INDUSTRIAL AND ENGINEERING CHEMISTRY
174 PRESSURE-TEhlPERATURE
RELATION OF TVO-COMPONENT
SYSTEMS.The pressure-temperature relationship of the following two-component systems was studied over the temperature ranges indicated: Benzene 1600 cc. Methang, 55 atmosphere initial (gage) Temperature range, 62" t o 261' C.
(73 mole 7) (27 mole '%)
Benzene, 1416 CC. Propane, 610 cc. (liquid) Temperature range, 100' t o 250' C.
(70 mole (30 mole
Benzene, 1440 cc. Propane, 605 cc. (liquid) Temperature range, 94' t o 256' C.
(70 mole 70) (30 mole %)
7)
2)
Hexane "practical", 32 weight Benzene, 68 weight 70 Temperature range, 150' t o 288' C.
The results obtained are given in Table I1 and Figure 3. As in the case of pure substances, the pressure-temperature curves exhibited well-defined regions or points of discontinuity, the section of the curve below the break being curved and the section above the break being practically straight. Samples drawn from the liquid-phase and vapor-phase sampling cocks a t temperatures above the break had approximately the same composition, indicating a one-phase condition, while samples drawn from the liquid-phase and vaporphase cocks below the temperature of break on the curve had different compositions, indicating a two-phase region. The points of discontinuity evidently correspond to the critical temperatures of the two-component systems studied, since analyses of samples taken above and below the temperature of discontinuity showed the existence of one-phase and two-phase regions, respectively.
Vol. 14, No. 2
Conclusions The results obtained with pure substances in either the presence or absence of hydrogen indicate that the critical temperatures may be determined with a fair degree of accuracy and by determining a sufficient number of points in the range of discontinuity the accuracy should be + 2 O C. The amount of material charged to the bomb may vary from 20 to 50 per cent of the total capacity of the bomb without any appreciable shift in the temperature of discontinuity. It is probable that a bomb of smaller size (200 to 300-cc. capacity) might serve equally well if no samples are removed. The smaller bomb could be easily rotated in a liquid heating bath and the temperature determined with greater accuracy than in case of the larger bomb used in these experiments. The pressure-temperature curves of two-component systems exhibited points of discontinuity similar to those shown by individual substances, and the critical temperatures of the systems probably lie within the break on the curves. Up to the present time, however, check determinations have not been made on two-component systems of known critical temperatures. Acknowledgment The authors are indebted to Lee Fischer and Charles Rohlfing for valuable assistance rendered during the course of the experiments. Literature Cited (1) Ipatieff, V. N., and Monroe, G. S., IND.ENQ. C H ~ M ANAL. ., ED., 14, 166 (1942).
PRESENTED before the Division of Petroleum Chemistry a t the 102nd Meeting of the AXERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Glass Electrode Measurements of the pH of Chromic Acid Solutions WINSLOW H. HARTFORD, Research Laboratories, Mutual Chemical Co. of America, Baltimore, Md. The effect of temperature, purity of acid, and water on the pH of aqueous chromic acid solutions has been measured and shown to be negligible, except for a slight variation with temperature in dilute solutions. A considerable variation in the readings obtained at pH below 2 with different types of measuring instruments exists. It increases as the pH of the solution decreases, and may amount to 0.2 pH at pH 0. A large portion of this discrepancy results from various liquid junction potentials, while a residual portion cannot at present be explained. Calculations show that the correct pH value is probably not far from that obtained with certain of the instruments used. All instruments are self-consistent and may be used for control purposes, if standardized against known solutions.
T
HE pH of chromic acid solutions has been measured by means of the glass electrode by Thompson (f7),Buzzard and Wilson (2), and Neuss and Rieman (f6),and by means of the hydrogen electrode in fairly dilute solution by Britton (1). (The term "chromic acid" is used in this paper to apply to both solid chromium trioxide and its aqueous solutions.) It is only under unusual circumstances that the latter method is satisfactory, and with the development of
modern equipment for glass electrode measurements, the glass electrode is practically universally used for such determinations, and has become of industrial importance in the control of baths for the anodic oxidation of aluminum (2), It is the purpose of this paper to show the effect of varying apparatus and conditions on the pH readings obtained with chromic acid solutions, and to present certain calculations which corroborate data obtained with one type of apparatus.
iipparatus The equipment used for these measurements consisted of six commercial-type instruments of three different makes with various types of electrode assemblies, as shown in Table I. All instruments were provided with a temperature compensator and used a saturated calomel electrode as reference half-cell, except Nos. 2A and 5A, in which a silver-silver chloride reference electrode was substituted. No difference was discernible between setups 1 and 2 and these were used unless otherwise specified.
