Platinum arsenites

Geminal dihalides undergo partial or total reduction. The latter can be achieved by catalytic hydrogenation over platinum oxide [512], palladium [512] or Raney nickel [63, 512], Both partial and total reduction can be accomplished with lithium aluminum hydride [513], with sodium bis(2-meth-oxyethoxy)aluminum hydride [514], with tributylstannane [503, 514], electro-lytically [515], with sodium in alcohol [516] and with chromous sulfate [193, 197]. For partial reduction only, sodium arsenite [220] or sodium sulfite [254] are used. 

When arsine is passed over a heated metal, such as the alkali and alkaline earth metals, zinc or tin, the decomposition of the gas is accelerated and the arsenide of the metal is formed. If platinum is used, the removal of arsenic from the gas is complete.3 The action of sodium or potassium on arsine in liquid ammonia yields 4 the dihydrogen arsenide (MHgAs). Heated alkali hydroxides in the solid form quickly decompose the gas, forming arsenites, and at higher temperatures arsenates and arsenides of the metals.5 The aqueous and alcoholic solutions have no appreciable action.6 When the gas is passed over heated calcium oxide the amount of decomposition is not more than that due to the action of heat alone. Heated barium oxide, however, is converted into a dark brown mixture of barium arsenite and arsenate, hydrogen being liberated.7 The gas is absorbed by soda-lime.8... 

For the amperometric detection of arsenite on the platinum working electrode, the applied oxidation potential can be kept constant. Figure 5.12 shows a respective chromatogram of a 10 mg/L arsenite standard that was obtained with an oxidation potential of -iO.95 V. 

Fig. 5-9. Analysis of arsenite with amperometric detection. -Separator column lonPac ICE-ASl eluant 0.5 mmol/L HCl flow rate 1 mL/min detection D.C. amperometry on a platinum working electrode oxidation potential +0.95 V injection volume 50 pL solute concentration 10 mg/L arsenite.

In this procedure the sample (100 ml) containing between 14 and 36 ppm DEA (1.4 to 3.6 mg) was reacted for various periods of time, between 20 and 60 minutes, with excess 0.01 M periodic acid in 0.03 N sulfuric acid medium. To this solution was added excess 0.02 N sodium arsenite and sodium bicarbonate. Unused arsenite was then back titrated with 0.01 N iodine solution using potentiometric titration using a platinum foil indicating electrode - calomel reference electrode. These results show that it is possible to determine DEA amounts down to about 3 ppm in the 100 ml of test solution, with a recovery of about 90% or higher of the amount added. ... 

Figure 13.1 Potentiometric titration of sodium arsenite with standard iodine solution using platinum/calomel electrode and automatic titrator. Reproduced from Author s own files)...

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