Properties of Detergent Solutions pH Studies on Modified Soda-Soap Solutions LESLIE R. BACON, JAMES W. HENSLEY, AND THOMAS H. VAUGHN The J. B. Ford Company, Wyandotte, Mich.
The pH measurements obtained by the hydrogen electrode, glass electrode, and two colorimetric methods on the systems modified soda-water and modified soda-soap water have been obtained and critically compared. Electrometric measurements were made a t 25', 40', 60', and 80' C.; colorimetric measurements were limited to 25' C. The modified soda concentrations encompass the full range of detergent interest; soap concentration has been held a t 0.1 per cent. Contrary to general acceptance, the modified soda examined raises the pH of soap solutions a t the usual whitework washing temperatures. pH measure-
ments, even of the electrometric type, cannot be used as a means of concentration control for modified soda systems with or without soap. Colorimetric measurements are misleading a t the lower builder concentrations of laundry interest because of inaccuracy and, further, because of inability to relate the values obtained to those prevailing under washing conditions. A survey of pH measurements made in the field discloses wide errors. Recent advances in the technique and standardization of pH measurements at elevated temperatures are discussed.
T
HIS paper is the first of a series concerned with the properties of detergent solutions and the measurement of detergent effects. We present here detailed pH data a t 25", 40",60", and 80" C.for commercial modified sodawater solutions over a wide concentration range, with and without soap. A number of reasons may be cited for particular interest in careful investigation of the pH behaviors of alkaline detergent solutions (26). In the first place, undue emphasis has come to be placed on pH as a practical method of control of detergent processes. To a considerable extent such reliance is misguided, particularly when it is dependent upon oolorimetric measurements. Most of the better pH studies on soap builders have been limited to concentrations largely above those of interest in laundry practice, and few have been made a t operating temperatures. pH values of soaps are often sensitive to temperature and to changes in fatty acid types represented. The dependable pH data on soaps are few, and little of major significance has appeared on pH values of soapbuilder combinations.
paraffin-air interfaces. Their work indicates that a t low soap concentration, well-defined minimum contact angles occur a t critical pH values, but with increase of soap concentration, contact angles fall to zero and spreading occurs under conditions completely insensitive to pH over wide ranges. Change of soap stocks gives a picture qualitatively similar but quantitatively very different. Extension of such measurements to soap solutions brought into contact with textile fibers coated with Nujol shows that wetting of the textile fiber with displacement of oil is, in addition, a function of the variety of textile fiber selected. Other simple changes, such as the addition of sodium chloride which produces substantially no pH change, do nevertheless have considerable effects on contact angles. Many other fundamental phenomena of detergency have more or less similarly showed behaviors critically dependent upon pH values in some instances and practically nonrelated in others.
Methods and Apparatus COLORIMETRIC. The two types of colorimetric equipment employed in this work were the Taylor nonportable model D3, with liquid color standards, and the Hellige portable comparator, with colored glass standards. Some workers can reproduce readings to *0.05 pH unit with these comparators. Such equipment is calibrated for use at room temperature (98). The indicators used were those regularly supplied by the manufacturers for the pH ranges involved, fresh and protected against undue exposure to light, contamination, o r concentration changes. ELECTROMETRIC. Of the numerous electrode systems that have been applied with more or less success to measurement of hydrogen-ion concentrations (2,3, Xi, 29,only two have proved capable of high precision in the alkaline range-
Relation of pH t o Detergency Excellent detergency can be obtained in alkaline solution, according to class of work, soap, builder, concentration, and other conditions selected, a t pH values from below 9.5 to 12 or perhaps even higher. Specification of pH alone, without further reference to composition, affords no guidance to detergent efficiency. The relations between pH and suspending power, emulsifying capacity, and other phenomena involved in detergent action have been investigated to some extent. Powney and Frost (22), for example, have investigated the relations between contact angles, pH, and type of soap at soap solution-
723
724
INDUSTRIAL AND ENGINEERING CHEMISTRY
namely, the hydrogen and glass electrodes. Since either of these methods might be in error under some conditions included in the present work, i t was considered advisable to make measurements with both electrodes in such cases,
FIGURE1. HYDROGEX ELECTRODE-CALOMEL ELECTRODE ASSEXBLY
With the hydrogen electrode, some workers have experienced difficulty in measurements on solutions containing soap. Halvarsan (9) attempted measurements on 5 per cent solutions of ricinoleate and other soaps, and found the readings with different electrodes or with the same electrode replatinized to be nonreproducible. This was attributed to catalytic reduction. Preston (66)found that saturated soaps a t very low concentrations may show long-continued drift in potential and changes which cannot be attributed to hydrogenation. He regards the hydrogen electrode as satisfactory if soap concentrations are as high as ordinarily used in the laundry. Bleyberg and Lettner refer to the formation of films on the hydrogen electrode made u p of saturated soaps formed by hydrogenation. We too have observed filming of the electrode at low temperatures but doubt that it originated entirely through hydrogenation. The production of foam due to bubbling of hydrogen through soap solutions has been suggested to be another cause of drift in potential in p H measurements by this method. The drift may result both from changes in concentration and from fatty acid-alkali ratio in the foam differing from that of the original soap. While such effects may become noticeable in extreme cases, Colton and Snell (4) recently observed that carbon-dioxide-free air bubbling through a soap solution failed to alter its pH; and after condensation, the foam showed the same value although the soap concentration had been increased. We have seen no evidence of these effects in our work. The drift in p H of a soap solution as observed with the glass electrode is approximately the same whether hydrogen is bubbled through or not, and in our experience appears to originate at least partially a t the liquid junction of the soap solution and saturated p o t a ~ i u mchloride.