3Iaterials The material used in most of the work was technical flake chromic acid, analyzing 99.8 per cent chromium trioxide, 0.03 per cent sulfate, and 0.017 per cent chromic oxide. Chromium trioxide was determined by electrometric titration according t o the method of Kelley (9), sulfate ravimetrically as barium sulfate after reduction of the hexavafent chromium by alcohol and hydrochloric acid, and chromic oxide by cerimetry according to
A N A L Y T I C A L EDITION
February 15, 1942
TABLEI. TYPES O F ELECTRODE ASSEUBLIES USED Instrument Glass Electrode -4-1 Quinhyrlrone-IICla -4-1 Sealeda 2A -4-1 Sealed 3 A-1 dealeda 4 A-1 Sealed 5 A-1 Sealed :A A-1 Sealed 2 A-2 Sealed A-2 Sealed A-2 sealed 9 .4-2 Sealed 10 B-1 Sealed 11 B-2 Sealed 12 B-:i Sealed 13 B-3 Sealed 14 C Quinhydrwe-HC1, 30 meg. Quinhydrone-"21, 30 meg. 15 C Qujnhydrone-HC1, 3 meg. 16 C Quinhydrone-HCl, 3 meg. 1; C a Sereral glass clectrodcs used.
So. 1 2
&
Liquid Junction Small ground-glass with pinhole Small ground-glasa( with pinhole Smallground-glass: with pinhole Fiber diaphragm Capillary junction Broad junction Broad junction Small ground-glass, as a h ~ w Fiber diaphragm Broad junction Flowing junction Broad junction Broad junction Fiber junction Large ground-glass Capillary junction Large ground-glass Capillary Junction Large ground-glass
a modification of the method of Willard and Young (20). For experiments special grade of c. p. chromium trioxide,
prepared by washing specially selected c. P. material with distilled water, was used. I t tested 99.9 per cent chromium trioxide, 0.0005 per cent sulfate, and 0.008 per cent chromic oxide. As reference standard for the pH measurements, Bureau of Standards potassium acid phthalate, 0.0500 M, was used. This sohtion had a pH of 4.00 at 25' C., and varies therefrom by about *0.008 pH at 19" or 31' C., accordin to the data supplied with the sample (14). Secondary PH stanfards were sodium acetateacetic acid buffers, 0.1 N hydrochloric acid solutions, and hydrochloric acid-potassium chloride buffers.. The meter was checked a t least once each day. Determinations were frequently performed on separate days on identical samples, and results were reproducible within 0.02 pH.
Effect of Varied Conditions TEblPER.4TURE. SOlUtiOnS were tested at 19", 25'7 and 31' C. (Table 11). It will be noted that the effect of raising temperature is to cause a slight decrease in pH, this effect is, however, nearly negligible except a t 1 0 concentrations ~ of chromium trioxide. WATER. Measurements of solutions of equal concentration in Baltimore city nTater and in laboratory distilled water showed no discernible difference. Slight corrections should be made in the case of waters of high alkali or calcium bicarbonate content. PURITYOF CHROMIUM TRIOXIDE.The technical grade and c. P. material described above were tested with the following results: LIole of CrOa/Liter 0.9696 0.1939 0,0485
p H , Technical Grade, 27* C. 0.10 0.80 1.39
pII, c. P. Grade, 27' C.
0.10 0.80 1.39
The small amount of impurities present in technical chromium trioxide has no effect on the pH of solutions.
