Evaluation of Methods for Trace Zinc Determination D. W. MARGERUM and FRANCISCO SANTACANA' Department of Chemistry, Purdue University, Lafayeffe, Ind.
b Methods for determination of microgram quantities of zinc are experimentally evaluated. The preferred method uses bis(2-hydroxyethy1)dithiocarbamate (diethanoldithiocarbamate) masking with dithizone extraction and a single color method; 30 pg. of zinc are determined with an accuracy of 99.0 =t 1.8% (for duplicate samples with 95% confidence limits) in the presence of a tenfold excess of each of the following: cadmium, cobalt, copper, mercury, nickel, lead, iron, manganese, chromium, and tin. Because zinc is a common contaminator of reagents, the true efficiency of zinc separation procedures is proved with zinc-65. Radiozinc also elucidates the difference in behavior of diethyldithiocarbamate and diethanoldithiocarbamate as masking agents in dithizone extractions. The use of Zincon for zinc determinations is not recommended unless a limited number of impurities are present.
T
compares and evaluates methods for the determination of microgram quantities of zinc in the presence of an excess of a Ride range of metallic impurities. The chemical behavior of these methods is elucidated where helpful. The methods considered were limited to those adaptable to the determination of 1 to 100 pg. of Zn in solution with concentrations as low as 0.1 p.p.m. and to those with good accuracy (relative error less than 10%). Determinations without separations did not meet these limitations and the only suitable separations were dithizone extractions in the presence of various masking agents, anion e l change resin elution with hydrocliloric acid, and certain sulfide precipitations. Since Zn is a very common contaminator of reagents and of water, it is not knomn whether many separation procedures are truly efficient or if they merely give reproducible distribution of zinc in the various steps. For this reason several separation procedures have been tested with radiozinc to determine the total loss of Zn in a determination and to locate operations which might be particularly susceptible to error due t o incomplete separation. HIS WORK
1
Present address, American Cyanamid
Co., Bound Brook. ?;. J.
Once zinc ion has been isolated by the dithizone separation procedure, it can be determined by the titration of Zn with dithizone, by the titration of zinc dithizonate with bromine, by the spectrophotometric mixed color method, or by the spectrophotometric single color method. The titrimetric methods were greatly inferior to the two spectrophotometric methods. When Zn is isolated by the use of an anion exchange column i t can be determined with dithizone or with a water-soluble color reagent such as Zincon. From the numerous published procedures eight were chosen as methods which have significant alteration in procedure and might meet the requirements specified. Table I lists the methods tested and indicates the main characteristic of each procedure which distinguishes it from the others. Those not specified as titrimetric are concluded by spectrophotometric means. One or more of the masking anions such as citrate, thiosulfate, acetate, and cyanide are used for most of the dithizone procedures. The procedures given in the references were followed closely except where specified.
those of each method were followed strictly. Four analyses run in parallel included a blank, a pure Zn sample, a Zn sample plus a n equal mole ratio of each impurity, and a Zn sample plus a tenfold excess of each impurity. The average analysis times are given for four samples and include preparation time.
EXPERIMENTAL
Spectrophotometric measurements were taken on a Beckman DU with readings between 0.15 and 0.60 absorbance unit. A photomultiplier attachment wits used below 600 mp and the slit widths were between 0.05 and 0.1 mm. Radiochemical measurements were taken with a scintillation crystal, so that the counting was done directly in the H20 or CC4 phase. Carrier zinc, 30 pg. in 10 ml. containing Zn65 with an activity of 0.1 pc., was used. The initial Zn65 solution u'as purified by Method 7 , then carrier added, and diluted. Carbon tetrachloride and aqueous solutions n'ere counted in small test tubes. For the niajor fraction, 100,000 counts (about 10 minutes) were taken. The standard ZnG solution was counted before and after the sample to give a 1 0 0 ~ reference. o Results are the average of two to three determinations.
All the usual precautions recommended in trace analysis work plus
Each value reported for the average Zn found and the per cent standard
Table I.
