REACTIONS I N FUSED SALT MEDIA. I1 STUDIES I N SOLVOLYSIS BY J. F. G . HICKS
WITH WALLACE A. CRAIG
I n a previous paper1 it was pointed out that the mechanism of the interactions between certain fused salts and certain solutes dissolved by them probably resembled hydrolysis quite closely. A study of the interactions of both lead oxide and lead chromate with fused sodium chloride furnished evidence for such a conclusion, as did also a study of the behavior of lead chromate in the presence of a fused mixture of sodium and potassium nitrates (4. v.). For such reactions the name “solvolysis” was suggested. It was the purpose of this investigation to ascertain whether these “solvolyses” were or were not true equilibria, thereby defining their relation to hydrolysis. I. Hydrolysis Experiments These were carried out for a twofold purpose: (1) t o furnish a means of comparison when studying the solvolytic (fusion) reactions from the standpoint of the mass law, and (2) to further establish the analogy between hydrolysis and solvolysis through the formation of analogous “basic lead chromates” by the two methods. A. Hydrolysis Under Ordinary Pressure: (a) Lead Chromate.-Increasing quantities of lead chromate were boiled with 100 grams of water, loss from evaporation being made up from time to time. The resulting “basic lead chromates” varied in color from dark yellow to very deep orange; no red ones were observed, nor did the hand-lens (Hastings triplet) reveal any red particles. At the end of each experiment a known quantity of ferrous ammonium sulphate was added to the filtrate from the insoluble “basic chromate” and the excess Fe ++ titrated with potassium permanganate in the usual manner. 1
Jour. Phys., Chem., 25, 545 (1921).
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The continuous increase in weight of initial samples has no particular significance. The samples were weighed out, arranged in order of increasing weight and then treated. This order of treatment was selected in order that the largest sample should be exposed to the action of a particular reagent for the longest time. The above arrangement holds true for all similar succeeding series of experiments. . Results of hydrolysis experiments are tabulated below :
TABLEI* Hydrolysis of Lead Chromate at 100'. Concentration of H20 Constant; of PbCr04 increasing Time (minutes)
10 20 30 40 -50 60 70 80 90 100 110 120 130 140 150 1GO 170 180 190 200
h i t . CrOs in filtrate mols) (weighed CrO3(mols) as PbCr04) (m) (M)
0.0018 0.0029 0.0039 0.0056 0.0076 0.0090 0.0105 0.0122 0.0130 0.0148 0.0159 0.0182 O.Ol9G 0.0213 0.0224 0.0251 0.0274 0.0287 0.0313 0.0322
0.053 0.065 0.058 0.0418 0.0444 0.0465 0.0491 0.03177 0.03133 0.03160 0.03184 0.03218 0.03245 0.03277 0.03284 0.03331 0.03353 0.03373 0.03391 0.03409
yo init. Cr03 removed by hydrolysis
0.17 0.17 0.21 0.33 0.58 0.72 0.87 0.96 1.02 1 .08 1.16 1.19 1.25 ' 1.30 1.27 1.32 1.29 1.30 1.25 1.27
K hydrolysis -- m
M-m
0.0017 0.0017 0.0021 0.0032 0.0058 0.0073 0.0087 0.0007 0.0103 0.0109 0.0111 0.0121 0.0127 0.0132 0.0128 0.0125 0.0131 0.0132 0.0127 0.0129
( b ) Lead Oxid.-Boiling with water for 6-8 hours did not dissolve sufficient PbO for the estimation of Pb++. The acidified filtrate darkened when saturated with H B S . *The writer gratefully acknowledges his indebtedness t o Mrs. Mary B. Hicks fo,r assistance in checking this and subsequent tables.
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( c ) Lead Chromate-Lead Oxid (50-50 molar yomixture).This mixture was hydrolyzed by boiling for seven hours as for the other hydrolyses. The reddish orange “basic lead chromate” contained 81.78Y0 PbO and 18.0670 CrOa. B. Hydrolysis Under Increased Pressure:” ( a ) Lead chromate and an excess of water were Chromate.-Lead heated in sealed bomb-tubes in the usual manner. Analytical procedure was the same as for the hydrolyses under ordinary pressures. Results are tabulated below :
No.
