ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 71 
starch, poasibly unaltered starch and glucose. There appear to be several 
modifications of dextrin, known as erythrodextrin, amylodextrin, achro- 
dextrin, &¢., which differ from each other in their molecular weights. 
Pure dextrin can be prepared by dissolving starch in moderately con- 
centrated sulphuric acid, quickly neutralising with alkali and precipitating 
with alcohol. 
The commercial dextrins are prepared by heating dry starch either 
alone or after sprinkling with nitric acid to temperatures between 100° 
and 280° CG. Nitric acid or a mixture of nitric and sulphuric acid are 
used (W. S. Hayward, Eng. Pat. 2,612, 1858). It is also produced by 
the restrained action of malt extract upon starch (B. J. B. Mills (T. Sim 
and E. S. Hutchinson), Eng. Pat. 933, 1869). The products vary according 
to the method of treatment, white dextrin containing soluble starch and 
possibly unaltered starch,while brown dextrin contains more or less glucose. 
Dextrin contains about 12 per cent. of water and about 0-2 per cent. 
of mineral matter. On heating with normal alkali solution the acetic 
acid liberated neutralises 15 to 24 per cent. of KHO. According to G. 
Malfitano and A. Moschkoff (‘ Compt. Rend.’ 1912, 154, 443-446), starch 
is converted into dextrins by drying over P,O;. When the desiccation 
is prolonged so that the water of constitution is removed the solubility 
isdiminished. They regard the molecule of starch as consisting of C;H,,0; 
aggregates linked together by OH.H molecules and that by the loss of 
the latter the molecule becomes disintegrated. 
Dextrin is only slightly precipitated by tannic acid; it is precipitated 
readily by alcohol, but is not readily salted out by electrolytes, only con- 
centrated solutions of sulphates and not those of other salts causing it 
to separate (S. Levites, ‘ Zeit. Chem. Ind. Kolloide,’ 1908, 3, 145-153). 
Dextrin is used to a considerable extent in finishing and calico-printing, 
imparting a very hard ‘ boardy ’ feel to cloth, which for certain purposes 
is much desired. It is also employed as an adhesive under the name 
of British gum, in place of gum arabic, but the solutions have much less 
- viscosity, and therefore stronger sols have to be employed. It has, how- 
ever, two advantages, that is, it can be spread in a thinner layer and is 
more readily softened when moistened; for these reasons it has largely 
replaced the natural gum. It is remarkable that, although a profound 
modification must have been made by treating starch at high temperatures, 
the structure of the granules is in no way affected until water is added, when 
they entirely dissolve. It is used to some extent in painting (Church, 
* Chemistry of Paints and Painting,’ p. 72), but it is rarely used for emul- 
sifying purposes. 
Pfeffer, using a membrane of copper ferrocyanide, found the osmotic 
pressure of dextrin to be 16-6 c.m. of mercury (Ostwald, ‘ Solutions,’ 
p- 94); Musculus and A. Meyer (Bull. Soc. Chim., 30, 270) found the rate 
of diffusion of dextrin through membranes to be for Y dextrin (achroo- 
dextrin) 7, and amylodextrin, 1; dextrose hydrate being taken as 100. 
Barium oxide is adsorbed by commercial dextrins, but it has been found 
that the amount varies with the percentage of starch present, the following 
results having been obtained : 
Starch. BaO adsorbed 
Dextrin 1 2 2 » 1:99 1°75 
” 2 E ‘ A 13°13 3°53 
3 i ; . 24°72 5°64 
Mardi Sst ese 100-0 23°61 