Method
Methods Tested
6
Dithizone Separations Citrate, thiosulfate, acetate Titrimetric with Bra Diethyldithiocarbamate Di-2-naphthylthiocarbasone used iiistead of dithizone H&, dimethylglyoxime, e k . Titrimetric with dithizone
8
Anion exrhange resin separation Zincon
1 2
3 4 5
7
Reference
Diethanoldithiocarbamate
(80)
(25)
Table II. Method 1 Citrate, thiosulfate, acetate uith dithizone Impurities. Cd, CO, Cu, Hg, Xi, Pb, Fe, Mn, K, S a , Ca, RIg, Al, Cr, S n .4verage analysis time 65 minutes Ratio of Accuracy, Zn Impurities 70 Relative Std. Dev., Present, pg to Zn .4v. Zn Found Error 70 31 8 0 Standardization 1 6 31.8 1 33.0 +3 8 3 6 64 0 Standardization 2 9 6 4 1 6.5 +I 6 3 0 6 4 10 6 6 +3 1 5 7
VOL. 32, NO. 9, AUGUST 1960
e
1157
Table 111.
Distribution of Method 1
of that initially present. The thiosulfate concentration is critical in preventing contamination from Cd, but cannot be increased without serious loss of Zn. d standard method used in water, sen-age, and industrial wastes analysis for “zinc determination in the absence of gross contamination” (19, 26), is a variation of Nethod 1. Because the first separation steps are omitted and the p H is raised, concentrations in parts per million of Cu, Cd, Co, Xi, Bi, and Fe as well as any oxidizing agents interfere. Therefore, this variation of Method 1 is not recommended. Iron(II1) is reported t o oxidize dithizone in this procedure, but if thiosulfate is absent, Fe(II1) in a p H 5 acetate buffer does not react with dithizone. This suggests that the Fe(II1) interference results from its slow reduction to Fe(I1) by the thiosulfate ion. On the other hand, in Method 1 the Fe(II1) citrate complex at p H 8.5 does cause oxidation of dithizone, but the oxidation product is separated when the Zn is stripped into HC1. Method 2 is complicated partly because it is designed for the quantitative determination of Cu and Co, as well as Zn. Cadmium and P b interfere and cannot be present. The separations are tedious and the final determination by oxidation of the metal dithizonates to diphenylthiocarbazone gives a poor end point, The results were fair, with an average recovery of 89% and an 11% standard deviation. With these drawbacks the method was not considered further. Method 3 uses diethyldithiocarbamate as a masking agent with dithizone extraction. Table IT7 shows the results of studies of the distribution of Zn65 between an aqueous and a CCl, phase when dithizone extraction from diethyldithiocarbamate is performed at p H 8.8. These results are compared to the per cent zinc dithizonate actually formed as measured spectrophotometrically. Both the carbamate and dithizone complexes of Zn are extracted into CCla. Some of the other metal carbamates are also extracted into CCl,, so this method gives a poor separation of Zn. The amount of zinc dithizonate
in
Zn Recovered,
70
Fraction Aqueous solution after 1st dithizone extraction CCl, after HC1 stripping Aqueous solution aft,er 2nd dithizone extraction from
0.2 0.1 1.0 97.5
SZOS - 2
Final Zn in CC14
(
)
X 100 were deterdeviation n* mined from three to six runs. The accuracy or per cent relative error is reported as positive if more Zn was found than in the standardization. RESULTS
Method 1 gives good results (Table 11). Spectrophotometric measurements were taken a t 530 mp because this wave length, which measures the amount of zinc dithizonate formed, gave better results by a factor of 2 to 3 than 620 mp, which measures the decrease in dithizone absorbance. Table I11 summarizes the results of Zn66 distribution determinations for Method 1. These results show a small but consistent loss of Zn to the thiosulfate complexing solution in the second extraction. Although results are slightly high in the presence of moderate amounts of impurities, the actual Zn recovered is only about 97 to 98%
Table IV. Distribution of ZnG5 between Extraction Phases Using Diethyldithiocarbamate as in Method 3 (30 pg. of Zn present)
Zn Diethyl- Dithizodithionate carbamate, Formed,
Zns Distribution,
70
blg.
