September, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
945
Effect of Sodium Chloride on Hydrogen-Ion Concentration and Stability towards Alkali of Chromic Chlorides’ By K. H. G u s t a i s o n WIDCN-LORD TANNING Co., DANVERS. MASS.
Addition of s m a l l a m o u n t s of s o d i u m chloride t o of increase in hydrogen ion DDITION of neutral solutions of c h r o m i u m chlorides causes a decrease in were found for the various chlorides to acid soluH-ion concentration; w i t h increase i n basicity a n d din e u t r a l c h l o r i d e s . The tions increases the hyl u t i o n the change is m o r e pronounced. Larger a m o u n t s drogen-ion concentration of change in actual acidity upon of s o d i u m chloride show an increase in t h i s figure. The standing resulted in the same the solution. This effect, the stability towards alkali is considerably reduced where decrease over the whole series, so-called “neutral salt effect,” dilution of solution a n d higher basicity of the s a l t and was thus independent of has in most. instances been exalso lessen t h e stability. plained satisfactorily by the the neutral salt concentraIt is concluded t h a t addition of s o d i u m chloride hydration hypothesis. This tion. causes a stabilization of chlorine in the coardinative explanation is comparatively T h e s e data were intersphere, explaining its lower H-ion concentration a n d simple for solutions exhibitpreted to indicate that the precipitation figure; b u t where large a m o u n t s of soing a constant p H value, such chief underlying factor was d i u m chloride are added t h e increase i n concentration removal of solvent by hydraas common aqueous solutions of acids, but solutions of of t h e c h r o m e s a l t by hydration of s o d i u m chloride is tion of the neutral salt, and chromium salts and similar evidenced by t h e increase i n H-ion concentration. that the hydrolysis of the types derived from a relachromic salt was not responNo relationship“ exists between H-ion concentration a n d stability t o alkali. tively weak base and a strong sible for the “neutral salt acid are complicated by the The d a t a a r e fully i n accord w i t h the hydration hyeffect.” slowly attained hydrolysis The presence of neutral pothesis in combination w i t h the concept of c h r o m e equilibria or the change from c o m p o u n d s on t h e basis of t h e coordination theory. chlorides in aqueous soluaquo base to hydroxo salt. t i o n s of c h r o m i c sulfates Moreover the possibility of tends to delay the precipitaconstitutional changes, affecting the physico-chemical be- tion of the chromic salt upon addition of alkali. This inhavior, has to be considered. Further intricacy is en- creased stability towards hydroxyl ions is evidenced by the countered in systems without a common anion-. g., so- greater amount of the reagent required to cause a permanent dium chloride-chromium sulfate-on account of a partial turbidity, and is believed to be due partially to the increase double decomposition. Consideration of this unknown factor in hydrogen-ion concentration on addition of neutral chlorides. was not deemed advisable, and chromic chloride and sodium The constitutional or coordinative factor has been found by chloride, the latter on account of its practical importance, the author to influence the physico-chemical nature of tho were selected for study in the present investigation. chromic salts. It was predicted that addition of small The effect of neutral chlorides upon the hydrogen-ion amounts of sodium chloride to chromic chlorides would concentration of aqueous hydrochloric acid has been studied decrease the actual acidity but that larger amounts would by Harnedz and Thomas and B a l d ~ i n . ~The , ~ increase in have the opposite effect. The constitutional changes were actual acidity, when plotted as pH, was found to be a linear considered to more than counterbalance the hydrating function of the salt concentration. The slope of the curve action of sodium chloride, tending to increase the hydrogenfor the 0.004 AT hydrochloric acid was greater than for the ion concentration. The point where precipitation of the in0.1 N acid. As a t greater concentrations the opposing soluble basic chromium chloride occurs was expected to deforces resulting from incomplete ionization are more promi- pend upon the substitution in the internal sphere and its relanent, the dilute solution shows better the rate of increase in tion to the actual acidity, was believed to be only accihydrogen ion with hydration of the added neutral chloride. dental. This investigation was therefore undertaken to test The order of increase in hydrogen ion produced by the differ- these views employing p H values and stability towards ent chlorides was the same as the degree of hydration-that alkali as criteria. is, increased with decrease in atomic volume of the cation. Materials This concordance and the excellent agreement between hydration values for the various cations calculated by this A C.P. grade of chromic chloride (CrCls.4 H20) served as maand other methods was strong evidence for the justifica- terial for the stock liquors. Upon analysis, however, this salt tion of the hydration hypothesis.& The behavior of solutions showed only 86.0 per cent acidity. This chromic chloride dissolved in boiling distilled water to give a concentrated of green normal chromic chloride under the same conditions was solution and the final liquor showed a CrzOs content of 210.2 was investigated by Miss B a l d ~ i nand , ~ here also the decrease grams per liter. This stock liquor was used as the basis in pH was a linear function of the molality of neutral salt of the liquors with reduced acidities-namely, 61.5 and 44.1 in the chromic solution, and the same regularity and order per cent.