Vol. 33, No. 6
One uncertainty connected with the use of hydrogen electrodes a t temperatures above 40" C. is the potential value to be assigned to the calomel electrode which usually forms a part of the cell. An indirect standardization procedure to be described was finally adopted to overcome this difficulty. The chief source of errors applying to glass electrodes is the uncertainty of values for sodium-ion correction. These corrections increase with pH, concentration, and temperature (6-8, 16,23). Corning 015 glass, from which glass electrodes are now commonly made, becomes unusable in very alkaline solutions and a t high sodium-ion concentrations. Since the experimental work reported in this communication was completed, h-ational Technical Laboratories have announced glass electrodes 1190-E and 4990-E, specially adapted to measurements in strongly alkaline solutions at room temperatures (20). Although these electrodes may be found to have certain limitations, such as slower attainment of equilibrium and drifting asymmetry potential, they show promise of extending the practicable range of p H measurements upward by several p H units or to sodium-ion concentrations many times those hitherto possible. I n our work with glass electrodes both the Leeds & Northrup potentiometer-electrometer 7660-8 and the Beckman laboratory model p H meter have been utilized, both with Beckman glass electrodes No. 1190. The large (&inch or 127-mm.) shielded electrode was used with shielded leads. The Beckman electrode 1190-T for intermittent use up to 90' C. was employed a t elevated temperatures. The Beckman meter was graduated to 0.1 p H unit, and readings could be estimated with an error not greater than kO.01. The potentiometer-electrometer was calibrated to 0.002 volt (0.03 p H unit) and could be read accurately to the equivalent of the nearest 0.01 p H unit. Hydrogen electrode potentials were measured by a simplified Queen potentiometer, with which readings could be made to the equivalent of less than 0.01 p H unit. Hydrogen electrode measurements on solutions containing no soap were made with the Leeds & Northrup Wilson type electrode. Thip did not function well in soap solutions. A type (Figure 1) in which hydrogen is brought in through a side arm, A , releasing bubbles which rise through the solution to contact the platinized surfaces, B, was found much better. Platinum electrodes of the wire type were the same in both cases. The saturated calomel electrode was found more convenient than the 0.10 N electrode and just as satisfactory, provided it was kept a t constant temperature for several hours before use to minimize hysteresis (50). The Beckman saturated calomel electrode 1170 is designed to minimize hysteresis effects. This electrode, in which the liquid junction is formed through a ground-glass sleeve, was satisfactory ith modified soda solutions at low temperatures. Because of difficulties involved in maintaining such electrodes a t a constant temperature during renewal of junction at higher temperatures and because of junction difficulties due to the ground-glass sleeve used, which a t higher temperatures proved particularly troublesome with soap solutions, a n open liquid junction, formed a t the end of a raised capillary tube of about 1-mm. bore and sealed into the bottom of the hydrogen electrode vessel, was adopted (C, Figure I). This assembly was in general the most satisfactory of many tried for both hydrogen and glass electrode measurements.
Hydrogen Electrode Measurements Closed Pyrex vessels were used in all cases. Except for early work at 25" C., temperature control was maintained within *0.03" C. by immersing the vessel to a suitable depth in a constant-temperature liquid bath. Calomel reference electrodes were maintained a t the temperature of the solution under test, as were all the salt bridge connec-
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
tions (Figure 1). When an electrode was taken to a higher temperature, it was first N e d with a solution saturated with potassium chloride and calomel at a temperature higher than the working temperature and then maintained at the working temperature for several hours before use (SO). Hydrogen electrodes were always employed in duplicate. Commercial hydrogen was found satisfactory without purification. The hydrogen was passed from a sintered glass disk (D,Figure 1) through a presaturating solution identical with that under measurement and a t the same temperature. Such preadjustment of vapor pressures of carbon dioxide and water effectively protected the solutions under test against changes in composition. Measurements by all methods were made on the 6ame day the solutions were prepared. The more concentrated modified soda solutions maintained a constant pH for long periods, but the more dilute ones and those containing soap showed a decrease after standing a day or more. Hydrogen electrodes were generally re latinized for each determination with a thin coating of black w&ch functioned best when electrodes were replatinized frequently.
