional Ammonium Salt,
M
None None None h-one None None Pione
pH 5 8 6 2 6 4 6 7 7 1 7 5 7 9 8 1 6 1 6 8 7 4 7 7 7 1 6 7 6 5 7 2 6 9 7 1 7 0 6 7 fi4
Gravimetric Results Zn found, Error, mg. mg. 45.18 - 0 05 45.15 - 0 08 46.14 - 0 05 44.70 + O 08 44.35 +o 09 45.88 4-0 14 43.35 + O 05 - 0 14 44.28 47.25 - 0 12 48.98 +o 01 49.76 4-0 04 47.04 + 0 03 - 0 06 47.28 - 0 05 47.88 47.13 - 0 04 46 11 - 0 84 48 16 - 0 90 45 49 - 0 99 46 12 - 0 93 46 01 - 0 76 - 0 51 46 84
Counting Results, Zn in Filtrate, Mg.
0.06 0.06 0.05 0.07 0 08 0.06 0 06 0 07 0 09 0 09 0 04 0 04 0 04 0 05 0.06 0 08 0 05
0 07
0 04 0 06 0 04
closely with expectations when the precipitation is made a t a pH close to 6.5. Effect of Additional Ammonium Salts. The effect of pH was studied in the presence of added ammonium sulfate. These results are shown also in Figure 1 and appear to indicate that a concentration of about 0.25 M of that salt narrows the optimum pH range slightly from 6.1 to 7 . 7 ; however, within this range the presence of ammonium sulfate has no discernible effect on the results (Table I ) except to produce a precipitate with improved physical characteristics. Concentrations of ammonium chloride up to 1.0 M (the highest used) had no effect on the results except to accelerate the transformation of the initial precipitate to the crystalline product. It would be hazardous to extend this conclusion to a situation other than the one described here since the counting results showed some increase in solubility with increasing chloride concentration and since the formation of zinc-chloride complex ions is well known-for example, the increased solubility of zinc sulfide in chloride solutions has been noted by Jpffreys and Swift (12). Very different results were obtained with ammonium nitrate, even when that salt had a concentration as low as 0.01 M, for although the counting resuIts showed (Table I) that the zinc had been removed from solution quantitativelv, the gravimetric results were low. A similar difficulty is apparent in the results of Dakin (6) though he made no comFent. The usual directions for the determination of zinc in a brass or German silver, following electrolysis of copper, avoid the interference by recommending the destruction of all ammonium salts and evaporation to sulfuric acid; if, hoivever, no other salts are present than the chloride and sulfate within the concentration h i t s noted, the step is unnecessary, if the zinc is to be precipitated by the phosphate procedure. Typical experiments on the effect of ammonium salts and of pH are given in Table I. Excess of Precipitating Reagent. The phosphate method for zinc is characterized by the use of an extraordinarily large excess of precipitating reagent: the amount recommended in standard references (IO, I S , l 7 j varies from 6 to 15 times the theoretical requirement. The size of the excess is all the more remarkable in view of the care normally taken to make the final precipitation of the corresponding magnesium compound in the presence of the stoichiometric quantity of phosphate and in view of the effort that is made generallj- to limit the amount and concentration of a reagent. The usual procedure calls for a preliminary adjustment of the p H to that of the pink color of methyl orange-i.e., a pH of about 3.5-before the phosphate is added and i t may be that the use of a large amount of reagent was recommended originally to ensure
that the pH is raised to the optimum range. The gross effect of adding the phosphate is to raise the pH, but a series of qualitative experiments demonstrated that an excess of two or three times the amount theoretically required for the zinc would raise the pH of the solution to the required value. The quantitative experiments (501, 521, 523, and 525) listed in Table I1 bear this out. The correct pH is not sufficient in itself, however, as shown by 517, 518, 513, and 514, in which the pH was adjusted with the help of appropriate indicators after the addition of phosphate. If the adjustment of the pH to the minimum satisfactory value of about 6 is left to the addition of the phosphate, the presence of ammonium sulfate lowers the pH and thus increases the error (compare 509 and 508, also 521 and 522), but this is true only in those cases in which the excess of phosphate is not as much as 2.5 times the theoretical amount. I t is possible to obtain quite satisfactory results using no more than twice the theoretiral quantity of phosphate, provided that the pH is adjusted after the addition of the reagent (519, 520, 515, and 516) even though the concentration of ammonium sulfate may be 0.25 M ; however, there appears to be no advantage in such a procedure since the use of two and a half to three times the theoretical requirement of phosphate avoids the difficulty and represents a very simple modification of the familiar method. On the other hand, there is nothing to be gained by the use of more than, say, three to five times the amount of diammonium hydrogen phosphate required by theory. The amount of zinc