THE DETERMINATION OF THE ISOELECTRIC POINT LYLE VERNON ANDREWS
AND
D. J. BROWN
T h e Chemical Laboratorg, University of Nebraska, Lincoln, Nebraska Received J a n u a r y 18, 1983
The isoelectric point may be calculated as outlined by Kuhn (1) and Simms (2) from the basic and acidic ionization constants of the oxide. These constants are often unreliable. This electrometric method is suggested as a means of measuring the isoelectric point of an amphoteric oxide. MATERIALS AND APPARATUS
A. Mercuric oxide. One of the best grades of commercial oxides, red variety, was used. Half-cells containing this commercial form of mercuric oxide gave potentials which agreed to within 0.0001 volt with half-cells containing red mercuric oxide prepared by decomposing purified mercuric nitrate. B. Lead oxide, red variety, was precipitated from a hot 12 normal solution of potassium hydroxide, as described by Smith and Woods (3) by adding powdered lead acetate to the hot alkali. The oxide was washed by decantation with hot 10 normal potassium hydroxide, then washed thoroughly and repeatedly with hot distilled water, and dried over sulfuric acid in a vacuum desiccator. Analysis for lead, according to the gravimetric method of Brown, Moss, and Williams (4) gave 99.7 per cent of PbO. A small amount of water was evolved when the oxide was heated strongly. C. Lead oxide, hydrated, was precipitated from a dilute lead acetate solution by the addition of a dilute solution of potassium hydroxide a t room temperature. The precipitate was washed by decantation with warm distilled water and dried over sulfuric acid in a vacuum desiccator. Analysis gave the ratio of hydroxyl to lead as being very close to 2: 1, showing the absence of a basic salt. The per cent of lead was 89.77, which agrees well with Mueller ( 5 )who found 89.75 per cent and represented the composition of the oxide by the formula (Pb0)3H20,which requires 90.40 per cent of lead. D. Potassium hydroxide solution was prepared as described by Ming Chow (6). A 50 per cent solution of one of the best grades of potassium hydroxide was electrolyzed in a tall form beaker, using a large sheet of platinum foil (8 X 10 cm.) for the anode and about 2 kilograms of re417
418
LYLE VERNON ANDREWS AND D. J. BROWN
distilled mercury for the cathode. The beaker was covered and the mercury was stirred during electrolysis. A current of 10 to 15 amperes was passed through until the amalgam had become so crystalline it could no longer be stirred. It was washed several times with distilled water, then transferred to a 3-liter flask, and washed a few more times in the absence of carbon dioxide. The flask was then filled with distilled water and connected with a filtering tube and another flask. After about two weeks the amalgam had decomposed and the solution was filtered through an asbestos filter. The filtering tube was removed and a carbon dioxide trap, which permitted filling of a weight buret, was attached. The solution a t no time gave a test for carbonate. The molality of the solution was determined by titrating with hydrochloric acid, standardized against sodium carbonate, and also by titrating against benzoic acid. Weight burets were used in these titrations. Solutions containing 0.1 mole of potassium hydroxide per thousand grams of water, were made up gravimetrically and the more dilute solutions made by diluting these, using calibrated pipets and volumetric flasks. E. Doubly distilled water, distilled in an all Pyrex still, first from alkaline permanganate and then redistilled from the clean still, was used. It was kept in 5-gallon bottles and was protected from the carbon dioxide of the air. Saturated solutions of the oxides were made in 200-ml. round bottomed, long necked flasks, fitted with ground glass stoppers and provided with rubber caps to prevent carbon dioxide from working past the ground glass stoppers. These flasks were almost filled with solvent, an excess of the solid oxide added, and then shaken in a constant temperature air bath a t 25.0 f 0.2"C. Electrode vessels were kept in a thermostat bath at 25.00 f 0.05"C. Each half-cell consisted of a Pyrex test tube with a side arm, and a tube sealed on at the bottom and bent upward, carrying a smaller tube with a platinum wire sealed in a t the lower end. It was stoppered with a rubber stopper coated with stopcock grease in order to prevent carbon dioxide from entering. The side arms of the cells were dipped in a beaker containing the same solution that was in the cells, and covered with a layer of paraffin to prevent evaporation. The only electrode vessel which presents enough novelty to warrant showing in a diagram, is the vessel shown in figure 1. This cell makes possible the measurement of potentials in very dilute solutions, even less than 0.0001 molal. The rate of diffusion in this vessel was tested with a dilute potassium permanganate solution. A number of days were required for the permanganate to diffuse from the right half into the left half of the cell.
