Spectrophotometric determination of arsenic (V) using molybdate, tetraphenylphosphonium chloride and polyvinyl alcohol
1973 Volume 22 Issue 7 Pages 850-856
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1973 Volume 22 Issue 7 Pages 850-856
Molybdoarsenic acid forms precipitate with tetraphenylphosphonium chloride (TPPC) from acid aqueous solution, but when polyvinyl alcohol (PVA) is added to the solution previously, water-soluble complex is formed, which can be used for the spectrophotometric determination of arsenic(V). The calibration curve of arsenic at 400 nm was quite linear over a range of 5×10-64×10-4 M, and the sensitivity was higher than either of the molybdenum yellow or molybdenum yellow extraction method. The recommended procedure for the determination of arsenic was as follows. To a sample solution containing less than 8 μmol of arsenic in a 20 ml volumetric flask, 4 ml of 0.5 M molybdate solution and 3 ml of 10 N nitric acid were added. The whole volume was made up to 14 ml with distilled water, and the solution was kept at 25°C for about 10 min. Then, 5 ml of 5% PVA solution and 1 ml of 0.02 M TPPC solution were added, and the final volume was adjusted exactly to 20 ml with distilled water. The solution was well mixed, and the absorbance (A) was measured at 400 nm within 30 min against the blank. The molar absorptivity of arsenic was 2.20 × 103l mol-1 cm-1. The moleratio of the water-soluble complex was found to be 1 : 1 (TPPC: molybdoarsenic acid) by means of the continuous variation method. The influence of diverse ions on the determination was examined, and it was found that iron(III), nickel(II), titanium(IV) and silicate ions should not be present. Phosphate ion also interfered seriously, but simultaneous determination of the two elements was successfully performed by the following procedure. The absorbance (B) corresponding to the amount of phosphorus in the presence of arsenic was measured at 420 nm according to the same procedure as mentioned above except that PVA and TPPC were not added. The sum of absorbance of arsenic and phosphorus was determined from the absorbance (A). The concentrations of arsenic (CAs) and phosphorus (Cp) were calculated by the following formulas, respectively.
CAs = k1A - k2B
Cp = k3B
where k1= 4.52 × 10-4, k2= 1.69 × 10-3 and k3= 1.62 × 10-3 mol l-1. This method was applied to the synthetic sample solutions containing 1.0 × 10-52.5 × 10-4 M arsenic and phosphorus with good results.
The water-insoluble precipitate described above was extractable into 1, 2-dichloroethane. Result of the continuous variation method indicated that the mole ratio of TPPC to molybdoarsenic acid in the extracted species was 3 : 1.