A
Presented before the Division of Leather and Gelatin Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April ti te 10, 1925. 2 J . A m Chcm. SOL.,37, 2460 (1915). ‘Thomas and Baldwin, Zbid., 41, 1981 (1919). 4 Baldwin, J . Am. Leather Chem. Assor., 14, 10 (1919). ‘Wilson, J . A m . Chem. SOL.,42, 715 (1920) 1
A dilute sodium hydroxide solution was employed as neutralizing agent, added slowly under constant stirring. As this procedure caused considerable dilution the liquors were again concentrated by boiling. Measurements of hydrogen-ion concentration were made from time to time until equilibrium was practically established. After “aging” for about 4 weeks the neutralized liquors were diluted to the required concentrations. The resulting liquors gave upon analysis 188.5 and 191.2 grams
I.VDUSTRIAL AND E.VGISEERliVG CHEMISTRY
946
Cr203 per liter for the 61.5 and 44.1 per cent acid liquors, respectively. Solutions of sodium chloride of 1, 2, 3, 4, and 5 molal strength were prepared, checked by analysis for chlorine, and tested for neutrality. Series of solutions with chromic concentration and salt content as noted in the tables were made up from these stock liquors by transferring an exactly measured quantity of the chromic chloride solution into 500-cc. volumetric flasks, and making up to the mark with the necessary amount of sodium chloride solution and distilled water. For example, the 86.0 per cent acid solution containing 4.2 grams CrsOs per liter and 1 M NaCl was prepared by pipetting 10 cc. of the stock liquor into a 500-cc. volumetric flask, adding 250 cc. of the 2 M NaCl, and making up to the mark with distilled water. The dilution figures were checked by the usual volumetric chrome determination.
MOLALITY OF ADDED NaC/ F i g u r e 1-H-Ion C o n c e n t r a t i o n of 44.1 Per c e n t Acid C h r o m i c C h l o r i d e S o l u t i o n I m m e d i a t e l y a f t e r A d d i t i o n of Various A m o u n t s of N a C l
Technic
Hydrogen-ion concentration was determined by means of a potentiometer, using saturated potassium chloride-calomel cell and a temperature of 25” C. In details of technic and checking of data the recommendations of Clark6were followed. The amount of alkali hydroxide required to cause a permanent turbidity of the chromic solution mas obtained by adding to 5 or 10-cc. portions of chromic chloride 0.1 N sodium hydroxide drop by drop under agitation, the quantity depending d permanent turbidity on the chrome content and acidity. upon looking through the test tube from the side determined the proper amount of sodium hydroxide, usually referred to as “precipitation figure.” In determinations of this figure for chromic sulfate, this point is best indicated when looking through the test tube from the bottom. For solutions of chromic chloride containing salt the behavior is similar to that found for chromic sulfates.
Results The effect of sodium-chloride a t various molalities upon the hydrogen-ion concentration and precipitation figure of chromic chlorides of varying acidities is shown in Tables I to 111. fi
“The Determination of Hydrogen Ions,” 1922.
Co.. Baltimore, Afd
Wilhams 81 Wilkins
Vol. 17, No. 9
44.1 Per c e n t Acid Chromic Chloride Table I NaCl molality
0
0.23 0.3 1
2 :i
4 2
---pH Immcdiately 2.63 2.73 2.76 2.74 2.59 2.43 2.2s 2.12
PRECIPITATIOX FIGURE,
VALVE-2 weeks 1.90 grams 2.97 3.09 3.15 3.17 3.07 2.91 2.i6
6 weeks
Cc. 0.1 S S A O H (10-cc. sample) Immediately 6 weeks
CnOs per liler 3.10 3.21 3.23 3.20 3.07 2.91 2.76 2.60
2.60 3.S3 grams Crdh per liler
2.5 1.9 1.5 1.3 1.1
0.9 0.8 0.8
2.4 2.0 1.6 1.4 1.2 1.1 1.0 1.0
11ninediatelg 0 2.41 0 . 2 5 2.49 0 3 2.51 1 2.49 2 2.39 3 2.2s 4 2.17 2.06 5
I veck 2 weeks 8 weeks Immediately 8 weeks 2.04 2.SS 3.01 4.9 4.8 2.78 3.03 3.15 3.6 3.7 2.S.i 3.04 3.12 3.1 2.9 2.s.5 3.04 3.09 2.8 2.4 2.80 2.93 2.96 2.4 2.0 2.69 2.7s 2.so 2.0 1.8 2.65 2.61 2.6:? 1.8 1.6 2.42 2.46 2.46 1.6 1.5 7.66 g r a m s Cr203 per liler lni ine(5-cc, sample) diately 3 hoiirs 24 holil-s 7 days G weeks Immediately 2.19 2.2s 2.33 2.68 0 2.95 4.1 2.31 2.33 2.40 2.76 0:25 2.97 3.2 2.46 2.24 2.31 0.5 2.82 2.99 3.1 1 2.24 2.30 2.46 2.79 2.96 2.5 2 2.15 2.22 2.44 2.71 2.80 2.1 3 2.06 2.13 2.38 2.59 2.64 1.9 2.30 2.45 4 1.97 2.05 2.47 1.d 2.23 2.31 5 1.88 1.OT 2.31 1.7 9.55 grams CrzO8 per liler Immediately 3 weeks 8 weeks Immediately 8 weeks 2.12 2.85 0 2.92 5.0 5.4 2.16 2.92 0.25 2.96 4.2 4.3 2.95 2.20 2.88 0.3 3.8 3.9 1 2.18 2282 2.91 3.1 3.3 2 2.12 2.69 2.76 2.5 2.9 2.60 3 2.03 2.54 2.3 2.7 2.43 1.93 2.39 2.2 2.4 4 2.27 1.64 2.23 2. I 2.2 10.10 grams CYZOS Oer Mer Imme1 week 6 weeks Immediately 6 weeks diatelv 24 hours 2.75 2.82 2.34 0 2.00 9.0 8.9 2.825 2.79 2.41 0 . 2 5 2.04 7.8 8.7 2.77 2.79 2.42 0.5 2.05 7.2 7.6 2.72 2.75 2.44 1 2.06 6.8 7.2 5.9 2.5s 2.60 2.39 2 2.01 6.6 2.45 2.45 5.7 2.32 3 1.94 6.2 5.5 2.31 2.31 2.21 5.6 40 1.86 1.79 5.2 5.4 2.15 2.15 2.09 38.20 grams CnO3 per liler Immediately 1 week ti weeks 1 week 2.63 15.2 2.63 1.83 2.59 14.6 2.60 1.84 2.56 14.0 2.57 1.84 2.53 13.5 2.54 1.83 2.40 12.6 2.41 1.s2 2.27 12.8 2.28 1.82 2.12 2.12 13.0 1.76 1.96 13.0 1.96 1.71