Glass Electrode Measurements Calomel electrodes for glass electrode measurements were handled as for hydrogen electrode work. When used at room temperature, glass electrodes were generally calibrated a ainst standard buffers twice daily. A t higher temperatures it was found desirable to check every hour or two. When the electrode was kept in distilled water between determinations, there was usually no change in asymmetry potential for a day; in some cases at lower temperatures, calibrations would remain constant within 0.01 pH unit for almost a week. Measurements were made on two solutions of each concentration and on several portions of each solution. Adequate precautions were taken to ensure that temperature equilibrium was reached. With some of the soap solutions having low modified soda concentrations the readings fell off rapidly, especially when the liquid junction was formed through a ground-glass sleeve. This drift amounted to as much as 0.4 pH within an hour. In these cases the initial readings (after temperature equilibrium were taken as correct and determinations were repeated severa times on fresh portions. These difficulties were largely eliminated by use of the open-type junction.
i
Colorimetric Measurements Determinations made in duplicate on several solutions of each concentration gave results which agreed to 0.1 pH unit or better, in general. For a given solution satisfactory readings were obtained with only one of the indicators for each instrument. Solutions were adjusted to approximately 25" C.
Shedlovsky (18). These workers give essentially the same pH for 0.05 M phthalate as that recommended by the Bureau of Standards.
BUFFERS.According to information furnished with standard sample 84a p H values on the activity basis for a 0.05 molal potassium acid phthalate solution may be calculated with an accuracy of b0.002 pH from 0" to 60" C. from the equation:
+
pH = 5.13 log T 1519.62/T where T = t o C. 273.16
+
+ 0.01092T - 17.039
This equation may be used tentatively to calculate p H values from 60" to 100" C. (11). We have used this material for standardization of glass electrodes a t elevated temperatures. The Beckman buffer 3506, marked pEI 4.00 * 0.01 at 25" C., has been found in agreement with i t a t 25', 40°,60°,and 80' C. within experimental error. Each glass electrode used after the standardization in this way was checked at 25", 40", and 60' against Beckman buffers 3501 and 3505, marked pH 7.00 * 0.1 and pH 10.00 * 0.01 a t 25" C. I n general, the p H values obtained were within 0.01 p H unit of those given by the manufacturer. Only those glass electrodes were used for obtaining data which gave the same values (within ~ 0 . 0 2pH unit) as the hydrogen electrode in the alkaline buffer standards. CALOMELELECTRODE POTENTIALS. I n glass electrode measurements, compensation for asymmetry and other peculiarities of the cell is provided for by calibration against standard buffers. The potential of the calomel reference cell is of no major consequence so long as it holds reasonably steady. In hydrogen electrode measurements the potential of the reference cell is required for calculation of pH. The literature reveals little uniformity of values for the saturated calomel electrode at 40" C. and upward and but few measurements. Vellinger (299) suggests the relation,
E, = 0.2622
- 0.00066t
where E, = potential of caolomel half cell t = temperature, C., while Leeds & Northrup consider as satisfactory values calculated from the equation,
Standardization Although there is still no universally accepted basis for pH standardization, some advances have been made in this direction within the last few years. The National Bureau of Standards has undertaken to reconcile and develop the necessary methods, data, and materials for standardization purposes (12, 81). The first s t a n d a r d i z e d material to become available under this program is potassium acid phthalate (standard sample 84a), serving now both as an acidimetric andpHstandard. We have used this as a primary standard in the manner to be indicated. The Nation$ Technical L a b o r a t o r i e s buffers are standardized on the basis of a value of 0.3358 volt for the 0.1 N calomel electrode at 25' C. in agreement with the work of Hitchcock and Taylor (14) and MacInnes, Belcher, and
725
E, = 0.2648 - 0.00076t
FINISHING ROOMIN
A
COMMERCIAL LAUNDRY
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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alkaline ranges. We conclude that the errors involved in our methods are considerably less than 0.05 p H and probably within +0.02 pH, or approximately the errors of glass electrode measurements themselves. We estimate the reliability of our pH measurements in general to be within 0.02 p H unit a t 25" C.; at 40",0.03; at 60°,0.05; and a t 80", within 0.10 unit of the correct value.
OF MODIFIED SODAUSED TABLEI. ANALYSIS
Constituent or Determination
Per Cent
40.94 45.15 0.15
0.02 0.01
13.73 40.60 30.05
Materials
COMPOSITIOX OF SOAPUSED TABLE11. ANALYTICAL Constituent or Determinationa, Equivalent t o (in Per Cent): Per Cent 1 0 . 2 Anhydrous soap 92.0 Alkalinity a s NaaO Total f a t t y acid 8 5 . 6 Silicate (0.74% Nan0 2.3% SiOn) 3.0 Si02 2 . 3 NaCl 0.8 NaCl 0 . 8 NazCOa 1.2 NaPCOa 1 . 2 Moisture and undetermined 3.0 Lose a t llOo C . 1.4 Titer of fatty acids, a C. 4 0 . 8 100.0 0 All determinations h y official methods of t h e American Oil Chemists Society.