Temp. ( ” C)
Time (hrs.)
Init. mols CrOt (weighed as PbCrOa) (M)
1 2
150-155 150-155
1. 3
0.19 0.27
Crop in filtrate (mols) (m)
0.009 0.013
% of M K hydrolysis removed m = by hyr drolysis M-m
-
4.7 4.8
0.049 0.054
Both “basic lead chromates” were deep orange in color, the second being slightly darker than the first. (b) Lead Oxid.-Lead oxid (PbO) was treated in the same manner as above. The acidified filtrate turned yellowish brown when saturated with H2S, but the resulting PbS could not be filtered out. It is probable that a larger quantity of PbO than indicated was in solution while the pressure was above normal, but predipitated when normal pressure was reattained. ( c ) Lead Chromate-Lead Oxid (50-50 molar yomixture).This mixture was treated as above, with excess water, in bombtubes. Results follow:
* Acknowledgment is hereby made to Mr. E. L. Gustus for the preparation of the bomb-tubes and the treatment of the samples hydrolyzed under increased pressure.
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TABLE 111 Hydrolysis of 50-50 Molar % PbCr04-PbO under Increased Pressure No‘
1 2
1
I
Temp. ( ” C)
150-155 150-155
I
1
Time (hrs.)
PbO ( %)
1 2
82.28 83.64
Color of residue
I
17.58 16.22
1
Deep orange-red Deep orange-red
11. Solvolysis Experiments A . Lead Chromate in “Nitrate Flux.” -Increasing quantities of lead chromate were “solvolyzed” in increasing quantities of a fused mixture of sodium and potassium nitrates (50-50
TABLE IV Solvolysis of PbCr04 in Nitrate Flux a t 225”-230’. Concentration of both PbCrOl and Flux increasing Time (minutes)
10 20 30 40 50 GO 79 80 90 100 110 120 130 140 150 160 179 180 190 200
Init. CrO3 (mols) (weighed as PbCrOa) (M)
0.0015 0.0016 0.0017 0.0019 0.0020 0.0022 0.0023 0.0026 0.0027 0.0028 0.0029 0.0032 0.0033 0.0037 0.0043 0.0045 0.0052 0.0054 0.00G9 0.0076
CrO3 in filtrate (mols) (weighed as PbCr03) (m)
0.0413 0.0423 0.0446 0.03103 0.03150 0.03245 0.03373 0.03543 0.03699 0.03788 0.03915 0.021129 0.021187 0.021372 0.021608 0.021675 0.021974 0.022038 0 022597 0.022862
% i n k Cr03 removed by solvolysis
0.8G 1.44 2.70 5.42 7.95 11.10 16.19 20.89 25.13 28.14 31.55 35.29 35.99 37.08 37.39 37.22 37.14 37.74 37. 66 37.66
K solvolysis =- m
M-m
0.01 0.01 0.03 0.06 0.11 0.10 0.21 0.24 0.85 0.40 0.45 0.52 0.57 0.81 0.59 0.67 0.63 0.59 0.60 0.62
’
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.
molar %, eutectic a t 218”)l after the manner of the previous work in preparing “basic lead chromates” by fusion methods2 The solvolyses were performed simultaneously in porcelain crucibles in an oven a t 225”-230”. The insoluble residues were bright red. The cooled fusions were extracted with water a t room temperature (to avoid hydrolysis, which would increase the concentration of (CrO,)’ in the filtrates), and the CrOa determined as PbCrOl in the usual manner. Results are tabulated in Table IV. In order to make sure that this “solvolysis” represented a true state of equilibrium, increasing quantities of lead chromate were fused with a nearly constant quantity of nitrate flux (37 g *O0.05 g = 2 mols, approximately) for two hours at 225”-230”, using the same procedure as before. The difference in weight between successive initial samples of PbCrOe were purposely made much larger than in the preceding series of experiments. Table I V would indicate that equilibrium is attained in about 100 minutes, hence a two-hour run should be sufficient. Should a true state of equilibrium TABLE V Check-runs for Equilibrium. Solvolysis of PbCrOc in Nitrate Flux a t 225 “-230 ’. Concentration of Flux Constant; of PbCrOI increasing Init. CrOr CrOI in filtrate :mols) (weighed mols) (weighed as PbCrOa) as PbCrOd) (M) (m)
No.
