This is given by the difference between the total acidity and the volatile acidity, care being taken to express the latter for this purpose in terms of tartaric acid. Example: -

Total acidity .... 0457 gram per 100 c.c, as tartaric acid. Volatile ,, .... 0 075 ,, ,, as acetic acid.

Then fixed acidity = 0457 - 0075 x 1.25 = 0363 gram per 100 c.c, expressed as tartaric acid.

In recent years a good deal of attention has been devoted to physico-chemical methods of examining wine. The acidity, for instance, has been studied from the point of view of the concentration of the hydrogen-ions, as determined by the velocity of inversion of sucrose. A given quantity of sucrose is dissolved in the wine (after any invertase present has been destroyed), and the mixture kept at a temperature of 76° by means of a bath of boiling carbon tetrachloride. From the quantity of sucrose inverted in a given time the inversion constant is obtained.1

A method of testing wine for free mineral acid, based upon the variation of the electrical conductivity when small quantities of alkali are added to the wine, has been described by Bosco and Belasio.2

For a lengthy study of the fixed organic acids of wine, with particular regard to lactic acid, see a paper by G. de Astis.3