+
__.
The modified soda used conformed to the A. S. T. M. specifications ( I ) and was obtained from a large stock, thoroughly mixed mechanically, and stored in sealed glass 'ars. An almost white, powdered high-grade commercial neutrai soap was used. Analyses of these materials are shown in Tables I and 11. All solutions were made up on a volume concentration basis (grams per liter at 24-26' C.). Conversion to other bases is made possible by the following analysis and the density data in Table 111, determined by pycnometer at 25" C.
TABLE 111. DENSITYOF MODIFIEDSODASOLUTIONS AT 25" C. (ACCURACY, +0.0002)
These different temperature coefficients lead to increasingly wide divergences a t temperatures above ordinary. Schollenberger (27) gives
Modified Soda Concn. Grams/liter Oz./gal.
Without soap
Density With 1 g./L soap
E, = 0.2804 - 0.00065 (t - 18)
which is in more general accord with Vellinger. Manufacturers of p H equipment are hesitant to commit themselves, but one (National Technical Laboratories) suggested the relation
E, = 0.2601
- 0.000606t - 0.00000109t2
from which working values were calculated and used in a manner t o be described in a section to follow. At higher temperatures these figures are in muoh better accord with Leeds & Northrup values than with the others mentioned. Entire reliance, however, was not placed upon particular reference electrode values. The hydrogen electrode-calomel electrode assemblies were checked against the Beckman pI-1 7 buffer a t each temperature. This procedure must be used with caution, but it is probably better than trying t o assign calomel electrode values above 40". Ordinarily the p H calculated from the above lvorking value for the calomel electrode agreed within 0.01 pH unit with the known pH. When disagreement was greater, different calomel electrodes were used. Readings with a particular calomel electrode were reproducible, but individual electrodes were occasionally found to be in error by as much as 2 mv. (0.03 pH). These calibrations mere made against the Beckman buffer of pII 7 at 25' C. rather than against potassium acid phthalate solution, first because of disagreement in the literature as to the reproducibility of readings obtained with platinized electrodes against potassium acid phthalate, and secondly because the ionic types existing in the neutral buffer approached more closely those of the unknown solutions to be measured. (Subsequent work in these laboratories has indicated that quite stable and reproducible readings can be obtained, especially with palladinized hydrogen electrodes, in Bureau of Standards potassium acid phthalate solution.) LIQUIDJUNCTION POTENTIALS. In theory it is desirable to calibrate the glass electrode against a standard buffer of approximately the same pH and composition as the unknown solution to minimize errors due to junction potential differences. Hamer (10) has shown that, for standardization against standard buffer arid measurements on relatively strong acid, the error can easily amount to 0.05 p H unit a t 25" C. No data have been found on similar errors occurring in the
Carbon-dioxide-free distilled water was used for the repart+ tion of all solutions. A pH of 6.9 at 25" C. was obtainefon this water by drawing it over into a closed Pyrex vessel in a manner entirely avoiding ex osure to the atmosphere. Before constant readings with the g&ss electrode were obtained, the water was changed approximately fifty times, but thereafter the readin s for successive samples were in agreement and a single sampfe would hold the same value for several minutes. The Corning 015 glass used has sufficient solubility to affect the pH in a short time. Powney and Jordan (24) reported 6.90 at 25" C. for the pH of distilled water, also measured with glass electrodes.
0.01
0.00I
0.I PER CENT Na20
1.0
OF PGBLISHED DATAON PH OF THE FIGURE2. COMPARISON SODIUM BICARBONATE-CARBONATE SYSTEM Curve Mol. Ratio Tpp., No. Source NaHC0a:NarbOs C.
1 2 3
RIenzel Menzel
1:l
2:l 1:1
6
9
10
2:l
0.91:l (approx.)
