Colorimetric Determination of Traces of Zinc in Zinc Oxide Jurgen M. Kruse Itek, Lexington Research Laboratories, Lexington, Mass. THE DETERMINATION of trace amounts of zinc metal and monovalent zinc in zinc oxide is of interest both in the field of semiconductor chemistry and photographic chemistry. In both these areas, even minor variations in the amount of zinc present can have profound effects on the properties of the zinc oxide (1). Because the differentiation between Zn(0) and Zn(1) in zinc oxide is chemically extremely difficult, the sum of these two components is usually determined. In this paper the term zinc is also taken to mean the sum of zinc(0) and zinc(1) in zinc oxide. Although there are several methods now available for measuring trace amounts of zinc in zinc oxide, these methods all require samples of over 1 gram, which in single crystal work may be more than the sample available. In photographic work, sample sizes of >1 gram also will often represent an unacceptably large fraction of the available sample. In addition to this requirement of a large sample, each of the existing methods has its own problems. Thus the method of Norman ( 2 ) proved to be simple and satisfactory as long as sufficient sample ( > l o grams) was available, the zinc content was >10 ppm, and zinc sulfide was absent. This last drawback proved quite serious, as nearly all samples of zinc oxide tested contained traces of zinc sulfide, so that a significant error in the measured amount of zinc was introduced. Another inherent drawback to the procedure of Norman is the fact that the zinc concentration is proportional to the difference of two measured quantities, so that the final measurement is by difference. Another procedure, and one which proved quite satisfactory with sufficient sample, is the method of Sachse and Nichols (3). This procedure has the advantages of employing a direct measurement and being less affected than the method of Norman by traces of zinc suUide, but does require a vacuum system and is not well suited to routine, repetitive analysis. A procedure by Allsop ( 4 ) avoids the interference by zinc sulfide ; however, this procedure requires an elaborate vacuum train, calibration with radiochemical standards, and reportedly is accurate to only 1 3 0 %. For studies in which the total available sample was less than one gram, as was true in our case, none of the above procedures was satisfactory. A more sensitive procedure, preferably with a direct measurement of zinc, was therefore required. Also, because most samples of zinc oxide contain traces of zinc sulfide, any new method should avoid or at least minimize the interference from this impurity. Thus the use of strong oxidants such as Cr(V1) appeared undesirable, as oxidation of sulfide to sulfate can occur. On the other hand, the use of a mild oxidant such as Fe(II1) appeared attractive, especially so because any Fe(I1) formed by the reaction with zinc could be measured colorimetrically with high sensitivity. (1) G. Holland, E. Mollur, and F. Stockmann, in “Solid State Physics,” F. Seitz and D. Burnbull, Ed., Academic Press, N. Y . , 1959, Vol. 8, p 191. (2) V. J. Norman, A m l y s t , 89, 261 (1964). (3) H. B. Sachse and G. L. Nichols, ANALCHEM., 33, 1349 (1961). (4) H. J. Allsop, A/zalyst, 82, 474, (1957).
A study of the factors which might affect a colorimetric procedure based on the reduction of Fe(II1) to Fe(I1) by the zinc in the sample was therefore undertaken. The conditions and reagents required for sample dissolution as well as the required reduction conditions and the effect of possible interference were studied.
EXPERIMENTAL Apparatus. A Cary Model 14 and a Beckman Model DK-I spectrophotometer were used for this work. Reagents. FERRIC PHENANTHROLINE SOLUTION.Dissolve 3 grams of FeNHr(S04)2.12H20and 3.5 grams of 1,lOphenanthroline monohydrate in 100 ml of 10% H2S04 (v/v). AMMONIUM BIFLUORIDE, 5 %. Dissolve 5 grams of NH4HFz in 95 ml of water and store in a plastic container. Procedure. Preparation of standard. Prepare a standard iron solution containing between 100 and 500 mg iron as Fez+ or Fe3+in a 100-ml volumetric flask. The iron standard can be either iron wire or Mohr salt. Add about 100 mg hydroxylamine hydrochloride and about 2 ml of a 0.5% 1,lO-phenanthroline solution. Adjust the pH of this solution to between 2 and 9 with ammonium hydroxide, and dilute to volume. Mix, and measure the absorbance at 510 nm. Calculate the absorbance for 1 ppm of Fe. Sample. Weigh a sample of about 500 mg to 11 mg into an Erlenmeyer flask. The optimum sample size is about 500 mg, but samples as small as 100 mg or as large as 1 gram can be used. Add some dry ice into the