A diagram of the Leeds & Northrup type K potentiometer is shown in figure 53, which, with the description that follows, illustrates the potentiometric principle and the operation of the instrument. In figure 54 the instrument is shown in use with a hydrogen gas-calomel cell. The standard cell, galvanometer, and dry cells also are shown. In the diagram the portions of the bridge by which measurements are made consist of fifteen 5-ohm coils in series in the circuit AD (contact made with M), and in series with them the extended wire DB, the resistance of which is also 5 ohms. The scale of DB, shown in the photograph, reads from 0 to 1,100. Contact with the extended Current from the battery, wire is made by the moving contact, M'. W, flows through these resistances and may be made exactly 0.02 ampere by means of the regulating rheostat, R. This is done by setting the double-throw switch to "std. cell", which connects the standard cell in series with the galvanometer, G, and the tapping keys, Courtesy of Leeds & Northrup Co. FIGURE 53. Wiring diagram of type K potentiometer R1, R2, and R., and with the point .5 on AD, to which one point on the double-throw switch is wired. Between A and O there is a series of resistances with a sliding contact, T. The resistance between the points .5 and A is exactly that which corresponds with the emf of 1 volt, and between 1.0166 and .5 a sufficient resistance is added to make the resistance between these points correspond exactly with an emf of 1.0166 volts. The small circular slide wire connected at 1.0166 makes contact with the standard cell through the contact, T. The resistance value of this slide wire is such that a practically continuous variation can be obtained in the standard-cell circuit voltages from 1.0166 to 1.0194, a range corresponding with the variations in different standard cadmium cells. To adjust the current to 0.02 ampere, with the switch thrown to the "std. cell" position, the contact, T, is set to correspond with the standard cell voltage and rheostat, R, is regulated until the galvanometer shows no deflection. The unknown emf is now measured by throwing the switch to the emf position and adjusting the resistances with M and M' by touching the contact keys, R2, R1, and R, until there is again no galvanometer deflection. After this measurement is made the working current may again be checked against the standard cell, as described. In figure 55 is shown a Leeds & Northrup potentiometer-electrometer with range from 0 to 1.100 volts. This portable instrument contains a stage of amplification and is suitable for pH measurements FIGURE 54.-Leeds & Northrup type K potentiometer with hydrogen-gas electrode, calomel electrode, etc. with all types of electrodes, including glass. Other types of potentiometers are illustrated in figure 59, A and B. (b) CALOMEL ELECTRODES Various forms of calomel electrode vessels are shown in figure 56. The bulb at the bottom of the cell (C, D, E) is filled with a layer of pure mercury. On this rests a layer of pure calomel mixed with mercury, and the filling of the cell is completed with a solution of potassium chloride having a definite concentration and saturated with calomel. One of three concentrations of potassium chloride is customarily used, either 0.1 M, 1.0 M, or saturated, and in reference to these concentrations the terms "tenth-normal", "normal", or "saturated" calomel electrodes are used as abbreviations. By means of side-tubes the cells communicate with a reservoir of saturated potassium chloride which by turning the stopcock at the top of the cell may be allowed to fill the side arm and form the necessary salt bridge between the calomel electrode and the unknown solution. Electrical connection between the electrode and the potentiometer is made through platinum wire sealed into the bottom of the vessel and either wired directly, as in D, or through a short column of mercury in the small glass tubes sealed at the bottom of the electrode (C and D). In the dipping electrodes, A and B, the calomel electrode proper is contained in the small inner tube, which, near the bottom and just above the calomel layer, has a small opening through which the electrode communicates with the potassium chloride solution contained in the outer jacket. This ihmis jacket is closed by a ground joint which is moistened with potassium chloride solution which passes the current when immersed in the test solution. It is essential that pure materials be used in preparing the electrodes in order that they may have the accepted emf values. Mercury may be purified by allowing it to fall in a fine spray through diluted nitric acid. It is then washed with distilled water, dried, and distilled in vacuo. Reilly and Rae [4] give the following directions for purifying mercury by electrolysis: "The mercury is placed in a large dish inside which is a small dish containing a small quantity of mercury. Platinum-wire electrodes sealed into glass tubes