Sodium arsenite. Standard solution

In the second method, the hypochlorite solution or suspension is titrated against standard sodium arsenite solution this is best done by adding an excess of the arsenite solution and then back-titrating with standard iodine solution. 

Excellent results are obtained by the following method, which is of wider applicability. When excess of standard sodium arsenite solution is treated with hydrogen sulphide solution and then acidified with hydrochloric acid, arsenic(III) sulphide is precipitated ... 

Sodium, D. of as zinc uranyl acetate, (g) 467 by flame photometry, 812 Sodium arsenite solution prepn. of standard, 390... 

Iodine in aqueous solution may be measured quantitatively by acidifying the solution, diluting it, and titrating against a standard solution of sodium thiosulfate, sodium arsenite or phenyl arsine oxide using starch indicator. The blue color of the starch decolorizes at the end point. The indicator must be added towards the end of titration when the color of the solution turns pale yellow. Prior to titration, iodine in the dilute acidic solution is oxidized to iodate by adding bromine water or potassium permanganate solution. Excess potassium iodide is then added. The liberated iodine is then titrated as above. 

Periodic acid or periodate solutions are standardized by adding an excess of potassium iodide and then titrating the liberated iodine against a standard solution of sodium thiosulfate, phenylarsine oxide, or sodium arsenite. One mole of periodate liberates an equimolar amount of iodine as per the following reaction ... 

To a solution of 280 mg of 3a,lip,17,17a-tetrahydroxy-13a-(2-cyanoethyl)-D-homo-18-noretiocholan in 17 ml of methanol at room temperature contained in a 25 ml volumetric flask there was added 162 mg of sodium metaperiodate in 8 ml of water. A small amount of water was added to bring the volume to 25 ml. After 15 min an aliquot was withdrawn and added to a measured amount of sodium arsenite solution containing sodium bicarbonate and potassium iodide. After 10 min, the solution was titrated with standard iodine solution. The solution was diluted with water and extracted twice with ethyl acetate, each extract being washed in turn with sodium combined, dried over magnesium sulfate and concentrated to dryness under reduced pressure, yielding 265 mg of a colorless, amorphous product of 4b-methyl-ip-(2-formylethyl)-2p-formyl-2-(2-cyanoethyl)-4p,7a-dihydroxyperhydrophenanthrene. 

Solutions of periodic add and of sodium metaperiodate in water are quite stable at room temperature. The periodate content is readily determined by titrating, with standard sodium arsenite solution, the iodine liberated from iodide in neutral solution.49 103-197 Periodate also may be determined accurately in the presence of iodate, since in neutral solution periodate is reduced by iodide to iodate. The reaction in the presence of a boric add-borax buffer is shown by the following equation. 

Analysis for Periodic Add.10 -106 Standard 0.1 N sodium arsenite solution containing 20 g. of sodium bicarbonate per liter and 0.1 N iodine solution are required. 

Transfer the solutions into 100-mL separators containing 25 mL of cold water, and rinse the beakers and pH meter electrodes with a few milliliters of cold water, collecting the washings in the respective separator. Add 2 mL of bromine TS, stopper, and mix. Add 2 mL of 2% sodium arsenite solution, stopper, and mix. Add 10 mL of n-butanol to the clear solutions, stopper, and mix. Finally, add 5 mL of p-Phenylenediamine-Pyridine Mixed Reagent, mix, and allow to stand for 15 min. Remove and discard the aqueous phases, and filter the alcohol phases into 1-cm cells. The absorbance of the solution from the Sample Solution, determined at 480 nm with a suitable spectrophotometer, is not greater than that from the Cyanide Standard Solution. 

Place the alkaline solution (about 10 ml) containing not more than 10 pg of CN in a 25-ml standard flask. Acidify the solution with glacial acetic acid and add 1 ml in excess. Immediately add 1 ml of bromine water. Mix the solution thoroughly, and let it stand for 10 min with occasional shaking. Add a 1.5% solution of sodium arsenite dropwise to reduce the excess of bromine, then 2-3 drops more. Add 5 ml of the benzidine-pyridine reagent, and stir the solution. After 30 sec, add 5 ml of ethanol, make up to the mark with water, and stir well. After 15 min, measure the absorbance of the solution at 530 nm against a reagent blank. 

Sodium arsenite (0.02 N) Dissolve 0.8 g of sodium hydroxide (Analar) and 1.0 g of arsenious oxide (Analar) in a minimum quantity of distilled water, warm in the beaker to obtain complete solution. Transfer quantitatively to a 1 litre standard volumetric flask and add 2.0 g of solid bicarbonate (Analar), swirl the flask until the solid is completely dissolved, dilute to the mark and mix thoroughly. 

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)...

Both sodium oxalate and sodium arsenite (prepared from A.R. AS2O3) are more convenient because they are stable in air and are primary standards. Ferrous sulphate is not a primary standard and is oxidised by atmospheric oxygen. So the operation has to be carried out in the absence of air. Sodium arsenite though poisonous, costlier and preparation of its solution being cumbersome, gives more reliable results than sodium oxalate because oxalic acid is decomposed at higher temperatures into CO and CO2. The extent of decomposition at a sulphuric acid concentration below 20% is however very small. 

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

The commercial solution of sodium hypochlorite contains 14-15% available chlorine, compared to 35-36% in bleaching powder. The concentration of hypochlorite in the bleaching bath generally varies from 1 to 3 g/1 available chlorine. The optimum bleaching conditions, however, depend on the degree of discolouration of the cloth and thus the temperature and time of reaction should be adjusted according to the requirement. The concentration of hypochlorite solution is normally estimated by means of standard thiosulphate or arsenite titration. 

---