4 5 7 8
1:l 1.6:l
Authors' d a t a Authors' data
1.39:l 1.39:l 1.39:l
18 18 18
38 18 18 30
25 40 60
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
727
in all cases readings of 10.0 (bottom of scale) or less were obtained. Modified 250 c. 40' C. 60' C. 8 e The electrometrio pH values are Hydro en G1aes Hydrogen also shown in Figure 2 (curves 8, 9, G1ase i k y Hydrogen electrode 22; eleotro5e trode electrode Grams/ and 10) in comparison with results of pH' Obsvd. E pH pHO pH0 Obsvd. E pHa 0 b s v d . E pH pH Liter previous investigators. The relation A . Modified Sod8 Solutions will be discussed later. Modified soda .15 ... 8.49 9 . 1 3 0.7845 0.012 solutions are characterized by re... 98.87 ... ... 0.03 ... 9... 9.58 .19 . . . .., 9.78 markably flat pH maxima a t con0.06 9.92 0 .'8361 9 : i o 0 :Si80 .41 9.39 9 . 9 1 0.'8'3'03 0.12 centrations between 0.5 and 1.0 gram 9 . 9 9 0.8354 10.00 0.30 ... 9.60 ... 9.68 10.00 0.8300 10.02 0.60 per liter at 25-60" C. At 25' a 0.'8444 9 . 8 5 0. s i 5 0 9 . 6 7 9.68 9.97 0.8350 10.01 1.2 9.92 ... 9 . 6 0 0.8512 9 . 6 1 ... two-hundred fold change in concen9.90 0.8300 3.0 9.73 9.55 9.50 9.83 9 . 7 8 0.8245 6.0 tration may be made within a pH 0 .'8303 9 . 6 3 0.8456 9 . 5 3 9.73 9.40 9.67 0.8190 12.0 range of 0.3 unit. Dilution bufferB . Modified sods-so8p Solutionaa ing capacity diminishes slightly with 0.8420 9 . 4 7 10.36 1 0 . 0 0.8523 9 . 9 8 9 . 2 4 10.4 0.8562 0 0.8416 9 . 4 7 9.98 ... 0.8520 9 . 9 8 increase in temperature but remains 10.38 0.8565 10.37 0.012 9.96 ... 0.8427 9 . 4 9 ... * 10.37 0.03 exceptional. pH diminishes regu. . . . . . ... 9.96 10.35 0.06 0 :si90 9.93 . . . . . . ... 9.95 10.30 O.b;5iO 10:29 0.12 larly with rise in temperature a t all ... ... 9.43 9.92 ... ... 10.18 0.3 9.44 ... 0 ..Si30 9.'64 9.89 ... concentrations. The low maximum 10.09 0.6 0 . ' 8 4 4 0 9 . 8 5 . . . . . . 9 . 4 0 9 . 9 8 9.87 10.00 0 .'8334 1.2 pH of 10.02a t 25" reflects the physio... 99 .. 07 89 ... ... 9.38 3.0 9.91 ... 9.33 0.'8495 9 . 6 9 9.83 9 . 8 2 0.'8246 0.0 logical mildness of modified soda 0 .'8305 9.133 ... 9.74 0.8450 9.52 9.58 9 . 6 9 0.8195 12.0 solutions. a Observed readings not corrected for sodium error. The presence of soap in modified 6 Containing I gram' of soap per liter. soda solutions produces interesting effects (Table IVA). At very low modified soda concentrations, the Calculation of pH Values pH values closely approximate those of the soap solutions. At high concentrations the behavior is practically that of the The pH values were calculated from hydrogen electrode builder alone (Table IVB). Intermediate concentrations data by the equation, show transitional values. At 25' C. the pH of soap is reduced by modified soda a t all pH (E Eb Ec) -i- 0.000198322' concentrations, the reduction increasing with concentration. where E = observed potential At 40" the behavior is similar but less marked. At 60",reE* = aqueous vapor pressure and barometric correction duction no longer occurs but all concentrations of modified @7) soda increase the pH of the soap solution, while a t 80" a still EE 3 calomel electrode potential greater increase occurs. Normal atmospheric pressure where the measurements were made was taken to be 740 mm. Variations of * 10 mm. I I I I 1 affect pH by less than 0.01 pH. Aqueous vapor pressures of solutions were taken as those of pure water. Values of E, based on the relation suggested by National Technical Laboratories are:
T A B LIV. ~ ELECTROM~TRIC PH VALUS~S FOR SOLUTIONS
czti.,
t;t:i
... ...
L.
..
...
. a .
...
...
...
tJ:zi
...
...
9 . .
+ -
.
Temp., EO
' C.
25 0.2443
40 0.2341
60 0.2198
These values were found to be in agreement with our calibrations generally.
Discussion of New Data The experimental data are presented in Table IV and Figures 2 to 4. The concentration range, 0.01-10.0 grams per liter, for practical purposes is equivalent to 0.001-1.0 per cent solutions. No corrections for sodium-ion error are included in the data. Such corrections are unnecessary a t 25' C.below modified soda concentrations of 1.2 grams per liter and the presence of 1 gram of soap per liter introduces no correction up to pH 10.5 (24). Values a t higher temperatures were not corrected since there is no satisfactory method for making this correction except by comparison with hydrogen electrode values. These were obtained only a t a sufficient number of points to establish the reliability of glass electrode methods and t o indicate sodium-ion errors. Colorimetric and electrometric values are compared in Figure 4. Indicators used with the Taylor set were phthalein red for values above 8.6, cresol red for those below; with the Hellige, phenolphthalein above 8.6, cresol red below. I n the case of those soap solutions having electrometric pH values above 10.0, samples were also tested with Taylor acyl red and Hellige nitro yellow but
I 20
I
I
I
I
I
1
30 40 50 60 70 80 TEMPERATURE-DEGREES CENTIGRADE
FIGURB 3. THERMAL PH COEFFICIENTS FOR SOAP AND SOAP-BUILDER COMBINATIONS
An argument long used against modified sodas for laundry purposes has been that they reduce the pH of soap solutions, and such a reduction is represented as diminishing the detergent activity of the soap. This argument can no longer be urged among laundrymen who wash cottons and linens in the vicinity of 70" C. (158" FJ, as shown in Figure 3 where pH has been plotted as a function of temperature for straight soap and for soap-builder ratios covering laundry practice. Figure 3 was plotted from data in Table IV.