Effect of Variation i n Measuring Equipment The effect of various pH cells and instruments for effecting the measurements was next studied, with the results shown in Table 111. The wide variations observed obviously require explanation. Several sources of inaccuracy are inherent in commercial glass-electrode instruments, and these will be discussed. ERRORS DTE TO HIGH d C I D I T Y b I i D CONCEKTRATION O F SOLCTIOSS. MacInnes and Belcher ( I S ) and Dole (5) have observed erroneous results from glass electrodes in highly acid solutions, especially a t pH below 0, and in solutions of high electrolyte content. The generally accepted explanation is that the effect is due to the fact that the hydrated proton, (H30)f, is transferred through the glass membrane, and that the Ion* activity of the water in these solutions is responsible
175
for the discrepancy. While no definite figures are available for the activity of the water in the chromic acid solutions under consideration, the concentration and pH are such that only slight indication of the effect would be expected. Approximate calculations indicate that the correction a t 1.O i v chromium trioxide should be of the order of 0.012 pH, which is less than the experimental error. Hubbard, Hamilton, and Finn (8) have indicated that the "acid error" may vary with the composition of the electrode; however, commercial instruments use Corning 015 glass for the electrode and any errors existing in the measurement would be common to all cells. This has been confirmed by the fact that measurements with assemblies identical except for the glass electrodes (Table 111, 1, 2 , 3; 14, 16; 15, 17) gave results identical within experimental error. The electrodes used gave normal readings in 0.1 M hydrochloric acid, and did not show the error reported by Hubbard, Hamilton, and Finn ( 8 ) . The same considerations which render p H measurements on chromic acid solutions feasible up to 1 M make it necessary to state that any values which may be obtained in more concentrated solutions-e. g., plating baths-are of value for comparison only. E~~~~~D~~ TO A~~~~~ OF cHRoarIc ON G~~~~ ELECTRODE. 15'atson (19) and Kerridge (10) have observed that glass electrodes cleaned in a mixture of chromic and sulfuric acids are not suitable for immediate use in the measurement of pH, and Dole (4) has observed increase in weight of glass cleaned in this manner, caused by adsorption of chromic acid. Laboratory cleaning solutions normally contain large quantities of sulfuric acid and might be expected to be dehydrating in action, with resultant change in the properties of the glass. No such action is to be expected from the relatively dilute chromic acid solutions under consideration, and this is borne out by the fact that electrodes with mhich several samples of chromic acid have been measured give accurate readings immediately mhen checked against a buffer. Tubbs (18) has reported high readings in the field
TABLE
'I.
pH
CrOt Mole/l. 1.00 0.80 0.60 0.50
0.40 0.20 0.10
0.05 0.04 0.03 0.02 0.01
SoLuT1oNs
OF CHRoMIC DI'*
EQUIPMENT 1 AND 2 7
190
c.
0.11 0.21 0.31
o:i1 0.77
1:io
1.51 1.61 1.82
..
.
310 C.
2&Hc. 0.10
..
0.08
0:40
0:io
o:i9 1.09 1.39
0:is
1:so
1:+7
2.08
2.06
..
1.07 1.37
..
..
TABLE111. EFFECTOF VARIATIONS IX MEASURIXG EQUIPMENT Apparatus NO.
(from Table I)
---Mole 1.00
of CrOa per Liter:-0.50 0.20
0.10
pH a t 2:" C. 1 2 2A 3 4 5
5 il
6
i9 10 11 1% 13
12 15 16 17
0.10 0.10 0.12 0.11 0.13 0.20 0.18 0.14 0.14 0.20 0.20
0.27 0.30 0.21
0.23
0.20 0.26
0.24
0.28
0.40 0.40
0.79 0.79
1.09 1.09
0:41
o:i9
1:oo
.. .. ..
.. ..
.. .. ..
..
..
.. ..
.. .. ..
..
.. 0133
0:ks
.. i:is
..
.. ..
....
0.47
.. ..
0.84
..
1.13
..
176
INDUSTRIAL AND ENGINEERING CHEMISTRY
with an electrode kept immersed in chromic acid instead of distilled water between readings, but such high readings may be due to diffusion in the liquid junction rather than any action on the glass. SLIDE-WIREINACCURACIES. Dole (3) has reported work by Blair indicating that errors exist in the slide-wire of a commercial pH electrometer, although another electrometer studied by Lanford and Kiehl (11) showed little difference when checked against the Type K potentiometer. Repeated checks with standard solutions of hydrochloric acid a t a pH of 1.08, where a variation in the observed pH of chromic acid of 0.04 to 0.09 pH (see Table 111, column 5 ) was noted with different electrode assemblies, indicate that a definite effect exists under conditions where both acid errors and slide-wire inaccuracies are completely balanced out. While it is possible that the variation of 0.2 pH noted a t pH 0.10 might be due to slide-wire inaccuracies, it seems unlikely that slidewire errors would amount to as much as 10 per cent over the range pH 0 to 1, or that the error would be nearly identical for all instruments of the same make. LIQUIDJUNCTION. Examination of the data in Table I11 indicates that the type of liquid junction is an important factor in determining the value obtained for the pH of chromic acid solutions. Algebraic analysis of the results indicates that approximately half the discrepancy is due to variations in the method of forming the liquid junction, while half is due to some factor at present unknown. The data show further that those liquid junctions possessing the broadest surface of contact give the highest readings. The increase in pH appears to be connected with diffusion in the broad junction, since calomel electrodes of the sleeve type in which diffusion has accidentally taken place invariably give readings of higher pH. The excellent check results obtained with all instruments on other solutions of low pH indicate that the variations observed are peculiar to the liquid junction between chromic acid and saturated potassium chloride. Extensive work on junction potentials (6, 7 , 16) has shown that the ground-glass sleeve or fiber-type junction is normally the least reproducible of liquid junctions; the author has found in the data given above that all junctions studied were reproducible within experimental error, but that diffusion effects appear to introduce a drift toward high pH in the broad types of junction. In this particular case, it would seem that the broad types of junction are more subject to error. This is confirmed in some degree by the calculations given below.
