Part 28 (2/2)
per cent.
per cent.
+----------+------+------+---------+
HCO_{3}
Cl
Na
NH_{4}
+----------+------+------+---------+---------+----------
0.18
8.17
4.62
3.73
43.4
95.1
0.31
7.65
3.39
4.56
55.7
93.4
0.51
7.13
2.19
5.45
69.2
90.5
0.92
6.79
1.44
6.28
78.8
85.1 +----------+------+------+---------+---------+----------
This table shows that the greater the excess of sodium chloride, the greater is the percentage utilization of ammonia (Point P_{2}); and the more the amount of sodium chloride decreases, the greater is the percentage amount of sodium chloride converted into bicarbonate. In the latter case, however, the percentage utilization of the ammonium bicarbonate decreases; that is to say, less sodium bicarbonate is deposited, or more of it remains in solution.
Consider, in the same manner, the relations for solutions represented by the curve P_{2}IV, which gives the composition of solutions saturated with respect to sodium bicarbonate and ammonium bicarbonate. In this case we obtain the following results:--
------+---------------------+
Initial composition
of the solutions:
grams of salt to 1000
Point.
grams of water.
+------+--------------+
NaCl
NH_{4}HCO_{3}
------+------+--------------+ P_{1}
397
496
--
351
446
--
316
412
--
294
389
--
234
327
------+------+--------------+ [Transcriber's note: table continued below...]
+----------------------------------+------+----------
Composition of solutions obtained:
in gram-equivalents per 1000 grams
U_{Na}
U_{NH_{4}}
of water.
+----------+------+------+---------+
HCO_{3}
Cl
Na
NH_{4}
+----------+------+------+---------+------+----------
0.92
6.79
1.44
6.28
78.8
85.1
0.99
6.00
1.34
5.65
77.7
82.5
1.07
5.41
1.27
5.21
76.4
79.5
1.12
5.03
1.23
4.92
75.5
75.1
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 {325} 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 P_{1}, 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 P_{2}, 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 P_{1}P_{2}). 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 P_{1}P_{2}I,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 P_{2}P_{1}IV, 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 P_{2}P_{1}.
Considered from the chemical standpoint, the results of the experiments lead to the conclusion that the Solvay process, _i.e._ pa.s.sage of carbon dioxide through a solution of sodium chloride saturated with ammonia, is not so good as the newer method of Schlosing, which consists in bringing together sodium chloride and ammonium bicarbonate with water.[395]
{326}
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
BaCl_{2} + 2NaNO_{2} = 2NaCl + Ba(NO_{2})_{2}
It was found that at the ordinary temperature NaCl and Ba(NO_{2})_{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. BaCl_{2}, NaCl, Ba(NO_{2})_{2}. These three phases, together with solution and vapour, const.i.tute 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 {327} composition of the solution, but by the necessity for the composition of the solution remaining constant.
[Ill.u.s.tration: FIG. 128.]
Barium Carbonate and Pota.s.sium Sulphate.--As has been found by Meyerhoffer,[397] these two salts form the stable pair, not only at the ordinary temperature, but also at the melting point. For the ordinary temperatures this was proved in the following manner: A solution with the solid phases K_{2}SO_{4} and K_{2}CO_{3}.2H_{2}O in excess can only coexist in contact either with BaCO_{3} or with BaSO_{4}, since, evidently, in one of the two groups the stable system must be present. Two solutions were prepared, each with excess of K_{2}SO_{4} + K_{2}CO_{3}.2H_{2}O, {328} and to one was added BaCO_{3} and to the other BaSO_{4}. After stirring for a few days, the barium sulphate was completely transformed to BaCO_{3}, whereas the barium carbonate remained unchanged. Consequently, BaCO_{3} + K_{2}SO_{4} + K_{2}CO_{3}.2H_{2}O is stable, and, therefore, so also is BaCO_{3} + K_{2}SO_{4}. That BaCO_{3} + K_{2}SO_{4} is the stable pair also at the melting point was proved by a special a.n.a.lytical method which allows of the detection of K_{2}CO_{3} in a mixture of the four solid salts. This a.n.a.lysis showed that a mixture of BaCO_{3} + K_{2}SO_{4}, after being fused and allowed to solidify, contains only small amounts of K_{2}CO_{3}; and this is due entirely to the fact that BaCO_{3} + K_{2}SO_{4} on fusion deposits a little BaSO_{4}, thereby giving rise at the same time to the separation of an equivalent amount of K_{2}CO_{3}.