INDUSTRIAL AND ENGINEERING CHEMISTRY
728
high; a t lower concentrations still within the range of detergent interests, the divergence becomes as great as 0.5 pH unit. 1T7emay conclude that a t lower concentrations, such as encountered in rinsing, ordinary colorimetric methods would give no indication of true pH. At the ordinary coacentrations, the Hellige method (phenolphthalein) seems more accurate in the presence of soap and the Taylor method (phthalein red) appears preferable in the absence of soap.
too 9.6 92
I,
/
8.8
8.4
,
/
I
I
2 5"
A/
,/
VOl. 33, No. 6
0- E L E C T R O M E T R I C
1
0- H E L L I G E C O L O R I M E T R I C
8.0
a-TAYLOR
COLORIMETRIC
I
Comparison of New Data with Published Results
Hastings and Sendroy ( I S ) , Kiehl and Loucks (16), Kolthoff and Bosch ( I 7 ) , and Menzel (19) have published hydrogen electrode data which by adaptation give some comparison with our results on modified soda. Our interpretation of their results appears in Figure 2. Density has been taken into con10.0 sideration and various devices of graphical interpolation used 96 in reducing all data to a common basis. It has been most 01 10 100 0 01 convenient to recalculate all results to per cent iYazO. The NiODIFIED SODA CONCENTRATION-GRAMS P E R LITER calomel electrode values, equations, and p H values given by FIGURE4. COMPARISON OF COLORIMETRIC AXD ELECTRO- the original authors have been followed. METRIC PH VALUES AT 25' c. FOR MODIFIED SODA SoLTJTIoNs Precise coniparisons fail in all but a few instances because (above) AXD MODIFIED SODA-SOAP SOLUTIONS (below) of differences in sodium bicarbonate-carbonate ratios or temperatures. This ratio for the modified soda used in this xork is on the molecular basis 1.39 to 1. After making allowances for differences in ratios and temperatures, we conclude For most practical purposes the thermal p H coefficient of that our data a t 25" C. and at 0.1 per cent NazO are perhaps a given solution of this class may be regarded as uniform be0.05 higher than those of Kolthoff and Bosch, in good agreetween 25' and 60" C. With decreasing soap-modified soda ment with Menzel, and perhaps 0.05 lower than those of ratios these coefficients diminish. The thermal p H coefficient, ApH/A.T, for straight soap is two to three times as Hastings and Sendroy. At higher concentrations our results accord better with Kolthoff aiid Bosch than Menzel, whose great as for soaps built to the extent used in laundry practice where soap-builder ratios rarely fall outside the limits 4:1 1:1 ratio curve is too flat and idlose values are increasingly high. The Kiehl and Loucks' values for the 1:1 ratio are into 1:2. consistent with those of Menzel, their curve appearing to be At a soap concentration of 0.1 per cent the practical laundry as much too steeg as that of Menzel is too flat. builder range of modified soda concentration can be taken as approximately 0.03 to 0 . 3 per cent (0.3 to 3 grams per liter). Powney and Jordan (23) have not presented their data on For t.his range Table IVB shows that the entire pH difference NaHCO3.NapC03213,O by the glass electrode method in a form which permits close comparison. They alone seem to would be 0.04 at 40" and 0.10 unit a t 60" C., yet great difhave worked at low concentrations comparable with our own. ferences in effective washing capacity result from the use of various compositions within this range. Such conditions do At the upper concentrations their results appear high in relation to all othez data; at the lower concentrations there is not permit practical cont,rol of detergent solutions by any no sufficient basis for drawing a comparison between their form of pH measurement, colorimetric or electrometric, but values and ours. titration or other means of analysis provides an effective means of control. Given proper titration data, the practical laundryman would predict with considerable accuracy which Application of pH Measurements to Laundry of a number of such solutions, all of the same pH value, would Control give the best results. Figure 4 compares pH values of modified soda solutions Our combined field and laboratory experience led us t o determined electrometrically and colorimetrically by two doubt seriously whether p H as measured in laundries actumethods a t 25' C. If we accept *0.2 pH unit as a fair standally had any close relation to correct pH values and whether ard for the method to attain and limit, ourselves for the mothere was any reasonable agreement among measurements by ment to builder concentrations within the range of laundry various laundries. Sccordingly, a solution of modified soda interest (0.30-3.0 grams per liter), the Taylor instrument just v a s made up having a pH of 9.92 at 25" C., checked by both fails to meet the specifications over this range, while the glass and hydrogen electrodes. Such a solution contains 3.0 Hellige instrument fails except at the higher concentrations. grams of modified soda per liter, falls in a \vel1 buffered region, At still higher concentrations, comparisons are better but the and is an easy system to measure. Samples were sent out to colorimetric methods do not indicate satisfactorily the falling service men and laundries without telling them the pH, with pH trend, At lower eoncentrations divergence from the electhe request that the pH of these solutions be measured in trometric values increases a t a rapid rate, being more than laundries known to be using this means of control. It was half a pR unit, a t 0.1 gram per liter and hopeless below that. asked that the cell measurenients be made in exactly the same The use of more refined colorimetric methods involving manner and under the same conditions as their regular conisohydric indicators would yield more accurate results for this trol operations. Thirty-tJvo returns representing measuresystem, but such procedure8 are not practical for laundry conments in ten cities were received, the values ranging from 9.2 trol work. to 11.0 and dktributed as follows: I n the presence of soap, results are entirely different but no better (Figure 4). IVithin the ranse of laundry interest both 9 2 g 6 9 6 9 7 9 8 9 9 100 pH values No. of returns 1 1 1 2 2 6 4 colorimetric curves cross the electrometric and are correct at single but considerably different concentrations. A t pH values 10 1 10 2 10 3 10 4 10 5 11.0 KO.of returns 8 3 3 2 3 1 higher builder concentrations colorimetric values become too
LA/ , , , , , , , I
0.01
0.1
,
!
