be used in the presence of strong oxidizing or reducing agents when a hydrogen electrode is useless. He also showed that the linear relationship of glass-electrode potential and pH is affected by strong concentrations of certain salts. Kerridge [8] found the glass electrode useful in the presence of substances of biological origin that poison a hydrogen electrode. A study of the composition of glasses was made by MacInnes and Dole [9], who consider as most suitable a glass composed of 22 percent of sodium oxide, 6 percent of calcium oxide, and 72 percent of silica. Glass of this composition, known as Corning 015, is made by Corning Glass Works, Corning, N. Y. The same investigators [10] produced membranes as thin as 0.025 mm by first blowing a bubble on the end of a glass tube until red and blue interference colors appeared. The end of a second tube, heated to dull redness, is placed against the bubble and the film fused to it. These membranes are fragile but have comparatively low resistance. Robertson [11] produced thinwalled bulbs by drawing 10-mm tubing to a tapering point and forming on this a small lump of glass by heating in a pointed flame. The bulb was then blown to a volume of 8 or 10 ml. Thompson [12] devised a metal-connected glass electrode of bulb or test-tube form by silvering the outer surface. The silver film was protected by lightly copperplating and the metal coating was then wired to a potentiometer. The test solution was added to the vessel and the chain was completed by means of a saturated calomel half-cell. No standard electrolyte is used in actual pH measurement, but each electrode must be calibrated before being used. This may be done with the help of standard buffer solutions, and the cell constant is determined. Accounts of the evolution of the glass electrode are given by Perley [13], MacInnes and Longsworth [14], and by Clark [3], while theories of the action of the electrode have been advanced by Haber and Klemenziewicz [6], Horowitz [15], Michaelis [16], Dole [17], and MacInnes and Belcher [18]. The Haber type of vessel, consisting of a thin bulb blown on the end of a glass tube, is now commonly employed. Various electrolytes have been used inside the electrode but a 0.1 N solution of hydrochloric acid (pH=1.0) saturated with quinhydrone is generally favored for routine measurements. The electrode tube is half-filled with the solution, connection with the potentiometer being made with a short. length of platinum wire sealed in a narrow glass tube containing mercury into which the potentiometer lead extends. The latter tube is supported in the rubber stopper of the electrode vessel. The bulb. of the glass electrode is immersed in the test solution and a calomel half-cell is used as a reference electrode. The concentration chain may be represented thus: Hg HgCl, KCl (sat.) solution 0.1 N HCl Pt. quinhydrone glass membrane The pH of an unknown solution is calculated from potential reading, E, as with the quinhydrone electrode, as described under (d), p. 289. That is, at 25° C. Tables giving pH values corresponding to potentiometer readings at various temperatures are supplied with some instruments, whereas others are calibrated to read directly in pH values. Since the resistance of the glass electrode is high, a sensitive detecting instrument must be used. Some workers prefer an electrometer, but with improved electrodes, a lamp-and-scale galvanometer with a sensitivity of 0.0005 ua per mm at 1 m is satisfactory, and an inexpensive potentiometer of the usual type may be used (fig. 59, A). The use of grounded metal supports for the electrodes prevents leakage currents from reaching the galvanometer. Various methods of thermionic amplification of the weak currents of the glass electrode have been reported by Goode [19], Elder and Wright [20], Partridge [21], Müller [22], DuBridge [23], Morton [24], Gilbert and Cobb [25], and others. In figure 55 is shown a potentiometerelectrometer in which the amplification is incorporated in the potentiometer housing. In some apparatus the entire assembly, consisting of potentiometer, vacuum-tube amplifier, dry cells, galvanometer, pH electrodes, etc., is housed in a single portable unit. One of these units is illustrated in figure 59 (B). With amplification, the employment of the glass electrode in automatic pH apparatus becomes possible. Longsworth and MacInnes [26] have described such a device for the automatic control of the addition of alkali solution to a growing culture of acidforming bacteria. A pH recorder for sugar juices, in which a glass electrode is employed, is described by Crites [27]. Electrodes made of the usual glass are considered inaccurate in the presence of sodium salts at ranges above pH 9.5; however, a calibration curve may be obtained for the range 9.6 to 12.5 which is said to be reproducible within 0.1 pH. Very recently the use of a different glass suitable for high alkaline ranges, has been reported [42]. (f) SILVER-SILVER CHLORIDE ELECTRODE The silver-silver chloride half-cell, represented by Ag|AgCl