70
CClr phase
0 2.5 12.5 25.0
100.0 96.6 90.8 76.6
101.5 101.9 101.0 101.5
HzO phase 0.2 0.3 0.1 0.4
Table V. Method 3 Diethyldithiocarbamate with dithizone Impurities. Co, Cu, Hg, Xi, Pb, Fe, Mn, Ca, Mg, All Cr, Sn Average analysis time. 55 minutes Ratio of Accuracy, Zn Impurities Av. Zn yo Relative Std. Dev., Present, pg. to Zn Found Error % 32.7 0 Standardization 2.9
32.7 6.5 6.5
1 158
1 0 10
ANALYTICAL CHEMISTRY
38.4
+ 8.3
10.8
+66.0
Standardization
5.4 5.1 29.6
formed will depend on the extent of the reaction between the impurities and diethyldithiocarbamate. Table V gives the experimental evaluation of Method 3. Cadmium interferes so seriously that it cannot be present. Despite its absence, other impurities can cause very high results. These difficulties seriously handicap Method 3 as a general method. Method 4 uses the naphthyl instead of the phenyl derivative of thiocarbazone to give a reagent similar to dithizone. Combining this reagent with diethyldithiocarbamate masking is reported to give a procedure almost selective for Zn (Table VI). Cadmium cannot be present and was not included as one of the impurities. Results were low when a large excess of impurities were present despite the presence of d i - 2 naphthylthiocarbazone in excess. This method is inferior to dithizone extraction separations. Method 5, the official AOAC method (15), gives good results Kith equal and tenfold ratios of impurities to Zn. The impurities include all the common interferences in Zn determinations. However, it is extremely time-consuming, involving many extractions and precipitations, and does not have special advantages. Since it has already been subjected to a critical collaborative study (SI), it was not considered further experimentally. Results of duplicate analyses from seven different laboratories were:
Zn,
Pb, Ni, Cd, Co, Bi Each,
pg.
pg.
2.0 15.0
80 80
70 Relative Error
(Combined Results)
Std. Dev.,
+4.0
14.9 8.7
0.0
%
The accuracies of the individual labcratory runs were not this good, but are comparable to Method 1. Good results are reported for microgram Zn determination with 0.2 mg. of P b and Cd, so it appears that the Cd interference is not as great as in Nethod 1. Method 6 uses the same type of separation as Method l except that cyanide is added and a higher p H is used. The higher p H causes Cd to interfere. The cyanide helps reduce the over-all free metal ion concentrations but aside from partially masking Co and Ni it does not serve as a selective masking agent. The titration of Zn with dithizone has a very erratic end point and the results were unsatisfactory. Method 7 uses the hydroxy derivative of diethyldithiocarbamate-namely, bis(2 - hydroxyethy1)dithiocarbamate (also called diethanoldithiocarbamate) - as a masking agent proposed by Serfass and Levine (29). This reagent is prepared by mixing diethanolamine with CSz. The results using
this reagent and dithizone extraction are very good. Table VI1 shows the ZnB5distribution between phases compared to the per cent zinc dithixonate formed. The Zn is in the form of t h e dithizonate complex and not present as mixed complexes as is the case with diethyldithiocarbamate. The experimental evaluation of Method 7 is given in Table VIII, which indicates its high accuracy and precision. The radiochemical test of the degree of separation is given in Table IX and indicates very little loss of Zn65 in the separation procedures of Method 7 . Diethanoldithiocarbamate is an excellent masking agent for Zn determination despite the fact that it forms precipitates with a number of metal ions. These precipitates are highly crystalline and do not interfere in the separation of the extraction phases. The addition of cyanide and citrate in this method prevents the precipitation of heavy metal hydroxides such as Fe(II1) and does not hold back a n y zinc. These reagents do not serve as selective masking agents but only reduce the concentration of metal ions. The Fe(II1) complexes will still oxidize dithizone (26) and the oxidation product is prevented from interfering b y stripping it with the excess dithizone. The use of a stripping procedure has been criticized (26) as causing loss of the Zn complex by dissociation. However, from the distribution of radioactive Zn in Table IX the loss of Zn due to stripping by NazS solution is negligible, and the single color method works satisfactorily. Even with 3 fig. of Zn and a 100-fold excess of each of 14 metal ions, the results are good (Table VIII). Method 8 utilizes the anion exchangehydrochloric acid separation procedure of Kraus and Moore (60)as adapted by Rush and Yoe (65). This method was first tested with only Co, Cu, Xi, Fe, and M n present as impurities (Table X). The high per cent standard deviation with no impurities present undoubtedly arises from column contamination, since these samples mere alternated with the separations where metallic impurities were present. Five samples of pure Zn solution with Zincon gave only 0.8% standard deviation. Table XI gives the results of the distribution of Zne5 in Method 8 and indicates that the tailing tendencies of Zn would predict slightly low Zn recoveries. Bccause of other interferences, the Zn determinations are high in some runs and much lower in others than the ZnG5distribution would predict. Zinc separation on anion exchange resins is particularly favorable with hydrochloric acid solutions, since it is one of the last metal ions to be eluted
Table VI. Method 4 Di-2-naphthylthiocarbazone extraction used with diethyldithiocarbamate masking Impurities. Co, Cu, Hg, Ni, Pb, Fe, Mn, Ca, Mg, Al, Cr, Sn
Average analysis time. 66 minutes Ratio of Accuracy, Zn Impurities Av. Zn % Relative Present, pg. to Zn Found Error 32.6 0 Standardization 32.6 32.6 16.3 16.3 16.3 6.5 6.5 6.5
1 10
0 1 10 0 1 10
38 0
Std. Dev.,
%
1.5 ...
