Thiocyanogen solutions

Submitted by William Howlett Gardner and Harold Weinberger Checked by D. T. EnglisI and E. C. PRicEf 

All methods that have been suggested for the preparation of thiocyanogen solutions involve the use of strictly anhydrous solvents, since the presence of moisture causes rapid hydrolysis and decomposition to take place. It is possible, howeyer, to stabilize thiocyanogen in aqueous solution to a limited extent by the presence of large quantities of potassium thiocyanate. 

The action of bromine upon various metallic thiocyanates gives thiocyanogen in better yield, but few solvents yield 

Plumbous thiocyanate is prepared by the interaction of sodium thiocyanate and lead nitrate in aqueous solution. It is recommended that both of these reagents be recrystallized to remove impurities. Two solutions are prepared, one containing 25 g. of purified sodium thiocyanate the other, 45 g. of the purified lead nitrate, each in 100 ml. of distilled water. Both solutions are cooled to 0 to 5°C., and the solution of the sodium salt added to that containing the lead nitrate. The precipitated plumbous thiocyanate is filtered, washed with ice water, and allowed to dry over a suitable desiccant in the dark. 

Acetic Acid-Acetic Anhydride Solution. One kilogram of glacial acetic acid is dehydrated in a 3-1.. flask over phosphorus pentoxide. The supernatant liquid is then fractionally distilled in an all-glass apparatus, and the portion boiling between 118 to 118.5° is collected. This procedure is repeated until the product has a melting point of 15.6°C. corresponding to that of 99.5 per cent acetic acid.f To the pure acetic acid 10 per cent acetic anhydride is now added. This solution should be kept in a stoppered container protected from moisture. 

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Estimation of Thiocyanogen.—Solutions of thiocyanogen in organic solvents can be titrated accurately by agitation with at least twice the equivalent quantity of potassium iodide and determination of the liberated iodine. 

Preparation of Thiocyanogen Solutions. To a suspension of 30 g. of plumbous thiocyanate in 300 ml. of the acetic acid is added 5 ml. of the bromine solution. The mixture is agitated vigorously on a shaking machine until it is practically colorless, and another portion of the bromine solution is added. This procedure is continued until all the bromine solution has been added. The mixture is then rapidly filtered through a dry fluted filter paper. ... 

The thiocyanogen solution is standardized by treating with an excess of potassium iodide and titrating the liberated iodine with standard sodium thiosulfate. The burette containing the halogenoid solution is protected against moisture by a tube containing calcium chloride or phosphorus pentoxide. 

Tellurium hexafluoride, 121 Tetrabromosilane, 38, 40 Tetrapyridino-ferrous chloride, 184 Thiocyanogen solution, 84, 86 standardization, 86 Thionyl bromide, 113 Thorium bromide, ammines, 54 anhydrous, 51... 

Thiocyanogen solution, 1 84, 86 standardization of, 1 86 Thionyl bromide, 1 113 Thionyl fluoride, 6 162 Thiophosphoryl bromide, 2 153 Thiophosphoryl chloride, 4 71 Thiophosphoryl triamide, 6 111 Thiosemicarbazide, 4 39... 

Diphenyl Tellnrium Bis[thiocyanate] A solution of thiocyanogen is prepared by shaking 4.5 g (14 mmol) of lead thiocyanate with 1.8 g (12 mmol) of bromine in dry diethyl ether for 10 min and filtering the mixture. The filtrate is protected from light. 2.8 g (10 mmol) of diphenyl tellurium are dissolved in 20 ml of dry diethyl ether and the thiocyanogen solution is added dropwise to the stirred solution of diphenyl tellurium. The mixture is stirred for 1.5 h, and the colorless crystalline product is filtered, washed with hexane, dried, and recrystallized from benzene/hexane (1/1, v/v) yield 2.9 g (78%) m.p. 129-132° (dec.). 

Improved preparation. Thiocyanogen can be prepared in 85-90% yield by use of a two-solvent system (water and waler-immisciblc toluene) for the reaction between sodium thiocyanate and gaseous chlorine. The thiocyanogen formed in the aqueous phase is extracted into the toluene. The thiocyanogen solution can be stored at reduced temperatures. 

Anhydrous acetic acid containing some acetic anhydride provides the most stable thiocyanogen solutions. Very pure lead thiocyanate is shaken in the dark with bromine in a mixture of 6 parts by volume of acetic acid (distilled from 1 % of chromic trioxide), 1 part by volume of acetic anhydride, and 3 parts by volume of pure anhydrous CC14, all contained in dry vessels that give up no alkali, i.e., that have been previously boiled out with hydrochloric acid. Filtration of the mixture through a dry filter, with exclusion of moisture, gives clear thiocyanogen solutions that are stable for several weeks. 

Thiocyanogen solution. In the manipulations to follow, it must be remembered that thiocyanogen is hydrolyzed rapidly with water, and all vessels and solvents must be absolutely dry. 

To a little thiocyanogen solution add a little concentrated aqueous potassium iodide and shake. Note that thiocyanogen is intermediate in activity between bromine and iodine. 

Take 0.5 ml of an unsaturated oil such as cottonseed oil or oleic acid, place in a small dry glass-stoppered Erlenmeyer flask, and add 10 to 15 ml of your thiocyanogen solution. Shake thoroughly and leave to stand in a dark place for half an hour. Note the disappearance of the thiocyanogen color. Add a little concentrated aqueous potassium iodide little if any iodine will be liberated. ... 

Shake portions of thiocyanogen solution with finely powdered iron and with mercury. Compare the action with that of a halogen. 

The reaction of thiocyanogen with unsaturated fatty acids has been introduced as a method of analysis. Excess of a standardized thiocyanogen solution is used, and the amount of unreacted reagent is titrated. Thiocyanogen reacts quantitatively with oleic acid but with only one of the two double bonds of linoleic add and with two of the three double bonds of linolenic acid. 

Thiocyanogen Solutions. Lead thiocyanate, used advantageously in the formation of thiocyanogen, is prepared from lead nitrate and sodium thiocyanate. To an ice-cold solution of 45 g. of lead nitrate in 100 cc. of water is added a cold solution of 25 g. of sodium thiocyanate in 100 cc. of water. Lead thiocyanate precipitates as a fine, white powder. It is collected on a filter, washed free of nitrates with ice water, and then dried in vacuum over phosphorus pentoxide in the dark. The product should remain perfectly white. 

One part by weight (in grams) of lead thiocyanate is suspended in 5 to 10 parts by volume (in cubic centimeters) of the deared solvent in a glass-stoppered flask. The solution is cooled to 5-10 , and a small portion of 10% bromine in the same solvent is added. The mixture is shaken vigorously until the color due to the bromine disappears. The process of addition and shaking is repeated until the calculated amoimt of bromine has been used. The suspended solids are allowed to settle, the thiocyanogen solution is decanted, and the residual solids are washed by decantation with small portions of the solvent. 

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