, ,,, I
I .o
I
June, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Of the thirty-two measurements, ten were made by the Hellige method and eight by the Taylor. Hellige values ranged from 9.5 to 10.5, and Taylor values from 9.8 to 11.0. Except for one return at 11.0, seven of the eight Taylor measurements ranged from 9.8 to 10.0. Three glass electrode determinations came in, ranging from 9.8 to 9.9 (uncorrected for sodium-ion error). Of the thirty-two returns, seventeen may be regarded as satisfactory by colorimetric standards; if the glass electrode measurements are eliminated, this number reduces to fourteen out of twenty-nine colorimetric measurements made.
w.4SH
729
builders, and in some cases, as we have shown, p H values fail entirely to reflect large composition differences in laundry detergent solutions. The individual pH characteristics of soaps are given insufficient weight in relation to the attention paid to alkalies used as builders in general; worse yet, most of the more reliable pH measurements on builders have been a t concentrations above those of laundry usage. These views result from attempts to gather a comprehensive system of pH data from the literature, covering exhaustively all the common builders and many of the soap and soap-builder systems.
ROOMIN A MVDERN INSTITUTIONAL LAUNDRY
In carrying out this test, particular consideration was given t o the errors which might arise. Several samples not sent out were checked every few days with the glass electrode and found to maintain a constant pH. When the unused portions of the samples were returned from the field, they were rechecked with glass electrodes. A11 values agreed with the original within *0.01 pH unit. We regard the accuracy of this group of pH measurements as representative OP or superior to the usual p H measurements made in laundries today. The portable glass electrode instruments of better construction afford a notable advance in pH measurements, but all models are not yet fully adapted to laundry service Temperature compensation should be provided to 80’ C. so that measurements may be taken on solutions directly under conditions of use. The glass electrode will be much more serviceable and dependable if it can be made less sensitive to sodium-ion concentration, elevated temperatures, and errors at high p H levels. Further improvements along these lines may be expected. Aside, however, from the question of ability to make corTect pH measurements, other considerations in our view limit the value of this method of control of wash wheel operations. The scientific and technical literature does not yet afford an adequate background of pH data, and relations between pH and detergent efficiency are much less understood. These difficulties may in time be largely eliminated but only after much further careful effort has been expended. Each builder has individual characteristics which distinguish it from other
Ordinary colorimetric p H measurements are subject to still further limitations. They are properly made only at room temperatures and do not correctly reflect conditions which prevail as detergent operations are carried out. Very few systems have been investigated to the extent that temperature or concentration corrections to colorimetric measurements can be applied, and any system of corrections would presuppose knowledge of the composition and concentration of the solution, frequently unknown. A further difficulty lies in the fact that concentrations are so low in many instances, particularly in rinsing, that the characteristics of the indicator predominate over those of the materials over which control is sought. We believe that simple titrations which are easily, inexpensively, and quickly made afford a far better primary means of concentratmioncontrol in this field than pH measurements.
Literature Cited (1) ,4m. SOC. for Testing Materials, Standard Specification or Modified Soda, D-457-39 (1939). (2) Bleyberg, W., and Lettner, H., Chem. Umschau Fette, Oele, Wachse, Harae, 39, 241-53 (1932). (3) Clark, W. hl., “Determination of Hydrogen Ions”, 3rd ed. Baltimore, Williams and Wilkins Co., 1928. (4) Colton, C. W., and Snell, B. V., Oil & Soap, 17, 33-8 (1940). (5) Dole, Malcolm, J. Am. Chem. Soc., 53, 4260-80 (1931). (6) Dole, Malcolm, J. Chem. Phys., 2,862-6 (1934). (7) Dole, Malcolm, and Wiener, B. Z., Trans.Electrochem. Soc., 72, 107 (1937). (8) Gardiner, W. C., and Sanders, H. L., IKD. ENG.CHEM.,Anal. Ed.. 9, 274-8 (1937).