pH VALUE-Figure 1 illustrates graphically the pH values of the six series a t various salt concentrations measured immediately after salt addition. The same data when equilibria are established (after 6 weeks) are shown in Figure 2 . The time factor is shown in Figure 3 for the series of solutions with a Crz03content of 7.66 grams per liter. These data bring out the fact that addition of sodium chloride in small amounts causes a diminution in hydrogenion concentration, with a minimum a t 0.5 M KaC1, but that solutions with greater molalities exhibit an increase in actual acidity. The increase in pH a t lower salt concentrations is favored by dilution and the increase of pH value with time is also greater for these dilute solutions. The two series of 19.1 and 38.2 grams Cr203 per liter show a slight increase in pH immediately after.dilution, but increase in hydrogenion concentration when equilibrium is reached. From Figure 1 it is evident that the increase in the actual acidity for solutions with large amounts of sodium chloride is more rapid for the dilute solutions, and the slopes of the curves vary. The same series illustrated in Figure 2 gives a t equilibrium almost parallel straight lines. indicating that the changes occurring upon aging tend to bring the same rate of increase in pH, independent of the chrome content of the solutions. A
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1925
decrease in hydrogen-ion concentration with time is evident in all instances (Figure 3). During the first 24 hours this decrease in actual acidity is much more rapid for solution containing large quantities of sodium chloride, and here the equilibria are reached in a much shorter time. Constant p H readings were generally recorded over the whole series after 6 weeks' standing. . PRBCIPITATION FIGuRE-The amount of sodium hydroxide required to bring about a permanent precipitation is considerably reduced by the addition of sodium chloride, indicating a more acid chromic salt when precipitating occurs. This decrease in precipitation figure is proportionately larger for salt concentrations less than 2 M and more pronounced with dilution of the liquor. Thus we find that for the two weakest chromic chlorides only about a third of the amount of alkali required for the blank is consumed. For the two strongest solutions there is much less decrease. The maximum decrease for the 19.1 and 38.2 grams Ci-208 per liter solutions is approximately 40 and 17 per cent, respectively, of the amount of sodium hydroxide consumed for the blank. The precipitation figure is fairly constant, tending to increase upon standing for the more concentrated solutions containing large amounts of XaC1 and to decrease for the dilute solutions of great XaC1 molality. As the hydrogen-ion concentration always decreases upon standing, a direct relationship between this property and the stability cannot exist. 61.5 Per cent Acid Chromic Chloride Table I1 ------pH Immediately
S aCI molality
0
Immediately 0 2.20 0 . 2 5 2.25 0 . 5 2.24 1 2.17 2.03 2 3 1.87 4 1.71 5 1.55
0 0.25 0.5 1 2 3 4 5
1.9
i.S
1.7 1.7 1.7
7.50 grams CrzOs p e r liter 1 week 2 weeks 6 weeks 2.41 2.65 2.73 2.46 2.68 2.73 2.48 2.67 2.71 2.46 2.64 2.65 2.37 2.50 2.50 2.24 2.34 2.33 2.11 2.18 2.18 1.97 2.01 2.01 9.425 grams CrlOa p e r liter 1 dav 2 weeks 6 weeks 2.27 2.63 2.63 2.32 2.62 2.65 2.29 2.60 2.64 2.27 2.58 2.60 2.26 2.54 2.56 2.11 2.40 2.42 2.01 2.25 2.26 1.88 2.08 2.08 1.75 1.91 1.91 15.00 u a m s CY203 per liter 1 week 2 weeks 6 weeks 2.30 2.52 2.54 2.35 2.53 2.54 2.36 2.54 2.54 2.35 2.48 2.48 2.2s 2.35 2.35 2.13 2.18 2.18 1.98 2.02 2.02 1.82 1.85 1.85
1 day 2.32 2.35 2.34 2.31 2.18 2.04 1.90 1.76
1 hour 0
1 week 6 weeks 3.77 grams Cr203 per liter
PRECIPITATION Frcwss, Cc. 0.1 -V NAOH (5-cc. sample) Immediately 6 weeks
2.50 2.53 2.53 2.50 2.35 2.19 2.03 1.87
0.25 0.5
0.25 0.5 0.75 1 2 3 4 5
VALUE-
2.24 2.26 2.23 2.21 2.18 2.04 1.89 1.76 1.62
1 day 2.10 2.14 2.15 2.11 2.03 1.92 1.80 1.68
Immediately 7.2 5.6 4.3 3.7 3.4 3.3 3.2 3.2 1 week 7.6 6.2 5.6 5 0 4.8 4.0 3.8 3.6 3.5 1 day
12.6 12.4 11.0 10.1 7.2 6.5 (3 , 4 6.3
Table I1 contains the experimental findings for the 61.5 per cent acid chromic chloride solutions. The same apparently anomalous shape of the curves at lower sodium chloride concentrations immediately after dilution is evident. The increase in pH, however, is less than that found for the previous liquor, and upon standing there is an increase hydrogen-ion concentration over the whole range of salt concentrations. The same regularity in decrease of the pre-