present in an analysis is of no particular consequence unless the volume of the solution is kept so small that the concentration of the rragent becomes large enough to increase the solubility of the zinc ammonium phosphate. Separate experiments showed that a safe limit for the concentration of diammonium hydrogen phosphate is about 0.1 JI (see also experiments 27 and 31 of Table 11). This concentration corresponds to the precipitation of as much as 0.2 gram of zinc in a volume of 150 ml. using a fivefold excess of diammonium hvdrogen phosphate. It is clear, however, that under the best conditions, several hundredths of a milligram escape precipitation so that a minimum amount of about 50 mg. of zinc should be taken in an analysis by the phosphate procedure if the relative erroi is to be about 0.1% or less. Wash Solution. It is common practice to wash the precipitate of zinc ammonium phosphate with a 1% solution of diammonium hydrogen phosphate, prepared by diluting the 10% reagent used in the precipitation. The reagent solution, however, is adjusted to a pH of about 8.5 corresponding to a faint pink color of phenolphthalein and the 1% solution will have a pH not much lower than that value (actual measurements gave 8.1 as the pH of the diluted reagent) and may, on that account, dissolve some of the precipitate. The effect of wash liquids was shown by measuring thrir activity after passing through the precipitate in several filtrations under the same conditions: While a 1% solution of the reagent (pH 8.1) dissolved about 0.7 mg. of zinc ammonium phosphate per 100 ml., a 1% solution of the reagent nrhose pH had been adjusted to 6.8 dissolved 0.04, 0.04, and 0.06 mg. and cold riater dissolved 0.23, 0 27, and 0.15 mg. of the precipitate. I n no case could solubility equilibrium be expected, but the relative effects indicate that water is a better wash liquid than the diluted reagent unless one adjusts the pH to the point corresponding to a minimum solubility of zinc ammonium phosphate. Dekin ( 6 ) stated that the use of asbestos in a filter crucible might introducp an error since some varieties of that mineral have a limited solubility in phosphate solutions; he suggested that this be tested after the final neighing hv dissolving the precipitate in dilute nitric acid and redetermining the weight of the empty crucible. -4lthough porcelain or platinum filter crucibles were used in the exprriments reported here, this point n-as checked It was found that large losses in weight were observed when the zinc pyrophosphate was dissolved on an asbestos mat with 2 S nitric arid, but if an empty Gooch, made from the same asbesto?
V O L U M E 25, NO. 4, A P R I L 1 9 5 3
613
rJrecirJitate becomes crvstalline k d {hen allow to s t a n i 4 hours or more. Filter on a porcelain Gravimetric Counting or platinum crucible and wash Results Results, free of chlorides with a 1% ' soluZn Zn zn in (SHa)zHPO4 (XHI)~SOI found, Error, Filtrate, Times pH of Added. Taken, Approx. EXPL tion of diammonium hydrogen Notes Grams No. mg. llg. theory concn., .tf Filtrate Mg. mg. phosphate whose p H has been 72.2 1 0 4 5 97 2 501 -24 9 0 01 0 adjusted to about 6.8 and fib 1 r, 87 62 -11 1 0 015 517 98 76 6 7 0 b nally n i t h a few milliliters of 107.47 1 .5 0 015 5 6 7 518 107 76 - 0 29 neutral 50% alcohol. The pre102 50 1 .i -2 2 0 015 7 0 0 104 73 513 !I8 76 1 .i 0 0i5 7 3 0 99 42 SI4 -0 70 cipitate may be weighed as zinc 100 80 -2.j y 1 6 0 016 4 7 3 509 126 7 3 ammonium phosphate after 3 4 108.80 -9 2 l i 0 017 1 118 01 ,508 98 47 '0 heating to constant neight a t 521 100 30 -1 8 0 02 0 6 1 92 YO 2 0 n 2 822 97 66 -4 8 0 02 5 110' C . or as zinc pyrophos100 60 0 02 518 100 78 - 0 18 6 4 0 phate aftrr heating at 500" to 120 09 0 02 5 7 0 520 120 08 + o 01 600" c. O R . 63 2 0 :.J 1165 106 99 59 0 02 7 2 T O 01 EL 106 5 8 - 0 21 2 0 0 02 7 2 io The experiments listed in 2 4 ,523 104 57 - 0 03 n 024 6 4 0 104 60 Table I11 were performed in2 5 100 3 2 -(I 0.5 0 025 6 2 5 ,?24 100 37 dependently by one of the au105 20 - 0 12 3 0 0 OR 105 3 2 a25 h 7 0 102 23 :i0 102 33 0 07 6 4 5 52fi -0.08 thors to confirm the modified 29 49 on .5 0 0 04 0 02G 7 2 0 49 01 +o 04 procedure. In these experi3n 10 0 46 95 i o 02 0 03 0 05 i 3 0 46 93 ments. Nunroe crucibles were 27 4 8 14 7 0 -0.04 0 06 20 0 0 10 0 48 18 31 47 01 0 33 50.0 7 6 0 25 0 47 18 -0.14 used. Kithin the limitations of a small number of observa" Standard procedure: U H adjiisted t o inethyl orange before phosphate added, p H adjusted with broniophenol blue after phosphate was added. tions, it may be concluded that pH adjusted with phenol red after phosphate was added. the use of a threefold excess of __ ~.. .. __ diammonium hydrogen phosphate leads to results which are slightly Io~r-erthan those obtained using a fivefold excess of reagent, but in either case the v a s w:.ash(d n-ith thv filtrate f1~01nthe precipitate, the lossrs wcre average deviation from the truth is less than 0.1 mg. much smaller. For example, when four Gooch crucibles containTable 11.
