Buffer for High pH Measurements W. M . TUDDENHAM AND D. H. ANDERSON Color Control Department, Eastman Kodak Company, Rochester, N. Y. A calcium chloride-hydroxide buffer system has been devised having a reproducible pH in the range of 11 to 12.65, depending on the concentration of calcium chloride. Such buffers are useful in checking the perSprmance of pH meters and electrodes in this pH range. A study of the precision of glass electrode measurenients in this range - indicates that their precision is largely dependent on the variability between electrodes.
I
K ISDUSTRIAL processes, many solutions are maintained a t
pH values as high as 12 to 13. The routine measurement of pH provides one analytical control for these solutions. I n the .past, these pH measurements have been made in this laboratory by standardizing with a borax buffer at pH 9.18 and checking the electrodes with another buffer a t 4.01. The pH measurements in the region '11 to 13, therefore, represented considerable extrapolation from the standardizing value, and this caused uncertainty a t times as to whether the solution measured or the electrode of the pH meter was in error. It was felt that a standard buffer in this pII range would assist in removing these uncertainties. A good buffer should be easily prepared, stable for long periods, and capable of prerise reproduction of pH, and should have a good buffer capacity. The buffer system investigdted was a calcium chloride solution saturated with calcium hydroxide. This system functions in the folloking manner: Addition of acid in small amounts results in neutralization of some of the calcium hydroxide. The latter is present as a slurry and as the dissolved calcium hydroxide is neutralized, more solid calcium hydroxide goes into solution, maintaining an approximately constnnt pH. On the other hand, addition of a base precipitates c:d(4um hydroxide, thus also maintaining the pH a t an approximately constant value. The effect of carbon dioside is to precipitate calcium carbonate. Again calcium hydroxide goes into solution to maintain the concentration of calcium ions constant and so the pH does not change appreciably until a large fraction of the calcium hydroTide has been converted to calcium carbonate. The solutions measured in this laboratory are in the neighborhood of 2 molar in sodium ion, and in order to minimize any variation in pH meter readings due to sodium ion error, oneof the buffet, solutions was made 2 molar in sodium chloride. It i s reasonable to believe that systems such as strontium chloride-hydroxide, harium chloride-hydroxide, and magnesium chloride-hydroxide would act in much the same manner, the pH range covered being largely dependent upon the concentration of the chloride and the solubility of the hydroxide in question. Preliminary experiments showed that the calcium chloride-hydroxide buffer had pH values in the range of primary'interest to this laboratory; therefore it was chosen for investigation.
1
2
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6
6
4
12,4A
12.2
-.
"' - 1
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I
0.2 0.4 0.6
Figure 1.
Molarity Calcium Chloride Effect of Calcium Chloride Concentration on pH
Solution of caloium chloride satureted with calcium hydroxide A . Calculated curve B . Curve based on experimental values
shows the calculated dependence of p H on molarity of calcium chloride. Three experimental values are represented by chcles. The difference between measured and calculated values is greatest in the more concentrated solutions. The solution prepared to contain 0.2 mole of calcium chloride and 2 moles of sodium chloride per liter gave a pH of 11.97 a t 20' C. and was considered most suitable for the intended purpose. This was called buffer 1. One or .2 days were required for the solutions to come % equilibrium. Calcium Hydroxide Buffer, 0.2 Molar in Calcium Chloride and 2 Molar in sodium Chloride. Pipet 25 ml. of 8.00 N hydrochloric acid into a 500-ml. volumetric flask. Add about 300 ml. of water. Add 7.5 grams of reagent quality calcium hydroxide. Add 58 grams of sodium chloride and dissolve by swirling the stoppered flask. Dilute to the mark with water. Transfer to a glassstoppered reagent bottle. Add 5 grams of calcium hydroxide and shake solution several minutes. Allow the solution to stand for 2 days before using.
APPARATUS AND REAGENTS
By varying the concentration of hydrochloric acid or omitting the sodium chloride, other pH values can be obtained.
