Hydrogen ion Concentration of Puer Liquors.—It has already been shown, in Chapter [II]., that fresh puer liquors have a certain acidity (7 c.c. N/10 per 100 c.c.) at the commencement of the operation, but that at the end they are alkaline (3 c.c. N/10 per 100 c.c.). If an artificially acid liquor be made by diluting hydrochloric acid until it shows the same number of c.c. by titration, it will be found far too “strong,” and will swell the skins. This brings us to the consideration of what is meant by the strength of acids.
Procter and Jones[64] have drawn attention to the point in their paper on “Acids in Tan Liquors.” As is well known, the ionic theory affirms that degree of acidity depends on the concentration of hydrogen ions, a strongly acid solution being one in which the hydrion concentration is great, an alkaline solution one in which it is extraordinarily minute, and if we adopt pure water as our standard of neutrality, a neutral solution is one in which the hydrion concentration is approximately 10-7 normal. A normal solution of hydrogen ions would contain 1 gram of hydrogen ions per litre; in the case of hydrochloric acid this would equal 1·35 N/1 HCl.
Sand and Law,[65] and Wood, Sand and Law,[66] have described the mode of estimating the hydrogen ion concentration in tan liquors directly by means of the electrometric method, and this method is especially applicable to the estimation of the hydrogen ion concentration in puer liquors. It can also be used to titrate the liquors, and we have already given some of the results in Chapter [II].
The method is based on the theory of Nernst, that the difference of potential between a metal plate and the solution of one of its salts into which the metal is dipping, depends on the osmotic pressure of the free ions of that metal in the solution (in other words, on the concentration of the solution). In the case of hydrogen ions, we use a plate of platinum coated with platinum black and saturated with hydrogen, and the difference of potential depends, therefore, on the concentration of the hydrogen ions in solution.
Fig. 9.—Diagram of Connexions of Electrometric Apparatus.
The accompanying figure (Fig. 9) shows the hydrogen electrode I, and the auxiliary electrode II, drawn to scale,[67] on the right, whereas the electrical apparatus is explained diagrammatically on the left.
The principle of the method of measurement consists in connecting the two ends P and Q, of a sliding rheostat to the terminals of a dry cell, D, and balancing the potential-difference to be measured against the potential-difference between one end, P, and the slider, S, by means of a special form of enclosed capillary electrometer, E. The value of this potential-difference is read directly on a delicate voltmeter, V. The connexions, which are found ready-made in the box, have been drawn out, whereas, those to be made by the operator are shown by dotted lines. The steps to be taken by the latter, consist first in taking off the capillary electrometer and manipulating it in such a manner, that on returning it into position the capillary may be partly filled with a thread of mercury and partly with the acid. The terminals, X and Y, marked battery + and −, are connected to a dry cell, and the terminals, Z and U, marked cathode and auxiliary respectively, to the hydrogen and calomel electrode. Very careful insulation of the connexion between the terminal marked auxiliary and the calomel electrode is necessary. The hydrogen is passed through the hydrogen electrode until a constant P.D. between it and the calomel electrode is obtained. This P.D. is measured by moving the slider up and down until no movement of the mercury in the capillary electrometer is observed on depressing the key K marked electrometer.
Fig. 10 is a view of the apparatus as set up for the titration of a puer liquor. H is a cylinder of compressed hydrogen;[68] I, the hydrogen electrode, dipping into the beaker C containing the liquor for titration; b, the burette, containing N/10 acid or alkali; II, the auxiliary electrode, the capillary of which is also seen dipping into the beaker C; P, the potentiometer box containing the sliding rheostat S and electrometer E; D, a dry battery. The acid or alkali is run in from the burette until the voltmeter shows 0·69 volts, indicating that the liquid is neutral or has a hydrogen ion concentration of 10-7.