(c) OSCILLATOR

Since the electrolytes in sugar solutions are usually of unknown composition and may affect the electrode polarization reactance in a different manner in different solutions, it is best to reduce this effect as much as possible by using electrodes coated with platinum or palladium black, and employing a frequency of at least 1,000 cycles per second. For extreme precision, measurements should be made at two or more frequencies and values so found should be extrapolated to infinite frequency [12, 15].

The most useful source of current to accomplish this is obtained by using a variable-type vacuum-tube oscillator. With this source, the frequency may be varied and errors due to bridge or cell design thus can be detected and often eliminated.

FIGURE 49.-Wiring diagram of electrical conductance bridge null-point indicator.

The human ear has a practically constant sensitivity at frequencies between 1,000 and 5,000 cycles. The oscillator should, therefore, be capable of supplying a current at any desired frequency between these limits if a telephone receiver is to be used as an indicator. In order to increase the precision of bridge setting, the oscillating current should have a pure sine wave form or be reasonably free from overtones, for if the bridge is balanced at the fundamental frequency and there remain notes of other frequencies in the detector branch of the bridge network, the sharpness of setting will be diminished and the time required to find the minimum will be increased. Since the current flowing through the cell heats the solution, and a potential difference between electrodes above 1.23 volts changes the bridge balance [12, 16, 44], the intensity and time of current flow should be reduced as much as possible.

(d) CONDUCTIVITY CELL

Since the conductance of a sugar solution is expressed in units of reciprocals of resistance and specific resistance has been defined as the resistance of a column of substance 1 cm long and 1 cm2 in crosssectional area, the specific resistance of any other column of the substance may be found from the equation

or, since k=1/p, the specific conductance may be found by equation

where R is the resistance in ohms; p, the resistivity or specific resistance; k, the specific conductance; l, the length in centimeters; and a, the uniform cross-sectional area in square centimeters. The values of 1 and a are usually combined and expressed as one quantity, l/a, which is defined as the "cell constant." The cell constant is best determined by measuring, in the cell to be calibrated, the resistance of a standard solution of known specific conductance, k, substituting in eq 63, and solving for 1/a. As shown later, page 275, these constants should be based on the resistance extrapolated for infinite frequency [12, 16].

15 CM.

F

A

FIGURE 50.-Precision-type conductivity cell.

F, Filling tubes; A, lead tubes; E, electrodes.

The cell must be so designed that the cell constant will remain unchanged under all conditions of the experiment [12, 18]. Unless the cell has been properly constructed, however, its constant changes with the specific resistance and dimensions of the solution being measured and with the frequency of the alternating current [19]. This change in cell constant with frequency (this should not be confused with the electrode polarization effect), sometimes called the Parker effect [20], results from the reactive shunts between parts of the cell of opposite polarity in close proximity, such as the filling tubes and lead tubes. This last factor can be made negligible by separating these parts by about 15 cm [18], figure 50. Errors due to the Parker effect may be expected when the "dipping type" cells, figure 51, are used. Since the error in measurement resulting from this Parker effect increases with frequency, and since the electrode polarization reactance decreases with frequency, it is recommended that both be checked by a method described below.

(e) ELECTRODES

Electrodes are usually made of platinum-iridium. They are sometimes platinized to decrease the errors caused by the electrode

903232 O- 50-19

polarization. Platinum black has the disadvantage that it is a good catalyzer and may change the conductivity of the solution by increasing the rate of oxidation of sugar therein [21]; therefore, if used, measurements should be made as soon after filling the cell as possible. Palladium black has less catalytic action and is considered more effective, especially at high concentrations [22].

Electrodes are platinized, in the completed cell, by filling it with 0.025 N hydrochloric acid solution containing 0.3 percent of platinum chloride and 0.025 percent of lead acetate. A direct current of 0.010 ampere from a battery is passed through the cell, with reversal of polarity every 10 seconds, until the amount of platinum deposited

E

SOLUTION

S

FIGURE 51.-Dipping-type conductivity cell.

is sufficient [23]. The platinum salt absorbed in the electrodes should be removed by immersing the cell for some hours in warm distilled water which is frequently changed. Removal of the last traces of platinizing liquid and occluded chlorine may be effected by placing the electrodes in a solution of sodium acetate or dilute sulfuric acid and passing a direct current through the electrolyte for about 15 minutes, with reversal of direction every minute.

Platinized electrodes which have dried are wetted with difficulty, subsequently introducing errors into the measurements by the layer of air bubbles on the surface. To prevent this they should be allowed to rest in distilled water when not in use [24].

The cell constant may change slightly with use, because of changes in the position and surface character of the electrodes, and should be checked regularly with a standard solution of potassium chloride. Should it become necessary to replatinize the electrodes, they may first be deplatinized by electrolyzing aqua regia in the cells for a few minutes reversing the current every minute. Palladium black may be removed in the same manner except that hydrochloric acid is used in place of aqua regia.

L, Leads; E, electrodes; S, electrode cell immersed in solution contained in jar.