Theoretical Neuss and Rieman (16), in connection with their measurements of the ionization of chromic acid, determined values for the ionization constants in a cell designed to eliminate liquid junction errors, and calculated the variation of the constants with the ionic strength, according to the DebyeHuckel theory. From their data, a t 25” C.:
where and where
pK’
= 1.64
+1 +0 . 7 6
(Concentrations are here denoted by brackets, activities by parentheses.)
Finally,
pH = -log (H+)
Vol. 14, No. 2 =
- (log[H+] + log y)
where log y is derived from values given by Scatchard (16) or MacInnes ( I d ) . If the chromic acid molality = M , and M - [H+] = y, since [H+] = [HCrOk-] 2[Crz07]--, on elimination of [HCr04-] and [Cr207]--:
+
0.01 0.02 0.04 0.08 0.10 0.20 0.40
0.0118 0.0248 0.0518 0.107 0.136 0.279 0.579
0.116 0.123 0.140 0.164 0.175 0.216 0.277
0.0178 0.0166 0.0145 0.0123 0.0117 0.0093 0.0074
0.921 0.888 0.870 0.848 0,842 0.838 0.863
0.000514 0.01051 0.00153 0.02153 0.00395 0.04395 0.00942 0.08942 0.0122 0,1122 0.0261 0,2261 0.0528 0.4528
0.00921 0.01776 0.0348 0.0678 0,0842 0.1676 0.3452
2.04 1.75 1.46 1.17 1.07 0.78 0.49
Plotting M against p H gives a curve which is in best agreement (+0.04 pH) with the data determined with instrument A with small junctions up to 0.2 M , which represents the highest concentration a t which the equations are valid. However, any of the assemblies employed in this work give self-consistent results, and no false e. m. fa’sare developed by the action of the chromic acid up to 1 M on the glass electrode in any case, as shown by the reproducibility of readings with standard buffer a t all times. Because of the acid error, the possible action of chromic acid on the glass electrode, the increase in liquid junction errors, the lack of any theoretical check on the measurements, and the fact that many commercial meters do not read below pH 0 or -0.5, it did not seem feasible to obtain pH values for higher concentrations of chromic acid.
Acknowledgments The author wishes to express his thanks to the follo.wing who have made pH measurements on chromic acid solutions: A. 0. Beckman, National Technical Laboratories, Pasadena, Calif.; F. R. McCrumb, W. A. Taylor & Co., Baltimore, Md.; and Mutual Chemical Co. of America, Jersey City, N. J. He is much indebted t o William Rieman I11 of Rutgers University for his assistance in the calculations of
Literature Cited Britton, H. T. S., J. Chem. SOC.,125, 1572 (1924). Buzzard, R. W., and Wilson, J. H., Natl. Bur. Standards, Research Paper 961 (1937). Dole, M., “Glass Electrode”, p. 36, New York, John Wiley & Sons, 1941. Ibid., p. 82. Dole, M., J. Am. Chem. SOC.,54, 3095 (1932). Ferauson, A. L., Van Lente, K., and Hitchens, R., Ibid., 54, 1285 (1932). Guggenheim, E. A,, Ibid., 52, 1315 (1930). Hubbard, D., Hamilton, E. H., and Finn, A. N., J. Research, Natl. Bur. Standards, 22, 339 (1939). Kelley and Co-workers, J. IND.ENQ.CHEW,9, 780 (1917). Kerridge, P. M. T., J . Sci. Instrumenis, 3, 404 (1926). Lanford, 0. E., and Kiehl, S. J., J. Phys. Chem., 45, 300 (1941). MacInnes, D., J. Am. Chem. SOC.,41, 1086 (1919). MacInnes, D., and Belcher, D., Ibid., 53, 3315 (1931). Natl. Bur. Standards, “Supplementary Certificate of Standard Sample 84a (for Use as a pH Standard)”, (Jan. 30, 1940). Neuss, J. D., and Rieman, W., J. Am. Chem. Soc., 56, 2238 (1934). Scatchard, G., Ibid., 47, 696 (1925). Thompson, M. S., J. Research, Natl. Bur. Standards, 9, 833 (1932). Tubbs, L. G., private communication. Watson, F. J., Chem. Eng. Mining Rev., 20, 59 (1927). Willard, H. A., and Young, P., J. Am. Chem. SOC.,50, 1322, 1379 (1928); Trans. Electrochem. SOC.(preprint), 67 (1935).