The different solubilities are shown in Fig. 128. In this diagram the solubility of the two barium salts has been neglected. A is the solubility of K_{2}CO_{3}.2H_{2}O; addition of BaCO_{3} does not alter this. B is the solubility of K_{2}CO_{3}.2H_{2}O + K_{2}SO_{4} + BaCO_{3}. A and B almost coincide, since the pota.s.sium sulphate is very slightly soluble in the concentrated solution of pota.s.sium carbonate. D gives the concentration of the solution in equilibrium with K_{2}SO_{4} + BaSO_{4}. The most interesting point is C. This solution is obtained by adding a small quant.i.ty of water to BaCO_{3} + K_{2}SO_{4}, whereupon, being in the transition interval, BaSO_{4} separates out and an equivalent amount of K_{2}CO_{3} goes into solution. C is the end point of the curve CO, which is called the Guldberg-Waage curve, because these investigators determined several points on it.
In their experiments, Guldberg and Waage found the ratio K_{2}CO_{3} : K_{2}SO_{4} in solution to be constant and equal to 4. This result is, however, not exact, for the curve CO is not a straight line, as it should be if the above ratio were constant; but it is concave to the abscissa axis, and more so at lower than at higher temperatures.
The following table refers to the temperature of 25. The Roman numbers in the first column refer to the points in Fig. 128. The numbers in the column [Sigma]_k__{2} give the amount, {329} in gram-molecules, of K_{2}CO_{3} + K_{2}SO_{4} contained in 1000 gram-molecules of water:--
SOLUBILITY DETERMINATIONS AT 25.
-----+-------------------------------------+-----------------------+
100 gms. of the
solution contain,
No.
Solid phases.
in grams,
K_{2}CO_{3}
K_{2}SO_{4}
-----+-------------------------------------+-----------+-----------+ I.
K_{2}CO_{3}.2H_{2}O + BaCO_{3}
53.2
--
II.
{ K_{2}CO_{3}.2H_{2}O + K_{2}SO_{4} }
53.0
0.023
{ + BaCO_{3} }
III.}
K_{2}SO_{4} + BaCO_{3}
{ 28.5
0.886
IV. }
{ 22.1
1.72
V.
BaCO_{3} + K_{2}SO_{4} + BaSO_{4}
17.81
2.485
VI. }
K_{2}SO_{4} + BaSO_{4}
{ 12.6
3.92
VII.}
{ 5.85
6.76
VIII.
K_{2}SO_{4}
--
10.76
IX. }
BaCO_{3} + BaSO_{4}
{ 7.35
0.602
X. }
{ 2.85
0.173
-----+-------------------------------------+-----------+-----------+ [Transcriber's note: table continued below...]
-----+-----------------------+-----------------+-----------
1000 moles
of water contain,
K_{2}CO_{3} No.
in moles,
[Sigma]_k__{2}
-----------
K_{2}SO_{4}
K_{2}CO_{3}
K_{2}SO_{4}
-----+-----------+-----------+-----------------+----------- I.
147.9
--
--
--
II.
147.8
0.051
--
--
III.}
52.58
1.296
--
-- IV. }
37.79
2.333
--
--
V.
29.11
3.220
32.32
9.03
VI. }
19.66
4.853
--
-- VII.}
8.724
7.995
--
--
VIII.
--
12.47
--
--
IX. }
10.43
0.676
11.11
15.0 X. }
3.828
0.184
4.0
21.0 -----+-----------+-----------+-----------------+-----------
The Guldberg-Waage curve at 100 was also determined, and it was found that the ratio K_{2}CO_{3}: K_{2}SO_{4} is also not constant, although the variations are not so great as at 25.
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