| Point. | Initial composition of the solutions: grams of salt to 1000 grams of water. | Composition of solutions obtained: gram-equivalents per 1000 grams of water. | UNa per cent. | UNH4 per cent. | ||||
| NaCl | NH4HCO3 | HCO3 | Cl | Na | NH4 | |||
| P1 | 397 | 496 | 0.92 | 6.79 | 1.44 | 6.28 | 78.8 | 85.1 |
| — | 351 | 446 | 0.99 | 6.00 | 1.34 | 5.65 | 77.7 | 82.5 |
| — | 316 | 412 | 1.07 | 5.41 | 1.27 | 5.21 | 76.4 | 79.5 |
| — | 294 | 389 | 1.12 | 5.03 | 1.23 | 4.92 | 75.5 | 75.1 |
| — | 234 | 327 | 1.30 | 4.00 | 1.16 | 4.14 | 71.0 | 68.6 |
As is evident from this table, diminution in the relative amount of sodium chloride exercises only a slight influence
on the utilization of this salt, but is accompanied by a rapid diminution of the effective transformation of the ammonium bicarbonate. So far as the efficient conversion of the sodium is concerned, we see that it reaches its maximum at the point P1, and that it decreases both with increase and with decrease of the relative amount of sodium chloride employed; and faster, indeed, in the former than in the latter case. On the other hand, the effective transformation of the ammonium bicarbonate reaches its maximum at the point P2, and diminishes with increase in the relative amount of ammonium bicarbonate employed. Since sodium chloride is, in comparison with ammonia—even when this is regenerated—a cheap material, it is evidently more advantageous to work with solutions which are relatively rich in sodium chloride (solutions represented by the curve P1P2). This fact has also been established empirically.
When, as is the case in industrial practice, we are dealing with solutions which are saturated not for two salts but only for sodium bicarbonate, it is evident that we have then to do with solutions the composition of which is represented by points in the area P1P2I,IV. Since in the commercial manufacture, the aim must be to obtain as complete a utilization of the materials as possible, the solutions employed industrially must lie in the neighbourhood of the curves P2P1IV, as is indicated by the shaded portion in Fig. 127. The best results, from the manufacturer's standpoint, will be obtained, as already stated, when the composition of the solutions approaches that given by a point on the curve P2P1. Considered from the chemical standpoint, the results of the experiments lead to the conclusion that the Solvay process, i.e. passage of carbon dioxide through a solution of sodium chloride saturated with ammonia, is not so good as the newer method of Schlösing, which consists in bringing together sodium chloride and ammonium bicarbonate with water.[[395]]
Preparation of Barium Nitrite.—Mention may also be made here of the preparation of barium nitrite by double decomposition of barium chloride and sodium nitrite.[[396]]
The reaction with which we are dealing here is represented by the equation
BaCl2 + 2NaNO2 = 2NaCl + Ba(NO2)2
It was found that at the ordinary temperature NaCl and Ba(NO2)2 form the stable salt-pair. If, therefore, barium chloride and sodium nitrite are brought together with an amount of water insufficient for complete solution, transformation to the stable salt-pair occurs, and sodium chloride and barium nitrite are deposited. When, however, a stable salt-pair is in its transition interval (p. [315]), a third salt—in this case barium chloride—will be deposited, as we have already learned. On bringing barium chloride and sodium nitrite together with water, therefore, three solid phases are obtained, viz. BaCl2, NaCl, Ba(NO2)2. These three phases, together with solution and vapour, constitute a univariant system, so that at each temperature the composition of the solution must be constant.
Witt and Ludwig found that the presence of solid barium chloride can be prevented by adding an excess of sodium nitrite, as can be readily foreseen from what has been said. Since the solution in presence of the three solid phases must have a definite composition at a definite temperature, the addition of sodium nitrite to the solution must have, as its consequence, the solution of an equivalent amount of barium chloride, and the deposition of an equivalent amount of sodium chloride and barium nitrite. By sufficient addition of sodium nitrite, the complete disappearance of the solid barium chloride can be effected, and there will remain only the stable salt-pair sodium chloride and barium nitrite. As was pointed out by Meyerhoffer, however, the disappearance of the barium chloride is effected, not by a change in the