(f) CONSTANT-TEMPERATURE BATH

Most solutions encountered in the measurement of conductivity have a high temperature coefficient of about 2 percent per degree centigrade. The desired precision determines the limits within which the temperature must be controlled. Thus with a precision of 0.02 percent, a temperature control to better than 0.01° is required. For this precision the cell should be immersed in a well-stirred bath with adequate regulation, and the bridge current should not produce an appreciable heating effect.

The use of oil in the bath is preferred to water. Oil decreases the errors caused by (1) capacitance bypaths between parts of the cell, (2) capacitance between the cell and the walls of the bath, and (3) electrical eddy currents in the liquid outside the cell. These errors may amount to 0.5 percent or more [11,12].

3. PROCEDURE

(a) EQUILIBRIUM WATER

All solutions on which conductivity determinations are to be made should be prepared from water of low specific conductance. It is possible to obtain water which has a specific conductance of 0.043 × 10-6 mho cm at 18° [25]. However, the instant water comes in contact with the air, CO2 is absorbed and the conductivity increases. When it is brought into equilibrium with the CO2 in the air, preferably by rapid aeration [26], and has no other impurities, it has a specific conductance of about 0.85X10-6 [27]. It also has the pH value 5.7, which can be readily checked with isohydric indicators [26]. (A diaphragm pump of suitable construction is very convenient for spraying outdoor air through a porous Alundum or fritted Pyrex aspirator immersed in the water. Laboratory compressed air generally carries along a spray of oil and impure water.) Such water is known as "equilibrium water." Water having a specific conductance greater than 3×10-6 mho cm-1 at 25° C may produce a precipitate in the sugar solution, which would alter its conductivity. However, it is possible to secure water of this or lower conductivity from an ordinary laboratory still. If such water is not available, equilibrium water may be prepared directly from tap water [27], but preferably from distilled water, by distillation in a Jena- or Pyrex-glass vessel to which a few milliliters of Nessler solution or alkaline permanganate [28] has been added, and condensing the vapors in a block tin condenser. This water is then thoroughly aerated (overnight generally sufficing) and should be stored in thoroughly steamed and seasoned Pyrex glass-stoppered bottles. It is quite stable over a period of several weeks.

(b) PREPARATION OF POTASSIUM CHLORIDE SOLUTION

The potassium chloride should be selected from the purest material available, recrystallized from conductivity or equilibrium water, separated by centrifugal drainage, fused in a platinum crucible, poured into a platinum dish, and transferred to a closed bottle while still hot. Solutions made from it should be carefully prepared according to the following procedure:

An approximate cell constant is estimated from a rough measurement of the dimensions of the cell. From this value the concentration of potassium chloride is selected from the recommended value in table 33. The correct amount of potassium chloride is weighed into a Pyrex vessel and equilibrium water is added to bring the weight to 1,000 ±0.02 g. The correction to be applied to convert both the weight of the potassium chloride and the solution to weight in vacuum is determined according to directions found in table 114, page 632

TABLE 33.-Specific conductance (at 1,000 cycles) of potassium chloride solutions, in reciprocal ohm-centimeter

1 When the corresponding standard solutions are used in cells that have a cell constant lying between the limits given in this column, the measured resistance will be between 1,000 and 50,000 ohms.

The specific conductances of solutions 1, 2, and 3 in table 33 were determined by Jones and Prendergast [29]. The value for solution 4 was calculated from the empirical equations of Davies [30], which are as follows:

A=149.92-93.85/C+50C at 25° C

A=129.67-79.55√T+35C at 18° C,

(64)

(65)

where A is the equivalent conductance, and C the concentration of the solution in moles per liter. Values calculated from these equations are in agreement with those experimentally determined by Shedlovsky [31], Davies [30], and Johnson and Hulett [32], at 25° C., and with those of Shedlovsky [33], as well as Davies [30], at 18° C. The specific conductance is determined from the equivalent conductance by equation

where k is the specific conductance, and the concentration in equivalents per milliliter (not per liter).

In order to calculate the concentration of solution 4 in grams of potassium chloride per 1,000 g of solution, the density of a 0.001 N KCl solution at 25° C was determined by methods of interpolation from data found in the International Critical Tables. This density is based on a solution which contains 0.074533 g of KCl per liter at 25° C.

(c) DETERMINATION OF CELL CONSTANTS

The resistance, Rolv., of equilibrium water from the same lot as that used in making the potassium chloride solutions is checked in a clean cell [12, 34]. The cell is rinsed two or three times with the potassium chloride solution and then filled therewith. It is allowed to remain 15 or 20 minutes in the constant-temperature bath. The resistance of the solution is then determined, and redetermined at the end of 5 minutes. If the two resistances check, the solution has reached thermal equilibrium. To verify the cleanliness of the cell, a second determination should be made after rinsing and filling it with fresh solution. The final value of the resistance, Roln., is used to determine the cell constant (uncorrected for solvent conductivity) by means of the equation

(a)

uncorrected = kCR2oln.,

(67)