1 2 3 4 5 6 7 8 9 10
’
0.0026 0.0042 0.0086 0.0162 0.0240 0.0306 0.0355 0.0459 0.0588 0.0621
0.0009 0.0014 0.0024 0.0040 0.004O 0.0054 0.0058 0.0065 O.OOG9 0.0072
of init. Cr03 removed by Folvolysis
34.62 33.33 . 27.91 24.89 20.42 17.68 16.34 14.09 11.73 11.58
K solvolysis = m M--m
_-
0.53 0.50 0.338 0.328 0.256 0.214 0.195 0,165 0.133 0.131
‘Landolt-Bornstein-Roth : “Physicalische-chemische Tabellen,” 4 Auflage, pp. 611-635.
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obtain, the CrOa removed by solvolysis should increase with the time, but the percent of a mol of CrOa so removed should decrease. Results are shown in Table V. These results indicate a true state of equilibrium between lead chromate and the fused nitrate flux. B. Lead Chromate in Fused Sodium Ch1orid.--Since could not be obtained in an a temperature of 850'-870' ordinary oven, these solvolyses could not be run simultaneously. The porcelain crucibles containing the several fusion-mixtures were heated in a resistance-furnace which accommodated but one crucible at a time. The insoluble residue was orange to light brown in color. The temperature was controlled by means of a Pt/Pt-Rh thermo-couple. In other respects the procedures were the same as before. Results of this series of solvolyses are tabulated below : TABLE VI Solvolvsis of PbCrOn in Fused Sodium Chlorid a t 850'-870". Relati;e Concentration of Both PbCrOl and NaCl .creasing Time (minutes)
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 1so 190 200
nit. CrOs (rnols) (weighed as PbCr04) (M)
0.0389 0.0397 0.02109 0.02114 0.02127 0.02132 0.02136 0.02139 0.02152 0.02155 0.02161 0.02178 0.02182 0.02189 0.02197 0.02203 0.02211 0.02217 0.02223 0.02229
:rO3 in filtrate (mol:) (weighed a s PbCrOd)
% init. CrOt removed by solvolysis
solvoIvois --m M-m
(m)
0.056 0.0411 0.0417 0.0427 0.0435 0.0441 0.0450 0.0458 0.0473 0.0486 0.03102 0.03121 0.03133 0.03144 0.03154 0.031G.5 0.0317.5 0.03180 0.03187 0.031F)l
0.67 1.13 1.43 2.18 2.76 3.11 3.68 4.17 4.80 5.61 G.33 6.87 7.31 7.60 7.82 8.13 8.29 8.30 8.38 8.34
*
,
0.007 0.01 0.02 0.03 0.03 0.03 0.04 0.05 0.05 0.06 0.07 0.06 0.06 0.06 0.06 0.06 0.06 0.05 0.05 0.05
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In order to make sure that a true state of equilibrium existed in the system PbCrOd-NaC1 under the above conditions, a series of check-runs was made, using a nearly constant quantity of sodium chlorid (30 g ~ 0 . 0 5g =: 0.5 mol+), exactly as for the preceding series of check-runs with nitrate flux. Wider differences in weight between successive initial samples of lead chromate were purposely made, as before. Results follow: TABLE VI1 Check-runs for Equilibrium. Solvolysis of PbCr04 in fused NaCI at 850"-870". Concentration of NaCl Constant; of PbCrOa increasing
No.