+, l e 6
27 .n -17 -. Standardization-'
19.9 11.9
...
3
3.8 6.5 8.4 3.1 . .
'-
f22.0 -27.0
Standardization
8.7 7.0
as the acid concentration is reduced and, therefore, has relatively few interferences. Rush and Yoe (25) attempted t o reduce the many hours of slow elution ordinarily required by using a flow rate of 2 ml. per minute which corresponds to 10 em. per minute and is about ten times as fast as that recommended by Kraus. Even with this increased flow rate Method 8 is much more time-consuming than Methods 1, 3, 4, and 7 . The radiochemical data in Table XI indicate that the Zn recovery in Method 8 should be better than that actually observed in the determinations finished with Zincon.
f34
...
f 8
Table VII. Distribution of between Extraction Phases Using Diethanoldithiocarbamate as in Method 7
Diethanoldithiocarbamate Zn (Aspuming Dithizo100% nate Yield), Formed,
Zn Distribution,
Rlg.
7c
CC1, phase
0.0 2.0 50.0 200.0
100 100 102 105
101 103 103 102
70
_-
H?O phase 0.5 0 3 0.1 0.2
Table VIII. Method 7 Diethanoldithiocarbamate with dithizone
Impurities. Cd, Co, Cu, Hg, Xi, Pb, Fe, Mn, Cr, Sn, Na, K, Ca, and hlg Average analysis time. 63 minutes Ratio of Accuracy, Std. Dev., Zn Impurities Av. Zn yo Relative Present, pg. to Zn Found Error %' 16.4 0 Standardization 2.4 16.4 16.4 32.7 32.7 32.7 3.3 3.3
1 10 0 1 10 0
100
16.0 16.7
-2.5 +1.8
32.4 32.1
-0.9 -1.8
Standardization
Standardization 3.5 +6
This seems to be due to Fe(III), which tends to tail in its column elution. This tendency is aggravated by the fast flow rate. Iron(II1) can cause slight positive or strongly negative results in the determination of zinc with Zincon, depending on the ratio of Fe and Zn t o Zincon. This accounts in part for the high per cent standard deviation in the standardization values in Table X. Rush and Yoe have considered only Co, Cu, Ni, Fe, and M n as contaminants, although the work of Kraus and Nelson (21) would predict
Table IX.
3.4 1.1 1.9 1.1 1.8 6.1 10.3
Distribution of Method 7
in
Zn Recovered, Fraction CCla after extraction of methyl red Aqueous phase after dithizone extraction Aqueous Na2S phase after stripping of excess dithizone Final CClr phase with zinc dithizonate
VOL. 32, NO. 9, AUGUST 1960
%
0.17 0.40 0.15 99.0
1159
Table X.
Anion exchange-Zincon Impurities. Co, Cu, Ni, Fe, and Mn Average analysis time. 180 minutes Ratio of hccuracy, Impurities Av. Zn yoRelative to Zn Found Error 0 Standardization 1 60.7 -4.6 10 62.7 -1.4
Zn Present, pg. 63.6 63.6 63.6
possible separation from other contaniinants of considerable interest in Zn determinations such as Hg, Pb, Sn, Cr, and perhaps even Cd. The authors tested these impurities as a group and then individually using the procedure of Method 8. Results were highly erratic even with a flow rate of half that used in Method 8. Some of these interferences could undoubtedly be diminished by using a much slower flow rate, but other interferences such as that caused by Hg(I1) and Sn(1V) are not a p t t o be greatly affected by the flow rate. Unless some hours are allowed for equilibrium conditions, the anion exchange separation method will not be generally applicable. Even then Cd will interfere unless a different solvent system is used such as the use of CH30H-HzO-HC1 elution recently reported (6).