947
cipitation figure exists although it is less marked. Concerning the changes in pH with time, the greater velocity of this reaction for the solutions of high XaC1 molality in the initial period is also here demonstrated. These results are shown in Figure 4. 86.0 Per cent Acid Chromic Chloride
I n Table 111 and Figure 5 are reviewed the data from the 86.0 per cent acid chromic chloride. Table 111 NaCl molality 0 0.25 0.5 1 2 3 4 5
0
0.25 0.5 1 2 3 4 5
0
0.25 0.5 1 2 3 4 5
PRECIPITATION FIGURE, Cc 0 1 N KOH (5-cc. sample) 1 week 6 weeks Immediately 6 Reeks 4 20 grams CrrOs per liter 2.38 2.39 2.35 2.28 2.13 1.96 2.4 1.80 2.7 2.4 1.63 8.40 grams C y 2 0 3 per liter 6 weeks 6 weeks Immediately 12.5 2.02 2.15 10.2 2.02 2.13 8.0 2.03 2.11 5.3 1.99 2.05 4.6 1.85 1.90 4.7 1.67 1.73 4.7 1.49 1.55 4.7 1.32 1.38
-PH Immediately
1 dav 1.6? 1.70 1.69 1.66 1.52 1.38 1.23 1.09
VALUE-
16.80 grams 3 days 1.73 1.74 1.74 1.69 1.55 1.39 1.23 1.09
cy208
per tiler
6 weeks 1.74 1.74 1.73 1.69 1.55 1.39 1.23 1.09
6 weeks 24.2 17.0 17.0 16.0 13.0 10.0 0.2 8.4
Only a slight decrease in the actual acidity is shown for this liquor with low salt content immediately after preparation,
Vol. 17, No. 9
INDUSTRIAL A N D ENGINEERING CHEMISTRY
948
and when equilibrium is reached increase in this figure is noted over the whole series. The change in pH with time is considerably less than for the previously examined less acid compounds, and equilibrium is reached in a shorter time. The diminution in precipitation figure is also here demonstrated. Rate of Increase of Acidity of Chromic Chloride Solution of from 2 to 5 M NaCl
The graphs from these three liquors (Figure 5 ) show that where the final sodium chloride concentration is from 2 to 5 M , a straight line results when the pH values are plotted
In the 44.1 per cent acid chromic chloride solutions the b values show a great difference immediately after preparation, indicating that the increase in pH occurs a t a greater rate for the more dilute solutions, as was also evidenced by the curves in Figure 1. After 24 hours, the values of b decrease over the whole range of observations, as was previously noted. When equilibrium, is established these values b vary only within narrow limits, evidently indicating the exclusion of factors other than hydration. For the more acid liquors, 61.5 and 86.0 per cent, better concordance is shown a t different time intervals, which is also illustrated in Figures 5 by the parallelity of the lines for the 86.0 per cent acid liquor of concentration’8.4 grams Cr203 per liter. Per cent Acidity at Which Precipitation Occurs
Table V gives a summary of the per cent acidity a t which the chromic chloride solution starts to separate insoluble “basic” salts, calculated from observations of precipitation figure made immediately after dilution to the final concentrsttions. These data are plotted in Figure 6. Table V 41.1 per cenf acid chromic chloride -CONCENTRATION OF CRzO3 SOLUTIONNaCl molality 0
0.25 0.5 1 2 3 4 5
0 0.25 0.5 1 2 3 4 5 0 0.25 0.5 1
2 3 4 5
Figure 3-Time Factor for 44.1 Per c e n t Acid Chromic Chloride Containing 7.66 Grams CrzOs per Liter
against the molality of sodium chloride. Accordingly, they are represented by a linear equation and, employing the same expression as proposed by Wilson,6we find: log (H+) = log a
+bm
where b is a constant, determined by the degree of hydration of the particular neutral salt employed, a is the hydrogenion concentration of the solution without any addition of neutral salt, and (H+) represents the measured hydrogenion concentration in the presence of m mols per liter of neutral salt. The values of b are given in Table IV. Grams CnO3/liter
1.90 3.83 7.66 9.55 19.10 38.20 3.77 7.50 9.42 15.00 4.20 8.40 16.80
Table I V Immediately 1 day 1 week 2 weeks 44.1 per cent acid chromic chloride
0.138 0.157 0.090 0.127 0.070 0.133 0.070 0.148 Q.060 0.143 0.02 0.150 61.5 Der cent acid chromic chloride 0.160 0.146 0.167 0.160 0.140 0.163 0.140 0.120 0,157 0.117 0.153 86.0 p e r cent acid chromic chloride 0.143 0.167 0.177 0.177 0.143 0.153
0.157 0.110 0.090 0.093 0.070 0.030
At equilibrium
0.157 0.160 0.159
0.157 0.167 0.163 0.177 0.150 0.150
0.170 0.163 0.170 0.163
0.173 0.170 0.170 0.163 0.167 0.177 0.153
1.90
3.83
7.66
9.55
g./L
g./l.
g./L
gJ1.
10.6 18.7 25.3 28.0 30.7 30.7 30.7 32.0
11.3 20.0 23.3 25.3 28.0 30.7 32.0 33.3
16.7 18.0 21.3 21.2 22.7 21.8 27.3 25.8 29.3 27.5 30.7 29.6 32.0 29.6 32.7 31.2 61.5 4er cent acid chromic chloride 3.77 &I. 7.50 g./L 12.7 10.0 30.1 23.5 32.8 32.2 36.3 35.5 36.9 38.3 38.1 39.0 39.6 39.5 38.1 39.6 86.0 per cent acid chromic chloride 4.20 %/I. 8.40 d l . 11.3 10 .-7 33.0 24.5 35.4 57.7 52.3 54.0 54.7 58.3 57.1 58.3 57.1 57.6 57.1 57.6
19.10
38.20
g./l.
g./l.