1 2 3 4
5 6 7 8 9 10
[nit. CrOs (mols) :r03 in filtrate (mols) (weighed as (weighed as PbCrOdJ PbCrOl) (M) (m)
0.00178 0.00377 0.00653 0.0117 0.0149 0.0158 0.0213 0.0227 0.0248 0.0294
'
0.00012 0.00016 0.0001s 0.00021 0.00023 0.00022 0.00028 0.00029 0.00031 0.00035
% init. Crdr iemcsved by solvolysis
K solvolysis =- m M-m
6.74 4.24 2.76 1.79 1.47 1.x\ 1.31 1.28 1.25 1.19
0.0724 0.0444 0.0284 0.0183 0.0150 0.0141 0.0133 0.0130 0.0126 0.0120
These results indicate a true state of equilibrium for the system PbCr04-NaCl under the recorded conditions. C. Lead Oxid i.n Fused Sodiwz Ch1orid.-It has been shown1 that one mole of lead oxid (PbO) completely dissolves in five mols of fused sodium chlorid a t 880" and that conversion to lead chlorid is practically complete in two hours. Hence one should expect a very nearly complete reaction at a temperature slightly above the meltingpoint of sodium chlorid ; the solvolyses were carried outeat 850"-870" as in the case of lead chromate. In view of this, one should also expect that the calculation of a constant of Jour. Phys. Chem., 25,545 (1921).
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solvolysis would be of no value, and that a series of checkruns, made in the previous series, should be omitted. The appended results would seem to justify this assumption and procedure :
TABLE VI11 Solvolysis of PbO in Fused NaCl a t 850'-870". Relative Concentration of PbO and NaCl both increasing Time (minutes)
Init. PbO (mols)
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
0.0008 0.0009 0.0010 0.0011 0.0012 0.0013 0.0014 0.0015 0.0016 0.0017 0.0018 0.0019 0.0020 0.0021 0.0022 0.0023 0.0024 0.0026 0.0027 0.0028
PbO (mols) solvolyzed to PbClz (weighed as PbCr04)
0.0001 0.0004 0.000B
0.0009 0.0010 0.0012 0.0013 0.0014 0.001.5 0 .OOlG 0.0017 0.0018 0.0019 0.0020 0.0021 0.0022 0.0023 0.0025 0.002G 0.0028
% init. PbO solvolyzed to PbC12
12.50 44.44 GO. 00 81.82 83.33 92.31 92.86 93.33 93 * 75 94.12 94.44 94.74 95.00 95.24 95.45 95.65 95.83 96.15 9G. 29 96.43
These results point to a completion of reaction, or at least to a set of conditions under which the reaction would soon be completed. The insoluble residue was white, containing a small quantity of admixed PbO (orange). From these and the previous observations it would seem practically certain that the reaction would be complete a t a temperature not much higher than that of the experiments. An inspection of the table will also show that the calculation of a constant of solvolysis would be without significance.
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TABLE IX Solvolysis of Silver Chromate in Nitrate Flux a t 230'-240'. Concentration of Flux Constant; Concentration of Silver Chromate increasing Time
(minutes)
10 20 30 40 50 60
io
80 90 100 110 120 130 140 150 160
170 180 190 200
[nit. CrOs (mols (weighed as AgzCrOd (M)
0.037536 0.0215071 0.0222607 0.02301% 0.0237678 0.0,245214 0.0252750 0.0260284 0.0267820 0.027535.5 0.0282891 0.0290426 0.0297962 0.0105497 0.0113033 0.0120568 0.0128104 0.0135639 0.0142175 0.0150710
CrO3 in filtrate (mols) (weighed as AgLrOd
% init. CrOs removed by solvolysis
K solvolysis -- m M-7??.
9.08 3.77 1.17 0,571 0.433 0.281 0.417 0.109 0.146 0.173 0.179 0.060 0.101 0.135 0.136 0.095 0.111 0.078 0.106 0.060
0.109 0.039 0.0175 0.0057 0.0044 0.0028 0.0042 0.0011 0.0015 0.00173 0.0018 0.0006 0.0010 0.00135 ' 0.0014. 0.00095 0.0011 0.0008 0.0011 0.0006
(m)
0.04742 0.04567 0.04262 0.04172 0.04163 0.04127 0.04220 0.0566 0.0599 0.04130 0.04148 0.0554 0.0599 0.04142 0.04154 0.04115 0.04142 0.04105 0.04151 0.0590
TABLE X Solvolysis of Silver Chromate in Fused Sodium Chloride a t 850 '-870 Constant Concentration of Flux; increasing Concentration of Silver Chromate
'.