Three types of instrument assemblies were used for pH measurements of the buffers described. One was a Leeds & Northrup, catalog No. 7673, thermionic amplifier, and a Rubicon Type B potenbiometer. Most of the pH values, however, were obtained with Beckman Model G pH meters. A few readings were taken with a Beckman hlodel H pH meter. The assemblies were standardized using borax buffer having a pH of 9.18 a t 25" C. Beckman Type E glass electrodes and saturated calomel reference electrodes were used throughout the entire series of measurements. Reagent quality chemicals were used throughout the study.
PROPERTIES
Reproducibility. BUFFER1 (2 MOLARI N SODIUMCHLORIDE). The pH values obtained for several mixes prepared to contain 0.2 mole of calcium chloride and 2 moles of sodium chloride per liter of saturated calcium hydroxide solution are given in Table I. These solutions were prepared at different times by different people and the pH of each solution was measured one or more times, as shown. I n all cases, excess calcium hydroxide was present as a slurry. The electrodes used were those in routine use with the respective instruments. The average of 24 values was 11.852 with a standard deviation of 0.030. The measure-
PREPARATION O F BUFFER WITH pH OF 11.97 A T 20' C.
The buffers were prepared by adding an excess of calcium hydroxide to a predetermined volume of standard hydrochloric acid. Several concentrations of calcium chloride were tried. Figure 1
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V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0 Table I. pH Values, Buffer 1 (Obtained at 23.8O f 0.4O C. for solutions 0.2 molar in calcium chloride, 2 molar in sodium chloride, and saturated with calcium hydroxide) Model G Potentiometer pH Meters Model H pH Meter Solution Solution Solution age, Solution No. age, days pH months pH age, days pH 1-1 1 11.810 5 ' d a y s 11.83 5 11.86 4 1 1 83 7 11.81 2-1 Fresh 11.896 4 11.85 Fresh 11.88 7 11.78 3-1 3 11.848 4-1 3 11.849 .. 5-1 3 11.848 .. 6-1 3 11.846 4 days 11.93 .. .. 7-1 5 days 11.88 7 days 11.85 3 11.87 4 days 11.87 8-1 5 days 11.86 7 days 11.84 3 11.85 . 4 days 11.88 9-1 11.85 7 days 3 11.84 '
..
11.3
20
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ze
14
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a6
28
Temperature
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30
32
thermionic amplifying unit. The temperature coefficient for buffer plus electrodes in the range 20" to 35" C. is about 0.03 pH unit per degree. The temperature coefficientsfor the other buffers tested appeared to be of the =me order of magnitude. If any of these buffers is to be used a t a temperature greatly different than 24" C., its pH value should be determined a t that temperature. Inasmuch as the pH a t any temperature is dependent upon the solubility of calcium hydroxide, one must be sure that the equilibrium condition has been realized a t the time the solution is used. -4s far as possible, these buffers should be used only a t the temperature a t which they are stored. Buffering Capacity. The buffer capacity was checked by titrating the buffers with 0.1 iV sulfuric acid and with 0.1 N sodium hydroxide. In doing this, care was taken to shake the buffer 90 that the sample titrated cont:tined some of the solid calcium hydroxide. The acid titration of buffer 1 represented by curve A , Figure 3, resulted in a pH rise that agreed well with the calculated rise due to dilution alone. Addition of sulfuric acid to the buffer dissolves calcium hydroxide and precipitates calcium sulfate. The solubility of calcium sulfate is low enough (Scm, = 2.25 X lo-' a t 25' C.) ( 4 ) that one can assume virtually complete precipitation. This leaves only the effect of dilution. Dilution with distilled water changes the molarity of the caIcium chloride, with the greatest effect on pH a t high concentrations. The pH change resulting when a solution 2 molar in calcium chloride and saturated with calcium hydroxide is diluted l to 1 is about 0.5 pH unit as determined from curve B , Figure 1. If a 0.2 molar solution is diluted 1 to 1, the pH change should be about 0.16 unit. If one adds an acid, such as hydrochloric, that will not cause precipitation of calcium ions, a different effect is encountered. Calculation shows that the results of addition of hydrochloric acid will depend upon the ratio of the concentration of acid used to the original molarity of calcium chloride. Taking activity
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OC.