20.1 23.3 24.9 25.9 28.3 28.8 29.4 30.2
23.8 24.6 25.4 26.1 27.3 27.0 26.7 26.7
15.00 g./L 19.5 20.8 24.2 27.2 36.9 39.3 39.6 40.0 16.SO d l . i3.i 37.8 32.5 37.8 46.8 65.8 58.7 60.7
The very low acidity to which chromic chlorides can be brought without showing any indication of instability is remarkable. The 10 per cent acid compound is relatively stable. The data from the solutions with no added sodium chloride show that for the less acidic chromic chlorides, such as the 44.1 per cent, the acidity of the precipitated compound is a function of its chrome content and increase of the latter gives rise to a more acid salt, showing tendency to precipitate; but that the more acid liquors, such as the 86.0 per cent, show practical independence of concentration of the chromic chloride in their stability towards alkali. Addition of sodium chloride in such a small quantity as to make the solution 0.25 M results in a tremendous increase in the acidity percentage a t which insoluble chromic salt commences to settle out for the dilute solutions; for concentrated systems only a slight increase is noted. This tendency for more acid salts to precipitate upon addition of sodium chloride is most pronounced in salt concentrations up to two M , and beyond this figure the acidity percentages lie closer together. pH Values at Which Precipitation Occurs
The p H values of the chromic chlorides brought to the precipitation limit were also determined immediately after precipitation. The data from the dilute liquors are given in Table VI.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
September, 1925 Table VI
NaCl molalitv 0 0 25 0 5 1
2 3 4 5
-44.1 1.90 g./L CrsOa . . 6.24 5.72 5.41 5.25 5.20 4.98 4.70 4.51
% ACID--
3.83 g/I. CriOs 5.82 5.30 5.22 5.00 4.77 4.56 4.46 4.30
6 1 . 5 % ACID 3 . 7 7 g./L CrzOa 5.72 5.07 4.87 4.71 4.63 4.55 4.40 4.24
86.0% ACID 4.20 &/I. CrzOa 5.61 5.08 4.92 4.75 4.54 4.48 4.31 4.10
In general, these data correspond to the acidity results and indicate that precipitation does not occur at a definite pH. It should be noted that the pH values from the blank and the two solutions with lowest NaC1 molalities fall on the alkaline side of the isoelectric point. It was therefore of interest to examine the time factor in regard to pH values, which are expected to show a considerable decrease immediately after addition of alkali. Relation between Time and pH Values
The 44.1 per cent acid chromic chloride solution containing 1.9 grams Crz03 per liter gave the following variation in pH values with time: TIME 0 After 3 minutes After 15 minutes After 24 hours After 3 days
pH Values 6.12 5.95 5.76 5.33 5.12
949
chloride with preponderance of the green forms in a concentration of 13.77 grams Cr& per liter was employed, and furthermore the lowest salt concentration was 1 M . No experimental work relating to the structure and physical chemistry of basic aquo-chromic chlorides has appeared, but their general character, with respect to the state of chlorine and coordination structure, is probably similar to that found for the normal chlorides, which have been elaborately treated by Bjerrum.’ The hexahydrates exist in three modifications: the hexaaquo-chromi chloride, according to Werner ICr (HzOhI Cb I forms violet solutions, containing a hydrated chromium cation; the chloro-penta-aquo-chromi chloride, or
I
(HzO)a
[cr
c1
I1
with the divalent internal sphere functioning as cation, forms a green solution; and the third member, dichloro-tetra-aquo chromi chloride
[cr
I
(HzO)r Clz
I11
is characterized by a monovalent chrome complex with two chlorine atoms directly attached to chromium, as cation, giving a green solution.
The hydrogen-ion concentration of this solution is still on the alkaline side of the isoelectric point of collagen after 3 days. It would be of great interest to investigate the tanning properties of such a solution, and the same chromic chloride would also be an excellent medium for study of the titration curve of chromic chlorides. Action of Magnesium Chloride
Finally, the action of magnesium chloride upon the hydrogen-ion concentration of the 44.1 per cent acid liquor was studied in the concentration range of 0 to 2 M MgC12 a t a CrtOa content of 9.15 grams per liter. Only a slight deviation from the linear relationship was found in the initial period a t lower molality of added salt. Its pronounced hydration tendency, as pointed out in the theoretical discussion will explain these findings.
Theoretical Discussion The data show clearly that the hydration effect of sodium chloride is not the only factor controlling the hydrogen-ion concentration and stability towards alkali of chromic chloride solutions. If the change in the potential hydrogen-ion activity were due chiefly to concentration, a linear relationship would exist between the recorded p H values and concentrations of sodium chloride, and further, the slopes of the curve, or the value of b, would be independent of the time and nearly the same for solutions differing in chrome content and acidity percentage. These conditions are not fulfilled and on that account and because the potential hydrogen decreases a t lower concentrations of added salt, particularly for the less acid liquor, another factor must be sought. The great tendency of these compounds to internal rearrangement is known and therefore an inclusion of the constitutional factor seems reasonable. The results recorded herein are in perfect harmony if these changes are considered from the concept for similar compounds advanced by Werner and his school, together with the concentration effect brought about by the hydration of sodium chloride. In Miss Baldwin’s investigation such irregularities in the hydrogen-ion activity as are here observed were not shown, which is easily explained by the fact that the normal chromic
MOLAllTY
Figure 4-H-Ion
OF ARDED Not/
Concentration of 61.5 Per cent Acid Chromic Chloride a-Immediate1 y b-1 hour old c-1 day old d-6 weeks old
The main findings in Bjerrum’s work are summarized here. Measurements of the immediate hydrolysis of the violet 7 Bjerrum, Kgl. Danske Videnskab. Selskab, Skrifter naturvidenskab. mafh. Afdel., [ 7 ] 4, 1, 76 (1907); in condensed Form in 2.physzk. Chem., 69,
336, 581 (1907).
