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 HCl 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 KH,PO, per liter. 0.2 M boric acid+0.2 M potassium chloride, one liter of the solution to contain 12.4048 g of H,BO, 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, KHC,HO. After the final crystallization the salt is dried at 110° to 115° C to constant weight. 1 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 hydroride.-To prepare the 0.2 M sodium hydroxide solution, 100 g of high-grade stick soda in a Pyrex Erlenmeyer 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 CO, 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 HCl... 46.7 39.6 32.95 26. 42 20. 32 14.70 9.90 5.97 2.63 2.2 2.4 2.6 2.8 3.0 3.2 50 ml of 0.2 M KHCH,01+X ml of NaOH Milliliters of 0.2 M NaOH. pH... 0.4 3.70 7.50 12.15 17.70 23.85 29.95 35. 45 39.85 43.00 45.45 47.00 50 ml of 0.2 M KH2PO1+X ml of NaOH Milliliters of 0.2 M NaOH. 3.72 5.70 8.60 12.60 17.80 23.65 29.63 35.00 39.50 42.80 45.20 46.80 pH.. 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 50 ml of 0.2 M H2BO3, KCl+X ml of NaOH 3.97 5.90 8.50 12.00 16.30 21.30 26.70 32.00 36,85 90.80 43.90 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 10.0 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 is made up to 200 ml with double-distilled water, the flask is stoppered, and the solution mixed. For storage the standard buffer solutions are transferred to resistant glass bottles labeled with the corresponding pH. The bottles containing the most frequently used standards may be provided, for convenience, with clean one-hole rubber stoppers, each carrying a 10-ml pipette. Indicator solutions.-Indicators recommended for the colorimetric determination of hydrogen-ion determination are listed in table 36 under the common names of the dyes. The list includes the indicators originally selected by Clark and Lubs [32], while those marked with an asterisk have been added as a result of the work of Cohen [33]. All are sulfonphthaleins with the exception of methyl red, an azo dye, and cresol phthalein. Methyl red, although replaced in the revised indicator list by chlor phenol red, is included in table 36 because it is prescribed in the method of Gillespie, as described later. Opposite the name of the indicator are given the molecular weight, the number of milliliters of 0.01 N sodium hydroxide needed to form the monobasic salt of 0.1 g of dye, pK (=log 1/K, where K is the dissociation constant of the indicator), useful pH range, and the colors of the acid and alkaline forms of the dye. The Determination of Hydrogen Ions, W. M. Clark, 3d ed. p. 94 (Williams & Wilkins Co., Baltimore, Md., 1928). *Added as result of later work by Cohen [33]. Stock solutions of the indicators are prepared by grinding 0.1 g of the dry powder in an agate mortar with the corresponding volume of 0.01 N sodium hydroxide given in column 3 of table 36. The alkali is added little by little, and when solution is complete it is diluted to 250 ml with distilled water to give a 0.04-percent solution. Methyl red (0.1 g) is dissolved in 125 ml. of neutral alcohol and diluted with water to 250 ml. Performing the tests.-Test tubes 15 mm in diameter and 120 mm long are calibrated roughly for uniformity of bore by adding 10 ml of water from a pipette and choosing those in which the water stands at approximately the same level. These are cleaned and afterward rinsed with distilled water, and drained. To separate tubes are added 10 ml. of each standard buffer mixture to cover a chosen pH range, and 5 drops of the appropriate 0.04-percent indicator solution. Meade [34] recommends that the corks used for closing the tubes be first boiled and thoroughly rinsed before inserting. To another calibrated test tube is added 10 ml. of the solution to be tested and 5 drops of the indicator. The matching of colors is best done with the aid of a comparator, a simple form of which is the block comparator described by Clark as having six deep holes of such size as to accommodate neatly the test tubes, drilled parallel in pairs, each pair being as close together as possible without breaking the wood. Through the center line of each pair and at right angles to them are drilled three smaller holes completely through the block so that light may pass horizontally through the pairs of tubes when introduced. The block is then painted a dull black inside and out. The back of the illuminating holes is covered with a ground plate of colorless glass to diffuse the light. The sample tube with indicator is placed in one of the middle holes and backed by a tube of water. Standard tubes containing the indicator are placed in the holes on either side of the sample, and each is backed by a tube containing the sample without the indicator, thus compensating for color and turbidity in the sample. When the sample appears to fall midway between two standards that differ by 0.2 pH unit, let us say, for example, between pH 7.2 and pH 7.4, then the value 7.3 may be taken as the pH of the sample. There are other forms of comparators arranged for quick and convenient exchange of color standards, such as that of Meade and Baus [35], which has a sliding rack carrying alternate tubes of color standard and water, and of such length as to cover the entire pH range of the indicator. A metal cover with suitably spaced apertures, and having sockets for holding tubes of the sample and the sample plus indicator, is provided. A white surface at 45° is placed behind the apertures and illuminated from above with a Daylite Mazda bulb. Care must be exercised in maintaining like volume of solutions and of indicators and like thicknesses of light-transmitting layer. Alkaline and neutral buffer solutions should be protected from absorbing carbon dioxide and should not be allowed to come in contact with the hands. (b) WITHOUT BUFFER SOLUTIONS GILLESPIE METHOD [36] This method for the determination of hydrogen-ion concentration without the use of buffer mixtures depends upon the ratio of the number of drops of a given indicator present in a solution wholly transformed into the alkaline form to the number existing wholly in the acid form. The indicators used are those of the original selection of Clark and Lubs [32]. The stock solutions of the dyes are prepared, except in the case of methyl red, by grinding 0.1-g portions with 1:1 equivalents of alkali solution (1.5 equivalents in the case of cresol red) to give the monosodium salt of the indicator acid. The equivalents of sodium hydroxide may be obtained from table 36, column 3. When the dye is in solution the volume is made up to 250 ml with water. Methyl red is dissolved as explained under (a), p. 296. In determining the data upon which table 37 is based, Gillespie |