419
DETERMINATION OF ISOELECTRIC POINT
All potentials were measured with a Leeds and Northrup type K potentiometer. The standard cell was checked frequently against a new cell, checked by the Bureau of Standards. EXPERIMENTAL PART
The rate of solubility and nature of a solution of lead oxide, as given in the literature, is so uncertain that a few investigations were carried out in connection with this problem. Randall and Spencer (7) determined the solubility of three lead oxides in alkaline solutions, ranging in concentration from about 0.05 to 0.3 molal. They claimed that in order to obtain saturated solutions of lead TABLE 1 Rate of solubility of (PbO)aH*O in water T I M E OF S T I R R I N G
5 minutes 15 minutes 50 minutes 4 hours 24 hours
I
MOLES OF
Pb
IN 1000 GRAMS OF SOLUTION
5.50 x 5.75 x 5.78 x 5.76 x 5.72 x
10-4 10-4 10-4
10-4 10-4
I
TABLE 2 Rate of solubility of PbO (red) in approzimately 0.1 molal KOH solution
20 minutes 3 hours 7 hours 25 hours 48 hours 122 hours
3.897 X 4.193 X 4.356 x 4.417 X 4.429 x 4.425 X
10-3
10-3 10-5 10-3 10-3 10-3
oxide, red variety, it was necessary to shake mixtures containing an excess of solid oxide for from twenty to fifty days. They shook the hydrated oxide, (Pb0)3H20, for from five to thirty days. We determined the rate of solubility of hydrated oxide in pure water, and the rate of solubility of red oxide in approximately 0.1 molal potassium hydroxide solution, by adding about 3 g. of the solid oxide to about 2 liters of the solvent contained in a 3-liter conical flask, fitted with a stirrer equipped with a vaseline seal to exclude carbon dioxide, a filter tube containing an asbestos filter, which permitted withdrawal of samples while the solution was being stirred, and an inlet tube connected with a source of carbon dioxide-free air. Stirring was just vigorous enough to keep the
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LYLE VERNON ANDREWS AND D. J. BROWN
major portion of the solid oxide suspended in the solvent. Samples were withdrawn through the asbestos filter a t various intervals from the time stirring was started and were analyzed for lead. The results of the measurements are given in tables 1 and 2. From the above measurements it would appear that saturated solutions of the hydrated oxide can be obtained in a few hours, and saturated solutions of the red oxide in a few days. The hydrated oxide was shaken for a t least twelve hours and the red oxide for from seven to ten days.
I n order to determine the basic characteristics of lead oxide and perhaps give a better idea of the nature of the solution, concentration cells of the type Hg
- HgO(,) - KOH 11 HzO - PbO(,) - HgO(.) - Hg
were set up and measured in electrode vessels of the type shown in figure 1. The electrolyte in the right half-cell consisted of a saturated solution of lead oxide in water, while the electrolyte in the left half-cell consisted of potassium hydroxide solution. The reaction in such a cell, when an electron current flows through the cell from right to left is Hg
+ 20H- + HgO + H20
42 1
DETERMINATION OF ISOELECTRIC POINT
in the left half-cell and HgO
+ 2H20 -+ Hg + 20H-
in the right half-cell. The total change is the formation of OH- in the right half-cell and consumption of OH- in the left half-cell. The E of this cell must then depend upon the activity of the hydroxide ion in the two half-cells. The molality of the potassium hydroxide was varied and the E of the cell measured. When E is zero, the activity of the hydroxide ion in the two half-cells must be equal. This must be almost the same as the molality of the potassium hydroxide when the measured E is zero. Under these conditions the liquid junction potential will be very small.
MOLALITY OF POTASSIUM H Y D R O X I D E
E,
0.0010 0.0004 0.0002 0.0001
0.0003
MOLALITY OF
POTASSIUM HYDROXIDE
i
TABLE 4 H20
- HgO(,) - NOH Ea
AVERAGE
0.0435 0.0255 0.0004 -0.0165
-0.0135
E.M.F. of cells Hg
0.0010 0.0004 0.0003 0.0002
Eb
0.0450 0.0260
I
0.0280 0.0088 O.OOO7
-
-
OF CELL
-
(PbO)aHzO(8) HgO(,) - Hg
1
Eb
AYERAOE
0,0190 0.0090 0.0009
-0,0068
E
0.0443 0,0257 O.OO04 -0.0150
-0.0075
E
OF CELL
0.0235 0,0089 0.0008 -0.0072
That mercuric oxide is such a weak base as to have no effect on the alkalinity was demonstrated by measuring the E of the cell. Hg
- HgO(,) - IiOH
(0.001 M)
11 HzO
- HgO(,) - Hg
This cell gave a value of about 0.24 volt, which proves that the mercuric oxide in the right half-cell was not basic enough to interfere with the basic properties of lead oxide. By plotting the measured E of the cell against the logarithm of the molality of potassium hydroxide, a straight line is obtained which cuts the zero line when the activity of OH- is the same throughout the cell. I n all measurements, a cell will be called positive when there is a tendency for an electron current to flow through the cell from right to left.
