March, 19661 CURTHOYS AND SIMPSON 195 Determination of Zinc in Trace-element Superphosphate by A.C. Polarography BY G. CURTHOYS AND J. R. SIMPSON (Newcastle University College, The University of New South Wales, Australia) Zinc has been effectively determined in “trace-element superphosphate” with an a.c. polarographic technique. This technique is both rapid and accurate and compares favourably with the atomic-absorption method. The zinc is maintained in solution by polarographing in an acid electrolyte of M hydrochloric acid a t a pH of less than 1. The method eliminates the time- consuming process of separation from interfering ions. The presence of the hydrogen reduction wave does not materially interfere with the zinc reduction wave as happens in conventional d.c. polarography .THE application of conventional d.c. polarography has been used by Heller et al.,l Walkley2 and Piper3 to determine zinc in soil and plant material. They extracted the zinc with dithizone and evaporated the extract to dryness. The residue was dissolved in a basal solution of ammonium chloride and potassium thiocyanate. The zinc was determined polarographically ; it has also been determined polarographically in biphthalate? ammoniaca15 and fluoride6 electrolytes. The above investigations were carried out in alkaline, or near-alkaline, solutions because of the interference of the hydrogen wave occurring close to the zinc wave. Breyer, Gutman and Hacobian’,* have shown that a.c. polarographic waves 40 mV apart are clearly separable and have, in fact, obtained well defined curves for zinc in 0.1 N hydrochloric acid and 0-5 N hydrochloric acid.METHOD REAGENTS- All the reagents used were analytical-reagent grade chemicals. Nitric acid, concentrated. Hydrochloric acid, 10 N. Suulphuric acid. Zinc sulphate solution-Prepare a standard solution by dissolving 0.2200 g of zinc sulphate heptahydrate in distilled water. Add to the solution 50 ml of 10 N hydrochloric acid and dilute the solution to exactly 500 ml with distilled water (1 ml of the solution = 0.0001 g of zinc). SAMPLES- A.O.A.C. ~pecifications.~ at 90” to 100” C for 4 hours and place them in a sealed bottle. Sample the normal superphosphate and “trace-element superphosphate” according to Crush the samples to pass through a 40-mesh sieve, dry them APPARATUS- Polarograph-A manual a.c.polarograph was used, similar to that described by Breyer, Gutman and HacobianlO with a few minor modifications. The cell consisted of a 100-ml squat beaker into which the test solutions were placed. The dropping-mercury cathode was lowered into the cell to within 15 to 20 mm of the mercury- pool anode. The head of mercury of 100cm gave a drop time of 1 drop per 4.0 second, with a mass (m) = 1.44mg per second. Standard polarograms were carried out at the same time as the experimental work and the temperature was maintained constant to within k0.5” C. The peak currents were measured from the interpolated base-line.196 CCRTHOYS AND SIMPSON: DETERhIINATIOS OF ZINC Ih’ [A?UdySt, vO1. 91 Zinc in the trace-element superphosphate samples was determined with an atomic- absorption spectrophotometer similar to that described by Box and Walshll and containing a hollow zinc cathode emitting a resonance line at 213-8 mp.PROCEDURE- Prepare five standard solutions by weighing 2.000 g of the normal superphosphate into a 250-ml beaker. Add to the solid 5ml of concentrated nitric acid, 5ml of concentrated hydrochloric acid and evaporate the solution to dryness. Dissolve the residue in 10 ml of 10 s hydrochloric acid and 70 ml of hot distilled water. Boil the solution, then filter it through a Whatman No. 41 filter-paper, and wash the residue with 6 small washings of hot distilled water . Cool the filtrate, dilute it to exactly 100 ml in a calibrated flask and mix it well. Transfer by pipette a 20-ml aliquot into a 500-ml calibrated flask with 48 ml of 10 N hydrochloric acid, dilute the solution to exactly 500 ml with distilled water and mix it well.Transfer by pipette a 50-ml aliquot of this solution into each of five 100-ml squat beakers. Introduce 1, 2, 3, 5 and 6ml of the standard zinc solution to the respective beakers and mix the contents well. Polarograph the solutions between -0.90 and -1.34 volt with a mercury-pool anode and an a.c. potential of 2.87 mV r.m.s. Transfer a 50-ml aliquot of the above solution to a 100-ml squat beaker and polarograph