Vol. 17, No. 9
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
950
solutions-i. e.,formation of compounds with “loose” hydroxyl groups (aquo-base) and thus excluding eventual secondary reactions, by conductivity and hydrogen-ion activity methods, assuming the hydrolysis to take place according to the ionic equation Cr+++ H20 Cr (OH)++ H+gave a value of 0.98 X for the hydrolysis constant at 25’ C. The theory as to the formation of CrOHClz upon hydrolysis was substantiated by the conductivity curve of the chromic chloride upon neutralization. To a 0.01M solution of hexa-aquo salt sodium hydroxide solution was added, the reaction allowed to proceed for 1 hour, and the conductivity determined after each addition of alkali. The conductivity decreased rapidly as hydrogen-ion was removed, until a value corresponding to CrOHClz was reached, after which a slow increase was noted up to complete precipitation of chromium hydroxide. These observations are substantial evidence for the existence of definite chemical compounds in basic chromic salts. The temperature coefficient is indicated by the following values of the hydrolysis constant for 0.1 M hexa-aquo compound.
+
+
Temperature, C. Hydrolysis constant ( X 100
0 0.22
25 0.98
50
75 10.3
3.4
100 26.4
In 0.1 M solution the degree of hydrolysis was: Temperature, O C. Degree of hydrolysis
0 0.015
25 0.031
50 0.057
75 0.096
100 0.150
The chlorine held in the complex is almost completely ionized in dilute solutions (by formation of intermediate “esohydrates”) a t room temperature; concentrated solutions show the reverse tendency. Earlier thermo-chemical measurements by Recoura,lO who found a greater heat of neutralization for the green solution, indicated the predominance of the green form with increase in temperature. Upon addition of the necessary amount of hydrochloric acid to form the normal salt, the neutralized solution gave a violet solution. This does not apply to basic chromic sulfates or basic chromic chlorides upon standing. Study of the reaction ICr (H20)d C ~ Z ] + 2 H10 Ft [Cr (H~O)B]+++ 4- 2 C1indicates that besides water of constitution, probably water of hydration is involved. As the number of ions is increased by formation of trivalent chromium cation, very likely more than two (H2O)s are involved, and assuming this number to be m, the equation is
+
where CI, CIII, C C ~and , Cazo denote, respectively, the activity of the violet and green cations, chlorine ions,and water. Thus it is evident that decrease in the activity of water tends to shift the equilibrium in direction of the green 1.0
I I
A comparison of the hydrolysis constants for the aluminium, chromium, and ferric chlorides in 0.1 M solutions is interesting. Salt Hydrolysis constant (X10‘) at 25” C.
AlCls 0.14
CrCls (violet) 0.89
FeCla 25.0
Upon heating, the violet color changes to green, but if the boiling is not unduly prolonged the violet color soon returns. The increase in hydrolysis-about fivefold for this temperature interval-explains this change. The dichloro compound shows great tendency to change into the violet form, and, therefore difficulties were encountered in obtaining concordant data for the hydrolysis constant. The hydrogen-ion determinations indicated a value of 4.3x and the conductivity measurements gave 3.25X 10 -6, showing that the degree of hydrolysis for this form is considerably less than for the violet solution and the hydrogen-ion activity is from one-fifth to one-seventh of the amount recorded for the violet solution a t corresponding molalities. Previous t o Bjerrum’s investigation the monochloro compound had not been isolated, and the reaction kinetics could not be brought into harmony with either the first, second, or third degree equations, but the isolation of this intermediate form brought concordance to the kinetic data. This monochloro compound was given a hydrolysis constant of approximately 8 X by extrapolation, which is considerably less than the value of the hexa-aquo form. The conductivity measurements indicated that in the hydrolysis product the hydroxyl groups take B position similar to internally attached chlorine, thus replacing one aquo group, which was also indicated in previous work by Werner,8and later supported by the work of others. Note-Bjerrum7 has first shown that the basic salts must be considered as hydroxy compounds and pointed out that the hydrolysis of hexa-aquo chloride is decreased in 0.1 and 0.2 M KC1 solutions. The basic chromic chlorides have been comprehensively studied in his “Studier iiber basiske Rromiforbindelser.” Dissertation, University of Copenhagen, 1908; particularly p. 138. (Summarized in Z. physik. Chem., 73, 724 (1910).
The negative catalysis in the rearrangement from green to violet forms was shown to be due to the retarding action of hydrochloric acid.