Figure 2. pH us. Temperature 2 M calcium chloride saturated with calcium hydroxide
ments were made a t room temperature, which was maintained a t 23.8' + 0.4: C. Part of the standard deviation repre&nts the reproducibility of preparing the solutions and part is contributed by the use of different glass and reference electrodes in these measurements. To give a better indication of the reproducibility, independent of a glass electrode, hydrogen electrpde measurements were made on solutions 3-1, 4-1, 5-1, and 6-1. The cell used included a 3.5 N potassium chloride salt bridge. The hydrogen and calomel electrodes were standardized with 0.01 M borax buffer. Because of the several liquid junctions involved, some constant error may have been introduced into these measurements. Because of this and other possible sources of error, the values obtained are reported merely to indicate the reproducibility of preparing buffer solutions of this type. These data, shown in Table 11, indicate that the buffers in question are reproducible as to pH. BUFFER 2 (No SODIUMCHLORIDE). The data obtained for saturated calcium hydroxide solutions that were 0.2 molar in calcium chloride but contained no sodium chloride are given in Table 111. Here again solid calcium hydroxide was present in the samples tested. The averagc! of 17 pH values a t 23.8" 0.5" C. was 12.063. The standard deviation of these data is 0.038. Hydrogen electrode measurenients of solutions 3-2, 4-2, 5-2, and 6-2 are shown in Table IV. Effect of Temperature on pH. Figure 2 is a plot of pH versus temperature for a third buffer, 2 molar calciuin chloride saturated with calcium hydroxide, containing no sodium chloride. The data for this plot were obtained using the potentiometer and the f
Table 11. Hydrogen Electrode Measurements of High pH Buffer Solutions at 25' C., Buffer 1 10.2 M CaCI,, 2 M NaCI: saturated writh Ca(0H)rl Solution NO. 3-1 4-1 5-1 6-1
PH 11.73 11.74 11.74 11.74
Table 111. pH Values, Buffer 2 (Obtained at 23.8" t 0 . 5 O C. 0.2molar in calcium chloride and saturated with calcium hydroxide) Model G pH Meters Model H P H Meter Potentiometer Solution Solution Solution Solution age, days pH age, months pH age, days pH NO. 1 12.08 5 days 12 10 5 12.10 1-2 4 12.08 7 12.02 Fresh 12.118 4 12.08 Fresh 12.10 2-2 7 .12 03 3 12.015 .. ... .. 3-2 ... 3 12.015 4-2 3 12.009 ... .. 5-2 . . .. 3 12.019 6-2 .., 4 days 12.12 7-2 5 days 12.06 7 days 12.04 3 12.08
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.. ....
..
..
... ... ...
Table IV. Hydrogen Electrode Measurements of High pH Buffer Solutions at 25' C., Buffer 2 [ 0.2 M CaCh saturated with Ca(0H)tl
Solution No. 3-2 4-2 5-2 6-2
PH 11.88 11.86 11.88 11.88
ANALYTICAL CHEMISTRY
1148 .coefficients equal to unity, if the normality of the acid is greater than twice the molarity of the calcium chloride, the pII should drop upon addition of acid in the presence of excess calcium hydroxide. If it is just twice the concentration of the calcium chloride, the pH should remain constant. If the normality of the acid is less than twice the molarity of the calcium chloride, the pH should rise. Experiment showed that .the titration of a buffer that was approximately 0.5 molar in calcium chloride with 0.1 N hydrochloric acid resulted in an initial rise in pH, as expected. The addition of sodium hydroxide to these buffers precipitates calcium hydroxide and dilutes the solution. Both effects tend to raise the pH. As is Seen in curve B , Figure 3, the pH change for buffer 1 is greater in the early stages of the alkaline titration than in the acid titration. For uae as a standardizing medium for high p H measurements, a buffer ought to have enough buffering capacity to ensure that the glass electrode attains its equilibrium condition rapidly. Perley has shown that the response time of glass electrodes in poorly buffered solutions is slow (3) The curves of Figure 3 show that the buffer capacity is adequate in this respect and in addition the buffer is capable of reacting, without an appreciable change in pH, with traces of previous solutions that may remain in the sample container or on the electrodes.
The standard deviation of duplicate determinations is about 0.009. The fact that the mean square for GE X R E is of the same magnitude indicates that there are no appreciable interactions among the electrodes tested. The much larger magnitude of the mean square for electrodes suggests that the variation between electrodes is the most important source of reduced precision in measurements of high pH solutions.
