Recently Eberle[27] has proposed to add a proportion of gall juice (bile) to a bate containing pancreatic enzymes, with a view of energizing the latter. This part of the subject will be dealt with in Chapter [V].
The Reactions of the Puer.—The organic acids are present principally in the form of the sodium or calcium salts. The intestinal juices contain sodium carbonate, but this is neutralized by lactic acid found in the intestines, so that the excreta usually have an acid reaction. The reaction of a fresh puer liquor is faintly acid to litmus, but this acid is immediately neutralized by the introduction of the skins, so that the rest of the bating process takes place in a neutral or alkaline medium, and the quantity of lime removed by the free acids is probably small.
The acidity of a filtered puer liquor made up freshly and titrated with N/5 soda, and phenolphthalein as indicator, was found to equal 10 c.c. N/1 acid per 1000 c.c. of bate. In the volume of liquor used, namely, 900 litres, this would neutralize only 250 grm. of lime (CaO).
The determination of the exact acidity or alkalinity of puer liquors by ordinary methods with indicators is not easy, as the end reactions are not sharp. The liquor above described was alkaline to methylorange and litmus, although acid to phenolphthalein, and the results also vary somewhat, according to the quantity of bate used for titration, and according to the strength of the acid or alkali used. The method used, both at the Trent Bridge laboratory and Messrs. Doerr and Reinhart’s laboratory at Worms, is as follows:—
The puer liquor is filtered through a Schleicher and Schulls hard filter, No. 602, 18 1/2 cm. diam.; 50 c.c. are taken, 4 drops of phenolphthalein solution (10 gr. dissolved in 300 c.c. alcohol) added, and titrated with N/5 acid or alkali as the case may be.
If the acidity be determined by adding excess of alkali and titrating back, a larger amount of acid is found than by direct titration. In the above-mentioned case, an indirect titration showed an acidity equal to 18 c.c. N/10 acid per 100 c.c. of liquor.
On adding the alkali to a fresh filtered puer liquor, a flocculent precipitate is produced, which appears to be due to the decomposition of proteid compounds of weak organic acids,[28] the alkali uniting with them and setting the proteids free. As these compounds are undoubtedly decomposed by the lime in the skins, it seems probable that the acidity available for neutralizing lime may be greater than that shown by direct titration.
A method which was suggested by me for the estimation of the acidity of tan liquors may be applied also to the examination of the bate liquors, namely, the method by which the electric potential between a hydrogen electrode dipping into the bate liquor, and a standard calomel electrode, is used to determine the neutral point (for details see Chapter [III].), using the potentiometer devised by Dr. H. J. S. Sand.[29]
Puer liquors, titrated in this manner with N/10 soda or HCl until a potential of 0·69 volts is reached (at which point phenolphthalein turns from colourless to red), gave the following results per 100 c.c. filtered liquor.
| No. | Description of Liquor | Reaction | c.c. N/10 |  | Alkali |  | Required for Neutralization |
| or | |||||||
| Acid | |||||||
1 | New puer before goods | Acid | 7 | ·4 alkali | |||
2 | The same after goods | Alkaline | 0 | ·57 acid | |||
3 | New puer before goods | Acid | 8 | ·1 alkali | |||
4 | The same after goods | Alkaline | 3 | ·25 acid | |||
5 | Used puer after goods | Alkaline | 5 | ·00 acid | |||
6 | Spent puer | Alkaline | 6 | ·6 acid | |||