KCl(m), where (m) refers to molar concentration, is analagous to the calomel half-cell for which it has been substituted as a secondary reference electrode, particularly in the study of reactions in chloride solutions. The electrode consists of a coating of silver chloride, intimately mixed with silver, on platinum or silver-plated platinum. The platinum support, which may be in the form of a small rectangle of foil or gauze or a wire coil, is sealed by means of a short lead of platinum wire to a convenient length of glass tubing, as in the construction of hydrogen electrodes. The silver-silver chloride coating may be produced in various ways, either electrolytically [37], thermoelectrolytically [38], or thermally [39]. The last-named process is simple and produces satisfactory electrodes. A coil of platinum wire is sealed into a glass tube and covered with a paste composed of 7 parts of silver oxide (precipitated and washed) and 1 part of silver chlorate and heated to decomposition in an electric furnace. After coating the electrodes, regardless of the process used, they are washed in many changes of distilled water to remove contaminants, and finally washed in the solution in which they are to be used. The period of washing and aging to stability may extend to as much as 20 days. Smith and Taylor [40] studied the reproducibility and stabilityof silver chloride electrodes and found an average agreement of 0.02 mv, among electrodes produced in the several ways referred to above. They also found that even after proper aging, the potential is sensitive to polarizing currents produced by as small as 0.1 to 0.2 mv. These authors indicate that the difference in values reported for the standard potential of silver chloride electrodes, quoted as varying from 0.2221 to 0.2238 volt, may be due to insufficient aging time and consequent lack of concentration equilibrium within the porous electrode materials. Values for the standard electrode potential of the silver-silver chloride electrode as given by Harned and Ehlers [41] for the temperatures 20°, 25°, and 30° C. are 0.2255, 0.2224, and 0.2191 volt, respectively. The utility of the silver chloride electrode has been realized in the construction of glass electrodes where the inner member of the glasselectrode assembly is a stabilized silver chloride electrode immersed in a chloride solution, the assembly being sealed to prevent evaporation. Polarization is said to be prevented by suitable electrical construction. 3. AUTOMATIC RECORDING AND CONTROL OF PH A scheme for the automatic recording of pH in lime-treated cane juice was described by Balch and Paine [28], wherein a tungstenmanganese sesquioxide electrode and a calomel half-cell in contact with the flowing liquid were connected with an automatic recorder. The quinhydrone electrode in special form also is used in certain industrial solutions. Reference has already been made to the employment of the glass electrode for pH control on a relatively small scale [26] and for recording the pH of sugar juices [27]. Electrodes of metallic antimony have been much used in industrial control equipment. These have the advantage of being rugged and not affected by flowing liquids. They may be set up in tanks and other vessels and used in sugar factories for both automatic recording and control of liming and gassing of beet juices. The electrode assembly consists of the antimony electrode and a saturated calomel electrode usually fitted into a chamber through which a sample of the juice flows continuously, the potential established being proportional to the pH of the juice. The electrodes are wired to a recording and controlling device which contains a potentiometer circuit in which the electrode potential is balanced automatically against an adjustable standard potential. Deviation of the pH from the control value unbalances the circuit, and the controller acting through a relay actuates a motor-drive unit, which in turn operates a feeder or valve to increase or decrease the flow of lime-milk or of gas. The antimony electrode should be made from pure metal and depends for its action upon the presence of very slightly soluble Sb(OH), formed by the action of dissolved air or oxygen. The exposed portion of the electrode is ground and polished at the start and is wiped clean daily. The electrodes are calibrated for the solutions in which they are to be used. The useful range of the antimony electrode under proper conditions is said to be 2.0 to 12.0 pH in continuous operation, with a limit of error of 0.2 pH. Antimony electrodes and their characteristics have been discussed by Perley [13, 29, 30]. Reference has been made in section 2 (e), p. 292, to the use of glass electrodes for pH control and recording. 4. COLORIMETRIC METHODS (a) WITH STANDARD BUFFER SOLUTIONS The standard buffer solutions used in the colorimetric determination of hydrogen-ion concentration, according to Clark [3], are mixtures of some acid or alkali with one of its salts, of such well-defined composition that they may be accurately reproduced, and with pH values accurately defined by hydrogen electrode measurements. Several such mixtures have been used, but the set of buffers devised by Clark and Lubs [31], as described here, have proved satisfactory and are conveniently prepared. Stock solutions.