Sampling thiosulphate

To determine the exact peroxide content of benzoyl peroxide (and of other organic peroxides) the following procedure may be employed. Place about 0 05 g. of the sample of peroxide in a glass-stoppered conical flask add 5-10 ml. of acetic anhydride (A.R. or other pure grade) and 1 g. of powdered sodium iodide. Swirl the mixture to dissolve the sodium iodide and allow the solution to stand for 5-20 minutes. Add 50-75 ml. of water, shake the mixture vigorously for about 30 seconds, and titrate the liberated iodine with standard sodium thiosulphate solution using starch as indicator. 

Procedure (iodometric method). Weigh out accurately about 5.0 g of the bleaching powder into a clean glass mortar. Add a little water, and rub the mixture to a smooth paste. Add a little more water, triturate with the pestle, allow the mixture to settle, and pour off the milky liquid into a 500 mL graduated flask. Grind the residue with a little more water, and repeat the operation until the whole of the sample has been transferred to the flask either in solution or in a state of very fine suspension, and the mortar washed quite clean. The flask is then filled to the mark with distilled water, well shaken, and 50.0 mL of the turbid liquid immediately withdrawn with a pipette. This is transferred to a 250 mL conical flask, 25 mL of water added, followed by 2 g of iodate-free potassium iodide (or 20 mL of a 10 per cent solution) and 10 mL of glacial acetic acid. Titrate the liberated iodine with standard 0.1M sodium thiosulphate. 

Among the most important indirect methods of analysis which employ redox reactions are the bromination procedures for the determination of aromatic amines, phenols, and other compounds which undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The metal-oxine complex can then be determined by bromine substitution. 

Bartlett et al. [55] used the method of Uthe et al. [70] for determining methylmercury. Sediment samples of 2-5g were extracted with toluene after treatment with copper sulphate and an acidic solution of potassium bromide. Methylmercury was then back extracted into aqueous sodium thiosulphate. This was then treated with acidic potassium bromide and copper sulphate following which the methylmercury was extracted into pesticide grade... 

The Sugar Determination.—Dilute the 5 c.c. sample in a measuring flask to 25 c.c. and pour 10 c.c. of this diluted solution into 25 c.c. of 01 N-iodine solution. Then add 40 c.c. of 0-1 N-sodium hydroxide solution (free from alcohol) and leave for twenty minutes. Make faintly acid with dilute sulphuric acid and titrate back with 0-1 N-sodium thiosulphate solution. One equivalent of iodine corresponds to 0-5 mole of reducing biose, or 1 c.c. of 0-1 N-iodine solution to 17-1 mg. of maltose. What is the course of the reaction ... 

Procedure Weigh accurately about 0.5 g of ferric ammonium citrate and dissolve the sample in 15 ml DW. Add to it slowly 1 ml of sulphuric acid and warm gently to attain a yellow colouration so as to decompose the iron and ammonium citrate complex completely. Cool and add 0.1 N potassium permanganate solution dropwise from a burette to obtain a pink colour that persists for 5 seconds. To the resulting solution add hydrochloric acid 15 ml and potassium iodide 2.0 g, shake well and set aside for 3 minutes so that iodine may be liberated completely. Now, add 60 ml of water and titrate with 0.1 N sodium thiosulphate solution while shaking the contents continuously till a colourless end-point is achieved. 

Industrially, the silver is recovered from either the wash water, or the bleach fix separately or from a mixture of the two using electrolysis employing a stainless steel cathode cylinder and an anode of stainless steel mesh. A typical wash solution composition contains silver (4 g L ), sodium thiosulphate (220 g L ), sodium bisulphite (22 g L ) and sodium ferric EDTA (4 g L ). At Coventry we have used a scaled down version of the industrial process employing 250 mL samples [46]. Electrolysis experiments were performed at ambient temperature with both wash and bleach fix solutions and in which the potential applied to the cathode and the speed of rotation of the cathode were varied. The sonic energy (30 W) was supplied by a 38 kHz bath. The results are given in Tab. 6.9. The table shows that the recovery of silver on sonication of the wash or bleach fix solutions is much improved especially if the electrode is rotated while ultrasound is applied. Yields with bleach fix (which contains ferric ions) are less since Fe and Ag compete for discharge (Eqs. 6.13 and 6.14). 

A sample of phenol glycerol injection was dilutee with water and an aliquot was taken and reacted with excess bromine generated from potassium bromide and potassium bromate solutions. The excess bromine remaining after reaction was reacted with potassium iodide and the liberated iodine was titrated with sodium thiosulphate. A blank titration was carried out where the same quantity of bromine was generated as was used in the titration of the diluted injection, potassium iodide was then added and the liberated iodine was titrated with sodium thiosulphate. From the following data calculate the percentage of w/v of the phenol in the injection. 

