Clifford C. Boyd and William K. Easley East Tennessee State College Johnson City
The Separation of Arsenic in Qualitative Analysis
A separation of arsenic as As(II1) has been developed based upon the observation that As(II1) is extracted selectively from hydrochloric acid solutions of the common cations encountered in the usual qualitative analysis scheme. The extraction is effected by a saturated solution of catechol in benzene. T o 6-8 drops of the unknown eontsjned in a amall beaker, add 2 drops of 4 M N&C1 and evaporate to dryness carefully. Extract the residue with 6-8 drops of 6 M HCI, decant if necesasry, and add 1-2 drops of 0.26 M NHJ. Warm the solution on a water bath until there is no further liberation of free iodine. Transfer to a 4 m l test tube, add an equal volume of a saturated solution of oatechol in benzene, md e x t r a t by shaking themixtnre for three minutes. Centrifuge the mixture and separate by withdrawing the benzene layer with s. pipet. Saturate the benzene extract with H2S. A precipitate of greenish yellow A& which tends to adhere to the glass confirms the presence of arsenic.
Experiments have shown that Cu(I1) and Fe(II1) will interfere with the reduction step in the test as described' above. The interference of Cu(I1) can be
384 / journal o f Chemical Education
avoided if 2-3 drops of 0.25 M SnClz are substituted for the NHJ normally used in the test,. Fe(II1) can he eliminated from the unknown by precipitation with base. I t is possible to use SnCL as the reducing agent in the place of NHJ except in the presence of mercury in the unknown. Moreover, the danger of reducing As(Tr) to elementary arsenic exists when SnClais ut.ilized. By this procedure it is possible to detect as little as 20 micrograms of arsenic. This is a much lower concentration of arsenic than is normally encountered by the student. An estimated value of the distribution coefficient for As(II1) between 6 M HC1 and a saturated solution of catechol in benzene is 0.92. This indicates the possible application of the extraction technique to the quantitative estimation of As(II1) by counter-current extraction or merely employing repeated extractions in the absence of counter-current extract.ion a.pparatus.