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LYLE VERNON ANDREWS AND D. J. BROWN
The data for this series of cells are given in table 3. By interpolation E is zero when log of molality of potassium hydroxide is -3.735. This and therefore the gives the molality of potassium hydroxide as 1.84 X activity of the hydroxide ion in the lead oxide is 1.84 X A similar series of measurements, table 4, was made with the hydrated oxide, (PbO)3Hz0. By interpolation the activity of the hydroxide ion in the hydrated lead If this comes from the primary basic ionizaoxide solution is 2.82 X tion of the oxide, it is possible to combine this data with the solubility data in table 1 and calculate the primary basic ionization constant of lead oxide. The activity of the lead ion in such a solution can be estimated from the value -0.122 volt for the Pb-Pb++ (a = 1) electrode as given in Gerke’s (8) compilation of electrode potentials, and the value - 0.508 volt for the Pb-PbO(,) - OH- (a = 1) electrode as measured by Smith and Woods (3). Ry substituting these values in the equation 0.0592 aPb++ E = E o + 7 log 1
we get log aPb++ =
-2 - (0.580 0.0592
- 0.122)
aPb++ = 3.2 X
when aOH- is unity, and the solution is saturated with lead monoxide (red). The solubility of the hydrated oxide is of the same order as the solubility of the red oxide, and we can assume that the product aPb++ X a O H - in a (Pb0)3H20solution is a number of the order of 3.2 X 10-l6. When aOH- is 2.82 X as given above, aPb++ must be about lo-*, which is so small that the secondary ionization of the oxide need not be considered. The ionization constant for the primary ionization PbO z HzO i$ PbOH+ OH- is
+
K =
aPbOH+ x aOHaPbO 2 H20
cxPbOH+ = aOHaPbO
HzO
K =
=
2.82
=
2.82 X
5.75 X lod4 - 2.82 X
x
= 2.93 X lo-‘
10-4.2.82 x 10-4 = 2.7 2.93 x 10-4
The above measurements and calculations indicate that a solution of lead oxide is a true solution, having an ionization constant characteristic of a moderately weak base.
423
DETERMINATION OF ISOELECTRIC POINT
I n order to investigate the amphoteric properties of lead oxide and to determine its effect upon the reference electrode, a number of cells of the type Hg
- HgO(,) - KOH 11 KOH - PbO(,) - HgO(6, - Hg
were measured. The molality of the potassium hydroxide was the same in each half-cell. These are concentration cells similar to the cells in the
MOLALITY OF POTASSIUM E Y D R O X I D E
Ea
0.0200 0.0100 0.0075 0.0060 0.0055 0,0050 0.0010
0,0035 0.0017 0.0016 0.0014 -0.0006 -0.0027 -0.0070
Ea
0,0036 0.0018 0.0016 0.0015 -0.0005 -0.0021 -0.0065
AVERAGE
E
OF CELL
0.0036 0.0018 0.0016 0.0015 -0.0006 -0.0024 -0.0068
FIG.2
previous series and the E of the cells will be zero whenever the lead oxide acts equally as an acidic and basic oxide. The data are given in table 5 and the graph in figure 2. The data in table 5 prove that in this range of alkalinity, the addition of red lead oxide does not destroy the reproducibility of the electrode. At
424
LYLE VERNON ANDREWS AND D. J. BROWN
concentrations of potassium hydroxide greater than 0.02 molal, the potentials showed a greater variation. The molality of potassium hydroxide at which the E curve cuts the zero line, read from figure 2 as 5.6 X 10-3 is called the isoelectric point of the oxide. SUMMARY
The rate of solubility and the amphoteric properties of lead oxide have been studied, and a general method for measuring the isoelectric point of an amphoteric oxide has been suggested. REFERENCES (1) KUHN:Z.physik. Chem. 114,44 (1924). (2) SIMMS:J..Am. Chem. SOC.48,1239 (1926). (3) SMITHAND WOODS:J. Am. Chem. SOC.46, 2632 (1923). (4) BROWN, Moss, AND WILLIAMS:Ind. Eng. Chem., Anal. Ed., 3, 134 (1931). (5) MUELLER:Z.physik. Chem. 114, 129 (1928). (6) MINGCHOW:J. Am, Chem. SOC.42,488 (1920). (7) RANDALL AND SPENCER: J. Am. Chem. SOC.60, 1572 (1928). (8) GERKE: Chem. Rev. 1, 377 (1925).