flask, and stopper it loosely to let the COz escape. Allow the COn to displace the air. At the same time deaerate the reagent with a little dry ice. Pipet 15 ml of reagent solution into the Erlenmeyer flask. After all the dry ice has sublimed, seat the stopper, swirl the flask to disperse the precipitate, and let the flask stand for 4 hours. Run a reagent blank without any zinc oxide in a similar manner. After the 4 hours, transfer the contents of the Erlenmeyer flask to a 100-ml beaker, and rinse the Erlenmeyer flask into the beaker. Add about 1-2 ml of the 5 % NHdHF2 solution to the blank. Neutralize the solution to a pH between 2 and 4 with 6N NHIOH. Transfer the solution to a 100-ml volumetric flask, and rinse the beaker into the volumetric flask. Dilute to volume with water. Allow the solution to stand 1 hour, and then measure the absorbance us. a water blank at 510 nm. Subtract the absorbance of the reagent blank from the absorbance of the sample, and calculate the concentration of zinc in the zinc oxide. Calculation:
Zn in zinc oxide Net A A of 1 ppm Fe
=
5.852 Wt sample in rng (1)
where 5.852 is the conversion factor for the iron-zinc stoichiometry assuming zinc(0) to be the impurity, i.e., mol wt Zn 2 X mwFe
100 X 100 1000
RESULTS AND DISCUSSION
Dissolution. Initial studies of the dissolution of zinc oxide single crystals and powders with a variety of nonoxidizing acids showed that only dilute sulfuric acid was satisfactory. ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
771
Present, mg Sample ZnO Zn 1 500 0.544 2 500 0.182 3 500 0.159 4 500 0.098 5 500 0.185 6 500 0.425 a Above amount present in ZnO. Four hours reaction time.
Table I. Recovery of Zinc in Zinc Oxide Reagent FeNH4S04.12Hz0, l,lGPhenanthroline,
x
.z
1 2 2 3 3 3
1.2 2.6 2.6 3.7 3.6 0.4
Table 11. Effect of Zinc Sulfide Found MgZn 1 ... ... ... 0.065 ... 2 ... ... ... 0.074 ... 3 500 ... ... 0.493 0.114 500 ... ... 0.468 0.104 4 5 500 ... 0.250 1.42 0.260" 6 500 0.835 ,.. 0.560 0.022" 500 2.80 .. . 0.748 0.074" 7 8 500 3.03 ... 0.898 0.116" a Found above the zinc naturally present in the zinc oxide. Sample
MgZnO MgZnS
MgZn
A,
Hydrochloric acid and organic acids such as acetic acid not only formed highly colored solutions which interfered with the final colorimetric measurement, but also did not react as rapidly with the zinc oxide as the sulfuric acid. A study of the sulfuric acid strength showed that acid concentrations of 5 to >20% were satisfactory; 10% sulfuric acid was therefore used in all further studies. Reagent Concentr'ation. The concentration of ferric ion in the sulfuric acid was found to have little effect on the recovery of added zinc as long as at least 0.25% Fe(II1) (added as FeNH4(S0&.12 H20) was present. At lower ferric ion levels, lower results for zinc recovery were obtained (Table I). However, initial tests with zinc metal added to the zinc oxide also showed that recoveries, Le., the amount of zinc reacting to give ferrous ion, were far from quantitative. These low recoveries could be attributed to overreduction of Fe(II1) to Fe(O), to the competing hydrolysis of zinc, to incomplete reaction of zinc metal, or to the oxidation of zinc or ferrous ion by residual oxygen in the sulfuric acid. The first possibility could readily be eliminated, as no metallic iron was observed even with a large excess of added zinc. Very thorough outgassing of the reagent with CO or Nz prior to addition improved recovery to almost l C O % when zinc metal only was present, but recoveries dropped to less than 70% for small amounts of zinc in the presence of a large excess of zinc oxide. The decrease in recovery was attributed to the incomplete reaction of zinc metal during the dissolution of the zinc oxide, and means of ensuring more complete reaction or more rapid dissolution of the sample and metal were sought. A longer reaction time overcame this problem. At reaction times of 2 hours or less, recoveries were still incomplete, and recoveries of >90 were obtained only with reaction times of 4 hours or longer. An additional problem to be overcome was the interference by the remaining excess ferric ion with the final colorimetric measurement. The addition of NH4HFz provided a satisfactory solution to this problem. Other complexing agents such as EDTA, citrate, etc. proved less satisfactory. The 772
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
Found Zn5 mg 0.292 0.156 0.134 0.099 0.181 0.195
Recovery6
.z
54 86 84 101 98 46
Table 111. Determination of Zinc in Zinc Oxide Wt Zinc Sample sample, found, designation grams PPm Method" 1 1,3648 428 Sachse and 1 2.200g 372 Nichols 24 10.00 22 Norman 24 10.00 20 Norman 24 0.401 30 24 0.409 24 Calcined 24 0.401