* Bcr.. 40. 272 (19071, Pfeiffer,’lbrd.,‘39,1876 (1906); Z. anorg. allgem. Chem., 68, 280 (1908); 113, 416 (1920). 9
Figure 5-H-Ion
M O L A L I T Y IN N U N Concentration of 86.0 Per c e n t Acid Chromic
Chloride
a-Immediately b-1 day old r-6 weeks old.
form. The activity of water, however, is first markedly changed in concentrated solutions, and consequently in dilute solutions the activity of chlorine ions plays the dominating role. For quantitative determinations of the various forms three independent methods were employed. The violet form was 10
Ann. chim. phys., 10, 32 (1887)
I.VDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1925
separated by precipitation with hydrochloric acid gas. Conductivity measurements of solutions immediately upon prepa. ration and when equilibrium was established permitted the computation of percentage of constituents. A third method was based upon the contraction occurring in this rearrangement. The change of dichloro compound into hexa-aquo salt in 1 M solution is accompanied by increase in specific gravity from 1.125 to 1.139. The 1 iM solution a t equilibrium consisted of 85.3 per cent (I), 12.2 per cent (11),and 2.4 per cent (111). The 0.01 N solution was composed almost entirely of form (I), only 0.25 per cent of green modification (11)being present. Upon heating this solution for 5 minutes and then cooling rapidly, a radical change in composition occurred, as evidenced by the data: 46.2 per cent (I), 54.6 per cent (11),and -0.8 per cent (111). The following table by Bjerrum illustrates the influence of concentration upon the composition of concentrated boiled solutions : 1 part CrCh 6H10 dissolved in: 3 1 / 1 parts HzO 1 I/Jparts 1120 1 part HzO 3/4 part H20 l / z part H20
Per cent 54.6 52 57 52 47.2
let color to green. Thus no uniformly hydrated chromium cation is expected in the basic salts derived from the hexa-aquo compound. Although the amount of chlorine in the internal sphere is expected to be considerably less for the salts derived from the hexa-aquo salt. The permutit method" shows the corresponding basic violet sulfate to possess a complex of about 20 per cent acidity, whereas the corresponding neutralized green form contains a complex of 33 to 35 per cent acidity. For basic chlorides under the same conditions the O'
I
I
I
I
I
CI .[~rg:~,)] -0.8
[Cr ( H z O ) ~C13 ] [cr ( H & ~ ) ] C I ~ Per cent 46.2 26 20 10 2.4
951
Per cent 22 23
38 50.4
The percentage of the dichloro form increases rapidly in concentrations over 1 M . The monochloro chloride has reached its maximum where equal parts of compound and water are employed. The violet form shows a steady, rapid decline, and a t the highest possible concentration only a minute quantity exists. These figures are in perfect concordance with the requirements of the mass action law. Bjerrum further studied the influence of neutral salts upon this reaction. The retardation of hexa-aquo salt formation caused by neutral salt had previously been observed and had been explained as a hydration effect. The chlorides and nitrates of potassium, calcium, and zinc were investigated in this respect. This retardation in case of neutral chlorides was increased in the order Zn, Ca, K. This order is the same as shown by the activity coefficients of these salts, but reverse to the degree of hydration. Nitrates showed only a slight retardation compared with chlorides, and followed the hydration series. The greater efficiency of chlorides was considered to be due to the increased activity of chlorine ions. The hydration effect is an additional factor, of course, but of little importance. This difference in behavior of chlorides and nitrates was further demonstrated in the color change produced in violet solutions after boiling in presence of these neutral salts. The nitrates gave little evidence, even in saturated solution, of causing change in color, but the chlorides in all cases caused a shift to the green, this tendency becoming more pronounced in the order Zn, Ca, and K. Hydrochloric acid, particularly, with increase in activity of chlorine ions gave the whole range from violet over bluish green and green to yellowish green. These findings brought further evidence that the activity of chlorine ions is the predominating factor. For the basic salts the explanation advanced for the normal salts cannot be applied. Immediately after neutralization the aquo base, in which the hydroxyl groups are ionizable predominates but the basic compound is stable where hydroxyl groups are directly attached to the central atom and accordingly upon standing the equilibrium is shifted in direction of this hydroxo salt. Computation from Bjerrum's conductivity measurements and the existence of hydroxyl in the complex of related compounds point in the same direction. That a radical change occurs in neutralization of the hexa-aquo salt is evident by the immediate change of the vio-
M O L A L i T Y IN
F i g u r e 6-Per
NaCI
c e n t A c i d i t y of P r e c i p i t a t e d C h r o m i c C h l o r i d e s
non-ionogen attached chlorine is only about a third of these figures, indicating a greater stability of sulfate than chloro compounds, which is the chief reason for the preference of basic sulfates in chrome tanning. Increase in activity of chlorine ions will, however, be similar in action t o the findings for the normal salts and in dilutersolutions a trivalent chrome cation is predominating by gradual change of hydroxyl and chlorine groups into the activated state. Addition of sodium chloride will favor the formation of compounds with chlorine coordinatively attached, and accordingly the increase in chlorine ions is evidenced by decrease in electric charges of the cation and by diminution of the hydrogenion concentration. The hydration effect a t low KaC1 concentration is small compared with the change in the chrome compound, and the decrease in actual acidity by this rearrangement more than counterbalances the small increase caused from removal of solvent by hydration of NaCl. In solutions of more than 2 ;M NaCl the hydration effect has reached a considerable magnitude and results in an increase in hydrogen-ion concentration. The constitutional changes are most pronounced up to 2 -If S a C l in the more basic liquors, and a compound with a maximum number of chlorine groups in the complex is obtained a t about this salt concentration. The molality of sodium chloride a t which this final equilibrium in chlorine migration occurs is expected to be a function of the acidity percentage of the chromium chloride, and as hydroxyl groups are equivalent to chlorine groups in the internal sphere, it is evident that with basic salts, where the primary valence forces are already satisfied by hydroxyl and chlorine, less chlorine will be changed into the non-ionogen 11
Gustavson, J. A m . Lealher Chem A s s o c , 19, 446 (1924)
952
.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
state than with salts with acidity percentage around 100. We would thus expect the equilibrium in rearrangement to be reached a t lower NaCl concentrations with decrease in acidity percentage. With the normal salt and generally with more acid salts this final form is first reached a t high sodium chloride concentration. The hydration effect is therefore more pronounced and the decrease in hydrogen-ion concentration is only noticed immediately after preparation and a t equilibrium only as a slight deviation from the linear relationship between pH and NaCl concentration. The results confirm this view. The decrease in hydrogen ion was found to be greater in the dilute solutions. Here, as previously mentioned, the formation of a normal chromium cation is possible, and the changes in the internal sphere are expected to be more radical than for stronger solutions where OH and C1 groups already occupy the internal sphere. Addition of sodium chloride, besides favoring the formation of a more heterogeneous complex, seems also to cause condensation in the chromium salt, as indicated by the lower diffusibility and greater similarity in precipitation to the basic chromium sulfates, which undoubtedly decrease in dispersity through interaction of hydroxyl groups by means of primary and secondary valence forces. The behavior of magnesium chloride in regard to hydrogenion activity is in accordance with its considerably less activity and greater hydration tendency compared with sodium chloride a t corresponding concentrations. The approximate pH values of the rearranged chromium salt in aqueous solutions are obtained by extending the straight line representing the pH values in solutions of 2 to 5 M NaCl to intersect the pH axis.