-The following stock solutions are used in preparing the standards: 0.2 M hydrochloric acid, 36.465 g of HCI per liter. 0.2 M sodium hydroxide, 40.005 g of NaOH per liter. 0.2 M potassium chloride, 14.912 g of KCl per liter. 0.2 M acid potassium phthalate, 40.836 g of KHC,H,O, per liter. 0.2 M acid potassium phosphate, 27.232 g of KH2PO, per liter. 0.2 M boric acid 0.2 M potassium chloride, one liter of the solution to contain 12.4048 g of H2BO3 and 14.912 g of KCl. The ordinary chemically pure salts are not considered suitable for making these stock solutions but are to be recrystallized three or four times from water that has been redistilled from a Pyrex flask and protected from absorbing CO2 by a soda-lime guard tube. Although the salts, the stock solutions, and even the buffer mixtures, especially prepared for pH determination, may now be purchased, the preparation of the various stock solutions and the buffer mixtures to cover the pH range 1.2 to 10.0 is briefly outlined here. Clark's directions for recrystallizing potassium acid phthalate state that the crystallization from the hot solution should be allowed to take place slowly at a temperature not below 20° C, since there is deposited at lower temperatures a more acid salt having the form of prismatic needles instead of the six-sided orthorhombic plates of the salt, KHCHO. After the final crystallization the salt is dried at 110° to 115° C to constant weight. Recrystallized potassium acid phosphate is dried at 110° to 115° C and potassium chloride at 120° C. The boric acid is air-dried in thin layers between filter paper, and the constancy of weight is established by drying small samples in thin layers in a desiccator over CaCl2. Preparation of 0.2 M sodium hydroxide.-To prepare the 0.2 M sodium hydroxide solution, 100 g of high-grade stick soda in a Pyrex Erlenmever flask is treated with 100 ml of distilled water which is used for rinsing any adhering soda from the neck of the flask. After solution and cooling, the flask is stoppered and allowed to stand until carbonate has settled. It has been found convenient to filter this strong solution by suction through purified asbestos (see Chapter XIX, 6 (a), p. 324) supported in a Jena No. 2 filter or in a Gooch crucible. From this point on, the solution is protected from absorption of CO2 from the air by careful manipulation and by means of soda-lime guard tubes. After a rough calculation, the clear filtrate is quickly diluted to a solution somewhat more concentrated than 1.0 M. Of this solution 10 ml is withdrawn and titrated with an acid solution of known strength. From this standardization, the dilution required to furnish a 0.2 M solution is calculated. The dilution is made with the least possible exposure and the solution is poured into a bottle thickly coated with paraffin wax and to which a calibrated 50-ml burette and soda-lime guard tubes have been attached. The solution is now carefully standardized. The purified acid potassium phthalate is recommended by Clark for this operation. Portions of the salt of about 1.6 g each are carefully weighed on an analytical balance with standardized weights and dissolved in beakers in about 20 ml of distilled water, and 4 drops of phenolphthalein are added. A stream of air free of CO2 is passed through the solution, which is titrated with the alkali to a faint but distinct permanent pink. It is preferable to use a factor with the solution rather than to attempt adjustment to an exact. 0.2 M solution. Preparation of 0.2 M hydrochloric acid solution.-A high-grade hydrochloric acid solution is diluted to about 20 percent and distilled. The distillate is diluted to approximately 0.2 M and standardized with the sodium-hydroxide solution described above. Preparation of the buffer mixtures.-The standard buffer mixtures used in performing the actual pH tests are made, as already indicated, by adding varying amounts of an acid or an alkali to a solution of its salt. Although in routine sugar-factory work only a limited range of buffer solutions may be required, the entire list is given here, since the occasion frequently arises for tests in other ranges. TABLE 35.-Clark and Lubs buffer mixtures, temperature 20° C Milliliters of 0.2 M NaOH pH. 3.72 5.70 8.60 12.60 17.80 23.65 29.63 35.00 39.50 42.80 45.20 46.80 8.0 In table 35 is shown, in the first horizontal line of figures, the number of milliliters of 0.2 M acid or alkali that must be added to 50 ml of a given salt solution to produce 200 ml of standard buffer mixture having the corresponding pH shown in the second line of figures. To prepare the solutions, 50 ml of the salt solution is pipetted into a calibrated 200-ml glass-stoppered volumetric flask, and the required amount of 0.2 M acid or alkali is run in from a burette. The solution |