Effect of Excess (NI&)~HPOI on Precipitation of ZnNH4PO4
::
-~
ing between 0.2 and 0.5 gram of zinc pyrophosphate were washed with about 100 nil. of 2 S nitric acid, the average loss in weight of the asbestos was 3.3 mg. vihile four similar crucibles washcd with a phosphate solution of thr same composition (obtained by filtering zinc ammonium phosphate prpcipitates on Nunroe crucibl(xs), v-hich had not been in contact with asbestos, lost an average of 0.07 mg. per 100 ml.of phosphate solution. Thus, if the solubility of the asbestos is to be testrd, it should be done with the solution itself rather than hj- dissolving the, precipitate on the asbestos mat. Weighing Form and Temperature of Ignition. Excellent results were obtained when zinc ammonium phosphate was the weighing form (after heating the hexahydrate a t 110' C.). I t vias found also that a constant weight of zinc pyrophosphate was reached promptly Then the precipitate was heated in a muffle furnace with a calibrated pyrometer a t 500" C.; additional heating periods of 1 hour a t 600", 700", and 800" of three samples caused no change in weight as large as 0.1 mg. This procedure for the conversion of zinc ammonium phosphate to the pyrophosphate differs considerably from that currently prescribcd by various authors, some of whom recommend temperatures as high as 900" to 1000" C. ( I O , I S ) but it is in reasonable agreement with the results of de Clerq and Duval (4)who studied the decomposition of zinc precipitates by means of a therniobalance and who found that the conversion of zinc ammonium phosphate to the pyrophosphate begins a t 167" C . ; between 410" and 610" C. the loss in weight was reported as inappreciable. These authors alfio reportcid that a constant weight of zinc ammonium phosphate can be obtained by heating the hexahydrate betveen 50" and 167" C. No published value for the melting point of zinc pyrophosphate could be found, but it was determined to be 880" to 890" C.; actual fusion of the pyrophosphate did not appear to cause a loss in weight, but this point was not checked in detail. As a result of these observations, a modified procedure for the determination of zinc by the phosphate procedure is recommended. To the cold solution, containing no more than 0.2 gram of zinc per 150 mi., and up to 5 grams of ammonium sulfate (but no nitrates), add methyl orange as an indicator and ammonium hydroxide or hydrochloric acid until just acid; the pH will be about 3 to 3.5. Heat the solution nearly to boiling and add 3 to 5 ml. of a freshly prepared 10% solution of diammonium hydrogen phosphate (just pink to phenolphthalein) for each 0.05 gram of zinc to provide a three- to fivefold excess of the reagent over the theoretical requirement; the concentration of phosphate should not exceed 0.1 111. Keep the solution warm until the
S U L F I D E PROCEDURE AIID SEP4RATIOY O F ZINC FROTI SEVERAL ELEMENTS
The sulfide precipitation of zinc is an old, well-known method of greater general applicability than the phosphate precipitation, serving a t once as a means of separation from many elements and as a means of determination. The variations in the procedure are principally in the choice of buffer solution and complexing agent for elements which might interfere. Among the weak acids suggested in recent years for the purpose are citric ( 6 . 11), chloroacetic (I,$), formic ( 7 ) , and the bisulfate ion (12). Probably the most common procedure is that of Fales and Ware ( 7 ) which employs both formic and citric acids with their salts, but for simplicity and general usefulness, the sulfate-bisulfate mixture recommended by Jeffreys and Swift ( 1 2 )appears to be even better. I n Table IV, the two last procedures are compared using zinc
Table 111. Determination of Zinc by Modified Phosphate Procedure (Initial adjustment of pH using methyl orange: final Munroe crucibles used) Wt. Zn (XHal2HPO4 (NHa)sSOa Exlit. Taken, Tinies .Iddcd, Xo, Gram Theory (;rams 0 0993 0 10io; 0 1082 0 1080; 0 1477 0 2057; 0 0438 0 1078: 0 1113 0 1304; 0 I494
volume about I50 ml.: JTt. Z n
Found, Grain n 09920 10i9; 0 1081 0,1079' 0 1476: 0 2055 0 0488; 0 10775 0.1113 0 1305;
0 1444
0 20382
n
3
2054
I.:rror.
lfz, -0 -0 -0
07 0;i
-0
03 09
-0 -0
11 12
+o
02
+o +o
ni 0'1
-0 O i i
no
-0 13
Table IY. Precipitation of Zinc Sulfide from Buffered Solutions Zn Taken, No. llg. 153 77 69 1.58 76 17 1,51 76 12 15.5 ~~. 76 71 1.59 73 03 156 81 06 I60 75 7.5 161 76 08
Esiit.
Gravimetrir Results Zn iound. l-:rror, n,p.
77 38 75 93
7fi 117
111e.
-0
24 03 04 19 - 0 01 -0 06 - 0 03
-0 ;a74 i? -0 81 -0 81 O i i . 5 6'4 76 03
rollntinp Results, Z n in Filtrate. >I&? 0 03 0 03 0 04
