Chemical constants of cellulose sulphocarbonate (solution).—In investigations of the solutions we make use of various analytical methods, which may be briefly described, noting any results bearing upon special points.
Total alkali.—This constant is determined by titration in the usual way. The cellulose ratio, C6H10O5: 2NaOH, is within the ordinary error of observation, 2: 1 by weight. A determination of alkali therefore determines the percentage of cellulose.
Cellulose may be regenerated in various ways—viz. by the action of heat, of acids, of various oxidising compounds. It is purified for weighing by boiling in neutral sulphite of soda (2 p.ct. solution) to remove sulphur, and in very dilute acids (0.33 p.ct. HCl) to decompose residues of 'organic' sulphur compounds. It may also be treated with dilute oxidants. After weighing it may be ignited to determine residual inorganic compounds.
Sulphur.—It has been proved by Lindemann and Motten [Bull. Acad. R. Belg. (3), 23, 827] that the sulphur of sulphocarbonates (as well as of sulphocyanides) is fully oxidised (to SO3) by the hypochlorites (solutions at ordinary temperatures). The method may be adapted as required for any form of the products or by-products of the viscose reaction to be analysed for total sulphur.
The sulphur present in the form of dithiocarbonates, including the typical cellulose xanthogenic acid, is approximately isolated and determined as CS2 by adding a zinc salt in excess, and distilling off the carbon disulphide from a water bath. From freshly prepared solutions a large proportion of the disulphide originally interacting with the alkali and cellulose is recovered, the result establishing the general conformity of the reaction to that typical of the alcohols. On keeping the solutions there is a progressive interaction of the bisulphide and alkali, with formation of trithiocarbonates and various sulphides. In decomposing these products by acid reagents hydrogen sulphide and free sulphur are formed, the estimation of which presents no special difficulties.
In the spontaneous decomposition of the solution a large proportion of the sulphur resumes the form of the volatile disulphide. This is approximately measured by the loss in total sulphur in the following series of determinations, in which a viscose of 8.5 p.ct. strength (cellulose) was dried down as a thin film upon glass plates, and afterwards analysed:
(a) Proportion of sulphur to cellulose (100 pts.) in original.
(b) After spontaneous drying at ordinary temperature.
(c) After drying at 40°C.
(d) As in (c), followed, by 2 hours' heating at 98°.
(e) As in (c), followed by 5 hours' heating at 98°.
| a | b | c | d | e | |
| Total sulphur | 40.0 | 25.0 | 31.0 | 23.7 | 10.4 |
The dried product in (b) and (c) was entirely resoluble in water; in (d) and (e), on the other hand, the cellulose was fully regenerated, and obtained as a transparent film.
Iodine reaction.—Fresh solutions of the sulphocarbonate show a fairly constant reaction with normal iodine solution. At the first point, where the excess of iodine visibly persists, there is complete precipitation of the cellulose as the bixanthic sulphide; and this occurs when the proportion of iodine added reaches 3I2: 4Na2O, calculated to the total alkali.