INDUSTRIAL A N D ENGINEERING CHEMISTRY Halvarsan, H. O., Proc. SOC.Ezptl. Biol. Med., 22, 358-61 (1925).
Hamer, W. J., J . Electrochem. SOC.,72, 45-69 (1937). Hamer, W. J., private communication. Hamer, W. J., and Acree, S. F., Natl. Bur. Standards, Research Paper 1261; J . Research Natl. Bur. Standards, 23, 647-62 (1939).
Hastings, A. B., and Sendroy, J., J. Biol. Chem., 65, 445-55 (1925).
Hitchcock, D. I., and Taylor, A. C., J . A m . Chem. SOC.,59, 1812-18 (1937).
Jordan, D. O., Tkans. Faraday SOC.,34, 1305-10 (1938). Kiehl, S. J., and Louoks, R. D., Trans. Electrochem. Soc., 67, 81-100, esp. 90 (1935). Kolthoff, I. M., and Bosch, W., Rec. trau. chin., 47, 819-25 (1928).
MacInnes, D. A., Belcher, D., and Shedlovsky, T., J . Am. Chem. SOC.,60, 1094-9 (1938). Menzel, Heinrich, 2.physik. Chem., 100, 276-315 (1922). Natl. Technical Lab., Pamphlet 34 (May, 1940). Perley, G. A., Trans. A m . Inst. Chem. Engrs., 29, 257-91 (1933).
(22) Powney, J., and Frost, H. F., J. Testile I d . , 28, TZ37-54 (1937). (23) Powney, J., and Jordan, D. O., J . SOC.Chem. I d . , 56, 133-7 (1937). (24) Powney, J., and Jordan, D. O., Trans. Faraday SOC.,34, 363-71 (1938). (25) Preston, W. O., Oil & Soap, 14, 289-95 (1937). (26) Rhodes, F. H., and Bascom, C. H., IND. ENQ.CEIEM.,23, 778 (193 1). (27) Sohollenberger, C. J., in Lange's Handbook of Chemistry, 3rd ed., p. 902, Sandusky, Ohio, Handbook Publishers, 1939. (28) Taylor. W. A., and Co., "Modern pH and Chlorine Control", 5th ed., p. 9 (1940). (29) Vellinger, E., Arch. phys. biol., 2, 119-22 (1926). (30) Wingfield, E., and Aoree, S. F., Natl. Bur. Standards, Research Paper 1018; J . Research Natl. Bur. Standards, 19, 16375 (1937). (31) Wingfield, B., Goss, W. A., Hamer, W. J., and Acree, S. F., A . S. T . M . Bull., pp. 15-20 (Jan., 1938). PRESENTED before the Division of Industrial and Engineering Chemistry st the lOlst Meeting of the American Chemical Society, St.Louis, Mo.
Nomograph for the Solubility of Sulfur Dioxide in Water
T
HE latest data covering the equilibrium solubilities of
sulfur dioxide appear to be those of Beuschlein and Simensonl, presented in tabular form and graphically as pressure-temperature curves for concentrations of 0.51, 1.09, 4.36, and 7.45 grams of sulfur dioxide per 100 grams of water. A study of the data was made with a view toward facilitating interpolation, and the following equation, connecting pressure and temperature, was developed,
+
0.61 2,3145 0.004434
1.09 2.1883 0.016033
4.36 1.7350 0.4684
dioxide and exhibits a partial pressure of 344 mm. of metcury at 50' C. The average deviation of partial pressures, as read from the chart, from those of the original data is less than 2 per cent.
t, 'C.
"3
and a and b have values corresponding to S, concentration, in grams of sulfur dioxide per 100 grams of water as follows: a b
D. S. DAVIS Wayne University, Detroit, Mich.
Temperature
p [l - 65 p 2 ] = b (t 20)a where p = partial pressuroe of SO2, mm. Hg t = temperature, C.
S
Partial Pressure of SO, P.
mm. ffq
E6o
7.45 1.2813 4.803
80 io0
Additional values of a for other values of S were calculated by means of the well-known La Grange interpolation formula, and a large-scale smooth curve was constructed. For all but the lowest values of S, values of b can be calculated from the equation: log b = 1.0106 SO.625
Vol. 33, No. 6
- 2.863
b may be read satisfactorily from a plot of log b us. for coslcentrations below 1.09 grams sulfur dioxide per 100 grams of water. These equations enable construction of the accompanying line coordinate chart which considerably extends the utility of the original data, since it places interpolation on a convenient and reproducible basis and includes solubilities both as grams sulfur dioxide per 100 grams of water and as grams of sulfur dioxide per 100 grams of solution or percentage concentration. The index line shows that a solution of 2.60 grams of sulfur dioxide in 100 grams of water tests 2.44 per cent sulfur 1 Beuschlein, W. L., and Simenson. L. O., J . Am. Chem. SOC.,62, 810 (1940).
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