Table XI.
Distribution of Znss in Method 8 Zn
Recovered,
Fraction From HCl complex solution placed on column 50 ml. 1M HC1 wash Zn fraction collected 1st. 30 ml. 0.01M HC1 2nd. 30 ml. 0.01M HCI Extra 30 ml. 0.OlM HC1 in wash
EFFICIENCY
OF
%
0.1 0.2
91.0 7 .O 0.7
THE ZINC SEPARATIONS
A11 four separation procedures tested with radiozinc showed that 97 to 100% of the Zn is retained in the final Zn fraction and that these separations are truly efficient and not merely reproducible distributions with sizable loss of Zn. However, in Method 1 there is a slight loss of Zn to the aqueous thiosulfate solution. Method 3 has good Zn retention in the CC1, phase but the Zn is present partly as zinc dithizonate and partly as zinc diethyldithiocarbamate. Method 8 has a slight loss of Zn on the anion exchange column due to the rapid flow rate. Method 7 has the smallest loss of Zn with a retention of 99% out of a 3O-pg. sample. 1160
Method 8
ANALYTICAL CHEMISTRY
Std. Dev.,
%
5.9 3.4 6.5
DIETHYLDITHIOCARBAMATE AND DIETHANOLDITHIOCARBAMATE
The data in Tables IV and VI1 show why diethyldithiocarbamate has not been satisfactory as a masking agent while the hydroxy derivative is very satisfactory. The zinc diethyldithiocarbamate complex is extracted into CCla and it prevents the complete formation of zinc dithizonate. On the other hand, the more polar hydroxy derivative does not form a carbon tetrachloride-extractable Zn complex. I n the same manner the Cd and Cu complexes of diethanoldithiocarbamate are not extracted and the hydroxy derivative effectively masks these ions while diethyldithiocarbamate does not. DlTHlZONE SEPARATION OF Pb, Cd, A N D Zn IN METHODS 1 A N D 6
Many procedures have used a dithizone extraction from an acetate-acetic acid buffered, thiosulfate solution to separate Zn from Cd and Pb. Although the acetate has been added as a buffer, it is in reality an important masking agent for Pb. The behavior of the dithizone-thiosulfate-acetate system as a function of p H is calculated from the dithizone instability constants (13, 12, 16, 24, 28) and from the constants available for the thiosulfate and acetate complexes (7, 8). Under the conditions of Method 1 (pH 4.1), P b exists primarily as Pb(OAc)$- and Pb(OAc)2and not as the thiosulfate complex. Cadmium is masked primarily as CdSZO3 and Cd(S203)e-2. The small amount of Zn retained in the aqueous phase is half ZnSz03 and half ZnOAcf. The effect of p H on the extent of extraction of each metal is different. Since the Pb(OAc)a- complex is important, increasing the p H favors its formation over the formation of Pb(Dz)z. Experimentally, even large amounts of P b do not react with dithizone from an acetate solution a t p H 5. On the other hand, the extraction of Cd increases with p H because thiosulfate complexation is not pH-dependent while the formation of cadmium dithizonate depends upon [H+]-z. While 0.6% of the Cd extracts at p H 4.1, 6% would be extracted a t p H 4.5. For this reason the use of a
higher p H such as in Method 6 causes Cd interference. The effect of p H on the Zn loss is not critical but small losses can be avoided if the p H is kept closer to 4.5 than to 4.0. This, of course, is not advisable in the presence of Cd. Another important effect of p H on the estraction is the time required to establish equilibrium. Whereas the Zn reaction with dithizone is complete in a few minutes at p H 4.9, the reaction can take as long as 30 minutes to reach equilibrium at p H 3.88 (18). The extent of interference from a masked ion such as Cd probably depends on the extraction time as well as the pH. LIMITATIONS
OF
ZINCON