make contact with the two portions of mercury, that in the smaller vessel being the cathode. An electrolyte, consisting of 90 parts of water, 5 parts of sulfuric acid, and 5 parts of nitric acid, is poured in so as to cover the upper edge of the smaller vessel to a depth of 2 cm. A current of 0.5 ampere is passed for 2 hours for each kilogram of mercury in the outer vessel. The mercury in the outer vessel is then removed, washed, dried, and distilled in vacuo." For the preparation of calomel a portion of the purified mercury is dissolved in pure nitric acid that has been redistilled and slightly diluted. The mercury nitrate solution is poured into a large excess of distilled water containing some nitric acid. To this solution is slowly added dilute hydrochloric acid purified by distilling pure 20percent acid, discarding the first and last portions of the distillate. The precipitate is repeatedly washed with distilled water, preferably by decantation. Some free mercury should be present throughout the process to prevent the formation of mercuric salt. At the completion of washing, the calomel should be intimately mixed with finely divided mercury by shaking. Half-cell I Half-cell III. Half-cell IV Half-cell V. Half-cell VI. Half-cell VII TABLE 34.-Arbitrarily standardized values for half-cells 1 (H+) = 1 | H2 (1 atm), Pt. KCI (sat.)| KCl (0.1 N), HgCl| Hg. KCl (sat.), HgCl| Hg. KCl (sat.) HCI (0.1 N) H2 (1 atm), Pt. 1 KCl (sat.) KH phthalate (0.05 M)| H2 (1 atm), Pt. KCI (sat.) Na acetate (0.1 M) H2(1 atm), Pt. (H+)=1, quinhydrone Pt. KCl (sat.) HCI (0.1 N), quinhydrone Pt. 1 W. M. Clark, The Determination of Hydrogen Ions, 3d. ed., p. 672 (Williams & Wilkins Co., Baltimore, Md., 1928). The calomel-mercury mixture, before being placed in the vessel, is repeatedly shaken with small quantities of the potassium chloride solution chosen for the cell. The potassium chloride solution used to complete the filling of the cell is also saturated with calomel. The calomel half-cell may be connected directly with the test solution or through a salt bridge formed usually by a saturated solution of potassium chloride flowing from a reservoir through a side arm of the vessel and making contact with the calomel-saturated potassium chloride of the cell. After a measurement, the solution at the end of the side arm may be flushed out by slightly turning the stopcock. In electrodes A and B (fig. 56) the larger outside tube contains saturated potassium chloride and the inner tube is the calomel electrode proper. Clark's [3] table of values for several half-cells commonly used in the determination of hydrogen-ion concentration is reproduced in table 34. It is pointed out by Clark that discrepancies may be found in certain values given in the table and that certain values are to be regarded as tentative. This applies particularly to the temperature error in the saturated calomel half-cell. "On the other hand, the potential of a cell composed of a hydrogen or quinhydrone half-cell and a saturated potassium chloride calomel half-cell has a small temperature coefficient. . . so that the temperature value may be in considerable error without causing great error in potential." (c) HYDROGEN ELECTRODES A hydrogen electrode is formed by saturating platinum black (coated on platinum foil or wire for rigidity) with hydrogen gas. When such an electrode is placed in contact with a solution containing hydrogen ions, a difference of potential is established at the electrodesolution interface analogous to the potential difference between a metal electrode and a solution containing its ions. The emf of such a half-cell is not directly measurable, but if two half-cells be connected so that the electrolytes form a sharp liquid junction, the total emf of the concentration cell so formed may be measured potentiometrically. If the emf of one half-cell is known, that of the other may be computed. The emf of such a concentration cell, ignoring the liquid-junction potential, is represented by E ᎡᎢ C (80) where R is the gas constant=8.31507 volt-coulombs in absolute units; T is the absolute temperature=273.1+t° C; n is the valency of the electrode element; and F (the faraday) is the charge on 1 g equivalent of the ion=96,500 coulombs. C and C are the ion concentrations in the two halves of the cell. Substituting and multiplying by 2.3026 to convert to common logarithms, we obtain T n (81) Since measurements of E are customarily made in terms of international volts instead of absolute units, the numerical value in eq 81 is converted by dividing by 1.00042 [3, p. 250], and we obtain T C E=0.000198322- log (82) For the hydrogen electrode, let us assume that the concentration in one of the half-cells is normal with respect to hydrogen ions (1.008) g/liter) and write C-1, and in the other half-cell let the hydrogen-ion concentration be unknown and write C1=[H+]. The valence, n=1, and eq 82 then becomes 323414°-42- -20 1 E=0.000198322T log [H+]' (83) |