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12 II 10
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n 9
8
7 6
IO
20
30
40
50
60
70
80
90
Table V. Duplicate pH Values
Miili I iters Added Figure 3. Buffer Capacity of Buffer 1
[Solution 0.2 molar CaClr, 2 molar NaCl and saturated with C a ( 0 H h obtained with eight oombinationa of four g1a.m and two calomel electrodes]
A . 50-ml. sample titrated with 0.1 N sulfuric acid B . 25-ml. sample titrated with 0.1 N sodium hydmxide
A 11.92 Reference electrode1 11.90 11.91 Av. Reference 11.93 electrode11 11.93 11.93 Av. Glass electrode aver11.92 ages Reference electrode range 0.03
Glan3 Electrode B C 11.95 1 1 93 11.94 11.94 11.94 11.94 11.95 11.93 11.96 1 1 95 11.96 11.94
D 11.91 11.90 11.90 11.92 11.93 11.92
11.95 11.94 11.92 0.02
0.02
0.03
Reference Electrode Averagea
Glass Electrode Range 0.04 0.04
11.94
The accuracy of glass electrode measurements a t high pH values is also limited, especially in solutions containing sodium chloride. If both the standardization medium and the solutions to be measured have approximately the same sodium ion concentration, this error should be minimized (2).
11.93
CONCLUSION
11.92 0.03 6.03
Total range
0,06
The data were analyzed ( 1 ) and are sumfnarized as follows: Source of Variation Glass electrodes Reference elecrrodea GE X R E Duplicates
Degree of Freedom 3 1 3
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100
Mean Square 0.0010396 0.0OO6563 0.0000896 0.0000812
In order to guard against any lowering of pH due to the presence of acid, some calcium hydroxide should be present as a slurry. No difficulty has been experienced with small amounts of the precipitate adhering to the electrodes. It is recommended, however, that the slurry be allowed to settle before the electrodes are immersed in the solution, and that the electrodes be carefully washed and wiped after each use. GLASS ELECTRODE MEASUREMENTS IN HIGH pH SOLUTIONS
The pH of buffer 1 (2molar in sodium chloride) was found to be 11.85 with a standard deviation of 0.03 a t 23.8“ C. It was stated that part of the standard deviation represents the reproducibility of preparing the solutions and part is contributed by the use of different glass and reference electrodes. With the high pH buffer, an attempt was made to determine the precision of glass electrode me&surements a t this high pH. Duplicate measurements were made on buffer 1 using eight combinations of four glass and two calomel electrodes. The measured values are shown in Table V.
Calcium chloride-hydroxide buffer systems may be prepaied covering the pH range from 11 to 12.65,depending on the concentration of calcium chloride. Alth‘ough the exact pH values of the buffers studied have not been determined, it is felt that the data presented are adequate to show the potential value of these buffers as standardizing media. The reproducibility and stability of the solutions are considered to be acceptable for such use. The addition of sodium chloride does not appear to affect the pH reproducibility or stability of the solutions to any extent. Although the pH values obtained with the glass electrode are probably somewhat in error, it is believed that they are of value as arbitrary standards. The precision of glass electrode measurements in the pH range 11.0 to 12.0 is largely dependent upon the variability between electrodes. ACKNOWLEDGMENT
The hydrogen electrode measurements were made by John McCallum, Physical Chemistry Department, Research Laboratories, and the statistical interpretations were made by Grant Wernimont, Color Control Department, Eastman Kodak Company, Rochester 4, N. Y. LITERATURE CITED
(1)Brownlee, K.A., “Industrial Experimentation,” p. 102, Brooklyn, N. Y., Chemical Publishing Co., 1948. (2) Dole, M., “The Glass Electrode,” pp. 135-7, New York, John Wiley & Sons. 1941. (3) Perley, G. A., ANAL.CHEM.,21, 559-62 (1b49). (4) Rieman, W., Neuss, J. D., and Naiman, B., “Quantitative Analysis,” p. 70, New York, McGraw-Hill Book Co., 1942. RECEIVED June 5, 1950.