Numerous published papers recommend Zincon for Zn determination; however, it reacts with too many other metals under the same conditions used to form the Zn complex. Zincon has been reported as giving blue complexes with Co(II), Ni(II), and Cu(I1). I n addition, the authors find that it forms blue complexes with Cd(II), Hg(II), Pb(II), and Fe(III), and also reacts with Cr(II1) and Sn(1V). As a result, if Sn(1V) and Cr(II1) are present in sufficiwt amounts t o consume the excess Zincon, low results are obtained in Zn determination. Since the molar absorptivity of the Fe(II1) complex is very small at 620 mp, its presence can lead to positive or negative results depending on its concentration and the ratio of Fe and Zn to Zincon. The molar absorptivities of the Zincon complexes of Pb, Cd, and H g are on the order of to that of the Zn complex. Therefore, the presence of these ions can also lead to positive or negative results. The use of Zincon must be limited to situations where very few impurities are present or where other ions are suitably masked. dlthough dithizone also reacts with many metals, it simultaneously provides a means of separation with a high p H dependence. Zincon does not. Zincon has been useful in animal and plant analyses where many impurities are known to be absent, but it should be used with caution. ROLE OF SULFUR-COORDINATING LIGANDS
The use of S coordination is very important in the extraction procedures for Zn determination. The so-called “natural order” (5) for the stability of divalent metal ion complexes with 0- and N-containing ligands is: H g > Cu > S i > Co > P b > Zn > Cd > Fe > >In. Dithizone is now believed (17, 233) to coordinate through one S and one N atom. For dithizone the stability order is changed somewhat with Xi(I1) and Co(I1) forming weaker
complexes, and Zn(I1) becoming more stable than Pb(I1) (I-S). With dithizone: Hg > Cu > Zn > Cd > Pb > Co > Ni Thiosulfate is one of the ligands used with dithizone to separate Zn from the other metals in this series. The thiosulfate complexes of Hg(I1) and Cu(I1) are sufficiently strong to prevent dithizone extraction of these metals. The stabilities of the thiosulfate complexes decrease very rapidly in a n order which reverses the positions found for Zn, Cd, and P b with dithizone (8). K i t h thiosulfate: Hg > Cu > Cd > Pb > Zn Co Si. log & 29.8, 12.3, 6.0, 4.2, 2.4 This allows thiosulfate to be used effectively to mask small quantities of Cd and P b without holding back Zn. Hen-ever, Cu does react with dithizone in the presence of thiosulfate and Co interferes more than Cd (98). The suggestion that thiosulfrtte compleiing occurs through 0 coordination (4) is not consistent with the order of stability constants for 0 ligands or with the order of stability of sulfate complexes. The thiocarbamate derivatives are also effective in Zn separations because of their stronger complexation with Cd(I1) and Pb(I1) than with Zn(I1). The order of stability of these complexes containing S coordinating ligands parallels the order of solubility of the metallic: sulfides where the pK,, values decrease in the order H g > Cu > Cd > P b > Zn S Co e Xi (8). The thioglycolate complexes also form Zn, Cd, and P b complexes which are stronger than the corresponding Ni and Co complexes (7,22). Sulfur coordination shifts the order of stability of complexes, increasing the relative stability of the Zn, Cd, and P b complexes a t the expense of the Co and Ni complexes. Sulfur coordination particularly favors the relative stability of the Cd complexes when compared to 0 or N coordination. Dithizone is intermediate in this effect, which might be expected, since it apparently chelates through both N and S atoms. COMPARISON OF METHODS
Method 7 is recommended as the best general procedure for the deter-
