Atomic absorptiometric determination of ppb level of arsenic in water by arsine-argon·hydrogen flame system combined with use of zinc powder tablets, potassium iodide and stannous chloride as reductant

Abstract

An atomic-absorption spectrophotometric method for the rapid, sensitive and precise determination of arsenic with arsine-argon·hydrogen flame system has been developed. Various factors effected on the evolution of arsine from ppb level of arsenic in water are investigated in detail. The arsine could be rapidly evolved from arsenic solution in 2 N hydrochloric or sulfuric acid by using zinc powder tablets together with potassium iodide and stannous chloride solutions as the reductant. The role of potassium iodide and stannous chloride in the reduction process of both arsenic(III) and arsenic(V) was made clear as follows: (1) Both the potassium iodide and stannous chloride enhance to a large extent the recovery of arsenic as arsine, and the same values in atomic-absorption signal can be obtained either from trivalent arsenic or pentavalent arsenic state, (2) only 90 sec gas sampling time is enough to obtain 100% recovery of arsenic as arsine, (3) the optimum acidity can be lowered and the degree of interference of diverse ions can be reduced to a considerable extent.
The sensitivity of the method for 1% absorption was evaluated to be 0.7 ppb of arsenic and the linearlity of the absorption vs. concentration was good up to 1 μg/20 ml (50 ppb). The precision was estimated to be 2.6% from the results of ten arsenic solutions (50 ppb). The maximum permissible amount of diverse ions was as follows; selenium(IV) 7 μg, lead(II) 150 μg, antimony(III) 220 μg and sulfide ion 200 μg. The following ions when present in 5 mg level did not interfere with the determination; Na⁺, K⁺, NH₄⁺, Ca²⁺, Mn²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Hg²⁺, Al³⁺, Cr³⁺, Fe³⁺, Co²⁺, Ni²⁺, Te⁴⁺, Bi³⁺, Cl^-, Br^-, I^-, NO₃^-, NO₂^-, CH₃COO^-, CN^-, SCN^-, MnO₄^-, Cr₂O₇^2-, SO₄^2-, SO₃^2-, MoO₄^2-, WO₄^2-, PO₄^3-, BO₃^3-. Recommended general procedure is as follows: Transfer 20 ml of a sample solution containing not more than 1 μg of arsenic into a reaction bottle. Add 2 ml of 12 N hydrochloric acid, 1 ml of 40% potassium iodide solution and 2 ml of 10% stannous chloride solution. Swirl the solution to mix thoroughly. After adding two piecies of the zinc tablet (about 0.5 g per a piecies), immediately connect the reaction bottle to the collection unit and allow the reaction for 90 sec at room temperature by agitating the mixture with a magnetic stirrer. Turn the four-way stopcock to the “sweep” position in order to introduce argon into the system, sweeping the hydrogen and the arsine into the argon-hydrogen flame. Simultaneously record the absorption signal (1937 Å) on a recorder. Finally return the stopcock to the “bypass” position.
The method was applied to analyses of water samples with satisfactory results.