Vol. 17, No. 9
The precipitation data show that stability towards alkali is not a function of the hydrogen-ion concentration of the solution. The progress of the agglomeration process, which finally leads to precipitation, is measured by the degree of condensation of the chrome compound, which is in its turn controlled by the nature of the internal sphere, primarily in regard to chlorine content. The per cent acidity a t which precipitation occurs is proportional to the ratio of chlorine to chromium in the complex cation. Accordingly, the increase in acidity of the unstable compound is very marked in salt concentrations up to 2 144 for the more basic liquors, a t which molality the rearrangement of the internal sphere is practically completed. Under these conditions chromium chlorides resemble in stability the corresponding basic sulfates. The decrease in electropositive charge which is associated with the increase in chlorine-ion activity due to addition of neutral chloride, is expected to increase the chrome fixation according to the theory of the reaction between hide substance and chromium advanced by the author. That this is the case will be shown in a later communication. This increase sometimes is considerable, amounting to more than 50 per cent for relatively weak, basic chromium chloride solutions containing 2 A4 NaC1. The neutral salt effect is generally considered to embody a retardation of chrome fixation by hide substance, but in this case the apparently paradoxical expression “the neutral salt effect as accelerating the chrome tanning” must be substituted. Acknowledgment
The author wishes to express his thanks to P. J. Widen for his support of this investigation.
Sulfite Liquor as a Protective Colloid’ By Eugene C. Bingham, Guy F. Rolland, and Guido E. Hilbert LAFAYETT~ COLLEGE, EASTON, PA.
APONINS have been used as detergents from time immemorial, m d generally they may be employed wherever the lowering surface tension is important, rn in producing foams, suspensions, emulsions, etc. Sulfite liquor contains saponins; hence there is nothing more natural than to employ sulfite liquor for these purposes. Of course, raw sulfite liquor is objectionable, but its defects can be largely overcome. The sulfur dioxide may be expelled by boiling. The calcium and magnesium may be precipitated by means of sodium or potassium carbonate. After decanting or filtering, purified sulfite liquor is obtained which contains varying percentages of solids depending upon the amount of evaporation. By use of a Donnan pipet and a light petroleum oil, such aa kerosene, the drop number has been determined for various concentrations. The drop number quickly increases to a maximum a t about 0.5 per cent solid sulfite. Curiously, this is about the same concentration as that required for the maximum drop number obtained with ordinary soap (sodium stearate). The drop number with sulfite liquor was found to be 371 for a 0.69 per cent solution. The value for a 0.12 per cent soap solution was found to be 582 and a mixture of these two showed a drop number of 783. There is not only a lowering of the surface tension when purified sulfite
S
1 Received July 24, 1925. Presented before the Division of Cellulose Chemistry a t the 70th Meeting of the American Chemical Society, Los Angeles, Calif., August 3 to 8, 1925.
liquor is added to water, but there is a greatly lowered surface tension when this liquor is added to soap. The values of the drop numbers are not additive, but later experiments indicate that this may be due to the presence of electrolytes in the purified sulfite liquor. Numerous experiments have been made to test the value of purified sulfite liquor as an aid in producing emulsions, suspensions, foams, etc., of various oils, clay, rouge, zinc oxide, lithopone, silver chloride, aluminium hydroxide, sulfur, graphite, lead arsenate, and finely divided metals. It is generally known that substances like sulfur do not readily form stable suspensions in water. I n the case of many substances used as insecticides, such as lead arsenate, soap is not permissible as an emulsifying agent on account of chemical reactions which take place, Prolonged grinding, even in the so-called colloid mills, is not sufficient in water alone. But in all such cases, purified sulfite liquor is found to be very efficacious. Patents have been taken out in the past for using glue, dextrin, or similar substances as emulsifying agents, but whereas they do increase the viscosity they are without very positive effect and are expensive a t the considerable concentrations which are necessary. Grinding flowers of sulfur for a short time in a ball mill with purified sulfite liquor causes all of the sulfur to become wet, and it will in part be permanently suspended. Experiments were made with some materials such as zinc oxide and sulfur to see whether the material could be separated