Time (minutes)
5 10 15 20 25 30 40
Initial CrOa (mols) (weighed as AgzCrO4)
0.03392 0.03754 0.02115 0.02151 0.02187 0.02226 0.02301
Cr03 equivalent to PPt. AgC1 . (mols)
0.03391 0.03'719 0.02112 0.02148 0.02179 0.02224 0.02287
% CrOs solvolyzed out
99.7 95.3 97.4 913.0
95.7 99.1 98.6
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D. Silver Chromate in Nitrate Flux.-The
procedure for this series of solvolyses was the same as for lead chromate except that the temperatufe of operation was 230"-240". This would lead one t o expect a state of equilibrium to be attained in a shorter time than in the previous solvolyses, which prediction was borne out experimentally. Equilibrium is attained at 80 minutes a t 230 "-240 " as against 100 minutes at 225"-230". The extent of the solvolysisreaction could not, of course be predicted, it depending upon the nature of the substance solvolyzed and the solvolyzing medium.
TABLE XI Solvolysis of Barium Chromate in Nitrate Flux at 230"-240'. Concentration of Flux Constant; of Barium Chromate increasing Time (minutes)
10 20 30 40 50 GO 70 80 90 100 110 120 130 140 150 160 170 180 ,190 200
.
Initial CtOI (mols) (weighed as BaCrOd) (M)
'
0.000987 0.00198 0.00296 0.00395 0.00494 0.00593 ' 0.00691 0.00790 0.00890 0.00980 0.01085 0.01184 0,01383 0.01381 0.01480 0.01580 0.01680 0.01779 0.01879 0.01975
CrOs in filttates (mols) (weighed as BaCrOa) (m)
0.000184 0.000187 0.000257 0.000201 0.000232 0.000238 0.000221 0.000218 0.000263 0.000340 0.000286 0.000338 0.000321 0.000356 0.000338 0.000296 0.000334 0.000327 0.000334 0.000411
init. CrOI removed by solvolysis
18.75 9.48 8.69 5.11 4.70 4.01 3.21 2.76 2.96 3.44 2.64 2.85 2.50 2.57 2.28 1.87 1.99 1.84 1.78 2.08
I
C solvoIysis
m M-m
=-
0.220 0.104 0.096 0,054 0.049 0.042 0.033 0.029 0.030 0.036 0.027 0.030 0.026 0.027 0.024 0.019 0.020 0.019 0.018 0.021
In order to save time, a nearly constant quantity of flux was used in each solvolysis, that is, the procedure of the checkruns for lead chromate solvolysis (Table V) was followed rather
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than that of the other runs (Table IV). Since a state of equilibrium was shown to exist in this and other similar experiments, it seemed safe to assume such a state in these solvolyses. I n this way double work was saved. Twenty samples of silver chromate varying in weight from 0.25 gram to 5.0 grams were solvolyzed with 25 grams ( * 0.05 g) of the nitrate flux. The CrOs removed by the solvolysis was determined as silver chromate in the usual manner. Results are shown in Table IX. E. Silver Chrmate in Fused Sodium Ch1orid.-Seven sam. ples of silver chromate were fused with a constant quantity (25 g *0.05 g) of sodium chloride at 850"-870". Complete solvolysis is' brought about almost immediately, as an in-
Fig. 1 No. 1. Hydrolysis of lead chromate a t 100". PbCi-04 increasing :HgO constant. No. 2. Hydrolysis of lead chromate under pressure a t 150"-155 '. No. 3. Hydrolysis of 50-50 molar percent PbO-PbCr01 under pressure. No. 4. Solvolysis of PbCrOc in nitrate flux a t 225"-230". Concentration of flux and PbCrO4 increasing. No. 5. Solvolysis OF PbCrOd in nitrate flux a t 225'-230°. Flux constant : PbCrOc increasing. No. 6. SolvoIysis of PbCrOI in fused NaCl a t 850"-870". NaCl and PbCrO4 increasing. No. 7. Solvolysis of PbCr04 in fused NaCl a t 850"-870". NaCl constant : PbCr04 increasing. No. 8. Solvolvsis of AgCrO4 in nitrate flux a t 23Oo-24O0. Flux constant : AgCrOl increasing. No. 9. Solvolysis of BaCrOd in nitrate flux a t 230'-240'. Flux constant : BaCrOl increasing.
