Determinations made in this way are more accurate than the colorimetric method above described. Puer liquors after use are found by the electrometric method generally on the alkaline side, whereas such liquors show neutral to phenolphthalein owing to some action of the bate constituents on the indicator.
Puer Liquors in Electrometric Apparatus.
No. | P.D. Before | c.c. of N/10 KOH | P.D. After | c.c. of N/10 HCl | Description of Liquor | |||||
A1 | 0 | ·622 | 1 | ·95 | .. | .. | New puer | |||
A2 | .. | .. | 0 | ·747 | 1 | ·9 | A1 after 2 packs skins | |||
B1 | ·607 | 3 | ·7 | .. | .. | New puer | ||||
B2 | .. | .. | ·770 | 3 | ·1 | B1 after 3 packs skins | ||||
C1 | ·580 | 4 | ·3 | .. | .. | New puer | ||||
C2 | .. | .. | ·762 | 2 | ·8 | C1 after 2 packs skins | ||||
D1 | ·580 | 5 | ·2 | .. | .. | New puer | ||||
D2 | .. | 0 | ·8 | ·680 | .. |  | D1 after 1 pack skins (still on the acid side of 0·69) | |||
E1 | ·570 | 6 | ·2 | .. | .. | D2 + 4 buckets more puer | ||||
E2 | .. | .. | ·720 | 0 | ·8 | E1 after 1 pack skins | ||||
F1 | ·610 | 3 | ·0 | .. | .. | New puer | ||||
F2 | .. | 0 | ·5 | ·680 | .. | | F1 after 1 pack skins (still on the acid side of 0·69) | |||
The potential at the neutral point, using an auxiliary electrode filled with N/1 potassium chloride, was 0·69 volts. Potentials below this point, therefore, indicate an acid reaction, and, conversely, potentials above 0·69 volts indicate an alkaline reaction. The apparatus is thus very useful in following the course of the bating process. The initial potential also enables the hydrion concentration of the bate liquor to be determined directly (see Chapter [III].). It was found that during the bating of a pack of skins the hydrion concentration of the liquor diminished from 10-5·2 to 10-7·4 normal.
Such acidities as are found by this method, e.g. 7–8 c.c. N/10 acid per 100 c.c., are too great if made up by taking a solution of the free acids. The ionization must be almost entirely repressed by the presence of sufficient neutral salts of the same acids in the bating liquid, in order to obtain a similar result to the puer.
The table opposite gives the values found for a series of puer liquors, made with dog manure from hunting-kennels.
The reaction of puer liquors (expressed in c.c. of N/1 alkali or acid per 1000 c.c. of bate), in a series in which the washed goods were run for one hour in old puer, out of which goods had been taken, and then transferred to a freshly-made puer, was as follows:—
| Bate | Before Goods | After Goods |
| Old puer | Neutral | 28 c.c. acid |
| Fresh | 10 c.c. alkali | Neutral |
| Old puer | Neutral | 40 c.c. acid |
| Fresh | 11 c.c. alkali | Neutral |