it as a blank determination on the normal superphosphate and the reagents used. Place duplicate 2-g samples of the “zinc-trace superphosphate” into 250-ml beakers and evaporate them to dryness with 5 ml of concentrated nitric acid and 5 ml of concentrated hydrochloric acid.Dissolve the residues in 10 ml of concentrated hydrochloric acid and 70 ml of hot distilled water. Boil the solutions and filter them through \$’hatman No. 41 filter-paper, washing with 6 small washings of hot distilled water. Cool the filtrates, dilute to exactly 100ml in a calibrated flask and mix them well. Transfer by pipette a 20-ml aliquot into a 500-ml calibrated flask together with 48ml of 10 N hydrochloric acid and dilute the solution to exactly 500 ml with distilled water. Transfer a portion of each to separate 100-ml squat beakers and polarograph between -0.90 and - 1.34 volt with a mercury-pool anode under the, same cell conditions as used for the standard. Draw a calibration curve from the standard polarogram for the zinc, and determine the zinc in the trace-element superphosphate.RESULTS Although the influence of the hydrogen ion reduction on the base-line is evident, it does not interfere with the zinc wave or the determination of its peak height. Zinc gave fairly well defined peaks in hi hydrochloric acid with a half-wave potential at -1.02 volt with a mercury-pool anode. No interference resulted from the presence of phosphate or iron. The polarograms for some of the additions of standard zinc solution to the normal superphosphate are given in Fig. 1 (a), ( b ) and (c), and show a corresponding increase in peak height with concentration. The blank determination, represented by Fig. 1 ( d ) , consisted of the reagents and the normal superphosphate and gave no wave for zinc.From the peak heights at different concentrations of zinc, as shown in Table I, the cali- bration graph was drawn. The calibration graph gave a straight line passing through the origin over the range 0 to 10.7 x 10-3g of zinc per litre. TABLE I P E A K HEIGHTS AT DIFFEREXT ZINC CONCESTRATIONS Concentration of zinc, Current, g per litre PA 1.96 x 10-3 3 3.85 x 10-3 5.5 5.66 x 8 9-09 x 10-3 13 10.7 x 10-3 15-5 The polarograms of duplicate samples for zinc in trace-element superphosphate carried out under the same cell conditions as the standards gave similar, well defined peaks. WithMarch, 19661 TRACE-ELEMENT SUPERPHOSPHATE BY A.C. POLAROGRAPHY I I -1.0 -1.2 1 I -1-0 -1.2 L 197 -1.0 -1.2 -1.0 - I .2 Potentia1,Volts Fig. 1 . Polarogram of zinc standards in “normal” superphosphate at 21.1” C.Base electrolyte M hydrochloric acid, a.c. potential 2.87 mV r.m.s. (a) 1-96 x g of zinc per litre; ( b ) 56.6 x g of zinc per litre; ( 6 ) 10.7 x g of zinc per litre; ( d ) “normal” superphosphate (blank) high zinc content, increased dilution was necessary. The peak heights were measured and the percentage of zinc was determined from the calibration curve. The concentration of zinc in the samples was also determined with an atomic-absorption spectrophotometer, the comparison of the results being shown in Table 11. TABLE I1 COMPARISON OF RESULTS Acid digestion, Atomic absorption, Sample per cent. of zinc per cent. of zinc Zinc - superphosphate . . 1.25 1.26 1-21 1-26 1.23 1.24 1.31 1.31 1.33 1.28 1.28 1-30 Mean = 1.24 Mean = 1.30 Standard deviation 0.015 Standard deviation 0.017 Zinc may be determined by a.c. polarography in superphosphate containing other trace elements, such as copper, by using a procedure identical with that described above.198 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. [Analyst, VOl. 91 CURTHOYS AND SIMPSON REFERENCES Heller, K., Kuhla, G., and Machek, F., Mikrochemie, 1937, 23, 78. Walkley, A., Aust. J . Ex$. Biol. Med. Sci., 1942, 20, 139. Piper, C. S., “Soil and Plant Analysis, Waite Agricultural Research Institute, South Australia, Jones, G. B., Analytica Ckim. Acta, 1954, 11, 88. Menzel, R. G., Jackson, M. L., Analyt. Chem., 1951, 23, 1861. Eve, D. J., Verdier, E. G., Ibid., 1956, 28, 537. Breyer, B., Gutman, F., and Hacobian, S., Aust. J . Scient. Res., 1950, 3, 559, --- , Ibid., 1950, 3, 567. “Offikial Methods of Analysis,” Ninth Edition, Association of Official Agricultural Chemists, Breyer, B., Gutman, F., and Hacobian, S., Aust. J . Chem., 1953, 6, 188. Box, G. F., Walsh, A., Spectrochim. A d a , 1960, 16, 255. 1950, p. 350. 1960, p. 6. Received November 20th. 1964