The volume of 0.1015 M sodium thiosulphate required to titrate the XS bromine after reaction with the sample = 22.4 ml. 

When the lactam ring is open it will react with iodine. 1 mole of the ring open form of penicillin will react with 8 equivalents of iodine, the intact lactam ring will not react. In this type of titration excess iodine solution is added to a sample of the penicillin and the iodine which is not consumed in the reaction is estimated by titration with sodium thiosulphate. The value obtained for the amount of hydrolysed penicillin in the sample should be no more than 5% of that obtained when all the penicillin in the same amount of sample is completely hydrolysed to the ring-opened form and then reacted with iodine. Most of the pharmacopoeial monographs for penicillins indicate that this test should be carried out. 

After taking the sample, the reaction was immediately stopped with 0.1 mL of 0.1 N sodium thiosulphate solution. Copper grids were exposed to 20-p, L samples for 15 min and washed with sterile distilled water. Finally grids were stained in 2% aqueous uranyle acetate for 8 min and dried. The grids were examined with a Zeiss EM 902 transmission electron microscope (LEO Elektronenmikroskopie GmbH). 

About 2 gm. benzyl iodide are weighed into a flask and then 50 ml. 20% alcoholic potash solution are added and the mixture refluxed for about an hour. At the completion of the saponification the contents of the flask are allowed to cool and then transferred to a 500-ml. flask and made up to volume with water. 100 ml. of the resulting solution are placed in a distillation flask and distilled in steam after adding 10 gm. ferric ammonium alum and acidifying with sulphuric acid. By this treatment, the ferric salt is converted to the ferrous condition, liberating iodine which is distilled over into 5% potassium iodide solution. At the end of the distillation, the free iodine in the potassium iodide solution is titrated with a decinormal solution of sodium thiosulphate. From this, the amount of iodine and so the quantity of benzyl iodide in the sample may be calculated. 

Further applications are described for sulfur (Fig. 2.6) and copper in water by reversed-phase or ion-exchange high-performance liquid chromatography . Also anions such as thiosulphate and polythionates were separated from environmental samples by HPLC (anion-exchange) . The sensitive detection was made possible by reaction with Ce(lV) after separation and fluorimetry of Ce(lII) ions. 

Lead Thiosulphate (Pb SgO,) is now used for the preparation of match compositions, which ignite on any surface. It is prepared by treating a solution of " sugar of lead ead acetate) with sodium thiosulphate it is a pure white, inodorous, tasteless powder, insoluble in water and not hygroscopic. It should be kept in well-closed bottles. Twelve kg. of lead acetate yield, when treated with 5 kg. of sodium thiosulphate, about 9-5 kg. of lead thiosulphate. A pure sample should contain no water-soluble matter. 

Sodium thiosulphate (Na3S,O3"5HjO) can be titrated easily by iodometry. Accurately weigh a 5 g sample and dissolve it in 500 ml distilled water to make a 1% sample solution. To 50 ml of 1% sample solution, add 50 ml distilled water and 3-5 ml 1% starch solution. Titrate with 0.1 N T solution until a blue colour is shown ... 

Extraction variant. Slightly acidify the sample solution containing not more than 0.5 mg of Co. Add 5 ml of the thiosulphate-phosphate solution and 5 ml of the thiocyanate solution, and adjust to pH 3.5. 0. Transfer the solution to a separating funnel, and extract the complex with two 10-ml portions of a mixture of diethyl ether and isoamyl alcohol (1+1). Make up the combined extracts to the mark with the solvent in a 25-ml flask, and measure the absorbance at 620 nm, using the solvent as reference. 

Extractive separation of In. To an acidic sample solution (15-20 ml, -1 M H2SO4), in a separating funnel containing not more than 50 pg of In, add -0.1 g of sodium thiosulphate and 5 g of potassium iodide. Extract In with two 10-ml portions of MIBK (shaking time 30 s). Wash the extract by shaking with 10 ml of 1 M H2SO4 containing 2 g of KI and 20 mg of sodium thiosulphate. 

To a weakly acidic sample solution ( 10 ml) containing not more than 50 pg of Mo, add 5 ml of cone. HCl and 2 ml of 20% KI solution. Mix, and let the solution stand for 5 min, then add 0.1 M sodium thiosulphate dropwise until the solution is decolorized. Add 2 ml of 20% citric acid solution and 2.5 ml of dithiol solution. After 10 min, shake the aqueous solution for 30 s with two 8-ml portions of amyl acetate. Dilute the green extract to volume with the solvent in a 25-ml standard flask, and measure the absorbance of the solution at 675 nm, using the solvent as reference. 

Low and erratic results obtained after the extraction of nickel a-furildioximate with certain chloroform samples are probably due to the presence of trace oxidants from the decomposition of CHCU- Preliminary shaking of the solvent with thiosulphate solution before the extraction prevents such interferences. 

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