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J . F. G. Hicks with Wallace A. Craig
spection of the results will show (Table X). Since the reaction proceeds to completion, a “constant of solvolysis” would have no signification, and was TIM IN MINUTES not calculated. Results are shown in TableX. F. Solvolysis of Barium Chromate in Nitrate Flux.-Increasing quantities of barium chromate were solvolyzed with constant quantity (25 g * 0.05 g ) of “nitrate flux’’a t 230 ‘-240 O , from 10 to 200 minutes, at tenminute intervals. The usual procedure was followed. The cold fusions were extracted with cold water, and CrOa determined in the filtrates in the usual manner. Results are shown in Table Fig. 2 XI. No. 1. Solvolysis of PbO in fused It had been planned to study NaC1 a t 850°-*700. NaC1 and the solvolysis of barium chroPbO increasing. mate in fused sodium chloride, No. 2. Solvolysis of AgCrOr in fused NaCl a t 850”-870°. NaCl but time did not permit. constant. AgCrOl increasing. For the sake of convenience of comparison a series of graphs have been prepared, in which the percentage of initial CrOa removed by solvolysis has been plotted against the time of solvolysis, Figs. 1 and 2. 111. Summary: Conclusions
(1) Solvolysis of lead, silver and barium chromates by a fused mixture of sodium and potassium nitrates (50-50 molar yo) represent states of equilibria analogous tb hydrolysis. Of these reactions the solvolysis of lead chromate is the most complete, that of barium chromate next, and that of silver chromate the least complete. The regularity with which the constants of solvolysis vary between consecutive observations follows the same order. The marked irregularity in the case
Reactions in Fused Salt Media. I1
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of silver chromate suggests the possibility of another factor entering into the reaction; this will be investigated later. (2) Solvolysis of lead chromate by sodium chlorid (fused) also represents a true state of equilibrium. In the case of silver chromate the reaction proceeds t o completion ; it was not tried for barium chromate. (3) I n the solvolytic reactions studied, equilibrium is attained in about 100 minutes. Constants of solvolysis can be calculated with a fair degree of accuracy, considering the conditions of experimentation. (4) Solvolysis of lead oxid by fused sodium chlorid proceeds practically to completion at 850 “-870 ”. This was expected, and confirms previous observations. The complete solvolysis of silver chromate by fused sodium chlorid a t 850”-870 O is brought about in a shorter time. (5) The present investig?tion would seem to show that lead chromate and lead oxid (PbO) combine by simple “molecular addition” affer part of the former has undergone solvolysis in the nitrate flux. In order words, the red “basic lead chromates” are “higher order” compounds. (6) The relatively small degree of solvolysis of lead cliromate and the practically complete solvolysis of lead oxid (PbO) by the same reagent under the same conditions will account for: ( a ) The failure to obtain red “basic lead chromates” when a fusion containing lead chromate, lead oxid (PbO) and sodium chlorid is quickly cooled. ( b ) The appearance of the red “basic lead chromates” when such a fusion as the above (a) is slowly coo1ed.l (7) A study of the solvolytic reactions apparently confirms the assumption of the dissociation PbzCr05% PbO PbCr04. This assumption was previously offered as an explanation of the failure to obtain red “basic lead chromates” when a fusion containing lead chromate, lead oxid (PbO) and sodium chlorid was poured into water, and agrees well
+
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J . F . G. Hicks with Wallace A . Craig
with the evidence offered by the thermal equilibrium diagram plotted for the system lead chromate-lead oxide1 (8) Graphs showing the rates of solvolysis for the different reactions studied have been plotted. Stanford University California 'Jour. Phys. Chem., 25, 545 (1921).