mination of microgram quantities of Zn. If duplicate samples containing 30 pg. of Zn are run, the results will be within 99.0 i 1.8% of the actual Zn present for a 95% confidence limit. These amazingly good results are from 20 runs where ten commonly interfering metals are each present in equal or in tenfold excess compared to Zn. There is a relatively small loss in accuracy for the determination of 3 pg. of Zn with 100-fold excess of each impurity. The fact that Zn is transferred only once between phases helps the reproducibility and accuracy of the method. Since Method 7 is relatively fast and is suitable in the presence of any common metal, there are very few cases where other existing methods should be substituted for it. The AOAC procedure for the sample preparation has proved satisfactory ( S I ) but it is recommended that the determination be concluded with Method 7 rather than Method 5. Method 5 might be used when greater than 1000-fold excesses of interfering metals are present. Under these circumstances it becomes inconvenient to attempt t o mask the impurities or to use large quantities of dithizone for extractions and the sulfide precipitation separation is more feasible. Method 1 is satisfactory, but does suffer from slight interferences, notably Cd. The separation part of Method 8 would be useful for obtaining micrograms of Zn from milligram quantities of Co, Cu, Ki, and Mn. However, the determination is best concluded with a dithizone extraction similar to Method 7 t o ensure no interference from other metals. Methods 2, 3, 4, and 6 were unsatisfactory in one respect or another as noted in the results. ACKNOWLEDGMENT
The authors are indebted to J. W. Cobble for the use of the radiochemical equipment and to B. A. Zabin for his aid in testing Method 8. LITERATURE CITED
(1) Babko, A. K., Pilipenko, A. T., J . Anal. Chem. U.S.S.R. 1 , 175 (1946). (2) Zbid., 2, 33 (1947). (3) Zbid., 5, 14 (1950). (4) Bailar, J. C., “The Chemistry of
Coordination Compounds,” p. 58, Reinhold, New York, 1956. (5) Basolo, F., Pearson, R. G., “Mechanisms of Inorganic Reactions,” LViley, New York, 19%. (6) Berg, E. W., Truemper, J. T., ANAL. CHEM.30,1827 (1958). ( 7 ) Bjerrum, T., Schwarzenbach, G., Sillen, L. G., “Stability ,,ConstaQts, Part I : Organic Ligands, Chemlcal Society, London, 1957. (8) Ibid., “Part 11: Inorganic Ligands,” 1958. (9) Butts, P. G., Gahler, H., ?*fellon, M. G., Metal Finishing 494,50 (1950). (10) Cholak, J., Hubbard, D. M., Burkey, R. E.. IND.ENG. CHEhl., AXAL. ED. 15, 754 (1943). (11) Cowling, H., Miller, E. J., Zbid., 13,145 (1941). (12) Geiger, R. W., Sandell, E. B., Anal. Chim. Acta 8,197 (1953). (13) Hibbard, P. I., ISD. ENG. CHEM., ANAL.ED. 10, 615 (1938). (14) Holland, E. B., Ritchie, W. S., J . Assoc. O&. Aqr. Chemists 22, 333 (1939). (15) Horjtz,: W,, Ed., “Official Methods of Analysis, An~oc.Offic. Agr. Chemists, Washington, D . C., 1955. (16) Irving. H.. Bell, C. F., J . Chem. SOC. 1952, 92y6. (17) Ibid., 1954, 4253. (18) Irving, H., Bell, C. F., JVilliams, R. J. P., Zbid., 1952, 357. (19) Kline, E. K., et al., eds., “Standard
Methods for the Examination of Water, Sewage, and Industrial Wastes,” Ani. Public Health Assoc., Kew York, 1055. (20) Kraus, K. A., Moore, G. E., J . A m . C h m . SOC.75, 1460 (1953). (21) Kraus, K. A,, Nelson, F., “Symposium on Ion Exchange and Chromatography in Analytical Chemistry,” Am. Soe. Testing Materials, Philadelphia, Pa.! 1958. (22) Leussing, D. I,., J . A m . Chem. SOC. 80,4180 (1958). (23) Morrison, G. H., Freiser, H., “Sol-
vent Extraction in Analytical Chemistry,” Wiley, New York, 1957. (24) Pilipenko, A. T., J . Anal. Chent. U.S.S.R. 8, 317 (1953); Zhur. Anal.
Khim. 8,286 (1953). (25) Rush, R. M., Yoe, J. H., ANAL. CHEW26,1345 (1954). (26) Sandell, E. B., “Colorimetric Dctermination of Traces of Metals,” Interscience, New York, 1950. (27) Sandell, E. B., IND.ENG. CHEM., ANAL.ED. 9, 464 (1937). (28) Sandell, E. B., J . Am. Chem. SOC. 72,4660 (1950). (29) Serfass, E. S., Levine, W.S., ChemistAnalyst 36,55 (1947). (30) Serfass, E. S., Levine, 1%’.S., Plating 36,818 (1949). (31) Taylor, L. V., Alexander, 0 . R . , J . Assoc. O$C. i l g r . Chemists 28, 271 (1945). (32) Young, R. S., Analyst 73, 413 (1948).
RECEIVEDfor review May 11, 1950. Resubmitted September 14, 1959. Accepted May 9, 1960. Division of Analgtical Chemistry, 135th Meeting, ACS, Boston, Mase., April 1959.
VOL. 32, NO. 9, AUGUST 1960
1161
