56 APPARATUS [Vol. 83 Apparatus A COMPACT UNIT FOR POTENTIOMET‘RIC AND “DEAD-STOP END-POINT” TITRIMEITRY ASSEMBLED mainly from radio compoiients, the transistor-operated arrangement shown in Fig. 1 is generally suitable for normal, differential and “dead-stop end-point” titrimetry. High-resist- ance titration systems, such as those involving the glass electrode, are excluded, since the titrimeter draws a current of a few microamperes from the system. Even with transistors selected a t random, the zero is stable and the response is closely linear. An input e.m.f. of 100 mV produces meter readings of about 10 and 50 PA, respectively, at minimum and maximum sensitivity. B,, B, = I-5-volt flashlamp dry cells M = 0 to 50-pA microammeter R,, R, = 3000-ohm resistance R, = 5000-ohm potentiometer (ZERO CONTROL) R, = 25,000-ohm resistance R, = 250,000-ohmvariable resistance (SENSITIVITY CONTROL) R, = 1000-ohm resistance R, = 500-ohm potentiometer ( i i ~ ~ ~ ~ - ~ ~ ~ * * CONTROL) S,, S, = Single-pole single-throw switches S3 = Double-pole double-throw switch TI, T, = PNP junction transistors, type CK 722 (Raytheon) Fig.1. Circuit diagram of titrimeter Transistor characteristics are markedly dependent on temperature. Two transistors are therefore used in a balanced type of circuit, the basic principles of which are discussed by Starke.1 Other transistor amplifiers of The 44-inch square 0 to 50-pA microammeter forms the front of the roughly cubical box that houses the entire unit and the two flashlamp cells required for energising the circuit. Current drain is very small, so that the cells last for 6 months or more.Set R, to give the desired sensitivity, close S, and open S,. Connect the potentiometric titration system so that the positive electrode goes to terminal 2 and, by adjust- ment of R,, restore the meter reading to zero. Proceed with the titration; if the meter reading falls instead of rising, reverse by means of S,. In difierential titrimetry, the e.m.f. of the electrode system is repeatedly brought to zero, so that no “backing-off” voltage is required. For this technique, open both S, and S,, operate a t high sensitivity and, after connecting to the electrode system, bring the meter reading to zero by means of R,. For “dead-stop end-point” titrimetry,7$*$g $lo close both S, and S,, operate at high sensitivity and obtain the desired polarising voltage by adjustment of R,.This apparatus was developed with the partial support of the US. Atomic Energy Commission. and unbalanced4 35 types have been described. Operation for normal titrimetry is as follows. Bring the meter reading to or near zero by adjustment of R,.January, 19581 APPARATUS 57 REFERENCES I. Starke, H. F., “Transistor Applications,” Raytheon Manufacturing Co., Newton, Mass., U.S.A., 2. 3. 4. 5. 6. 7. 8. 9. 10. p. 19. Phillips, J . P., Chemist-Analyst, 1955, 44, 80. Wieme, R. J., Bull. SOC. Chinz. Biol., 1956, 38, 801. Perryman, P. W., and Richards, D. H., J , Clin. Path., 1956, 9, 273. Phillips, J. P., Chemist-Analyst, 1956, 45, 107. MacInnes, D. A., and Jones, P. T., J . Amer.Chenz. SOC., 1926, 48, 2831. Foulk, C. W., and Bawden, A. T., Ibid., 1926, 48, 2044. Stock, J. T., Metallurgia, 1948, 37, 220. -, Ibid., 1952, 46, 209. -, Ibid., 1957, 55, 48. DEPARTMENT OF CHEMISTRY UNIVERSITY OF CONNECTICUT STORRS, CONNECTICUT, I:.S.A. JOHN T. STOCK Received June 27th, 1957 A SUCTION-OPERATED DIFFERENTIAL ELECTRODE SYSTEM FOR POTENTIOMETRIC TITRATION SINCE the introduction of the differential potentiometric electrode system by MacInnes and Jones,l various modifications have been described.2~3~4 ~ 6 ~ 6 Developed in a programme of studies on non- aqueous titrimetry, the suction-operated micro-assembly shown in Fig. 1 is easily constructed. rnrn 0 Fig. 1 Scale mm 0,. s,, , ,I? Fig. 2 Fig. 1. Differential electrode system: (a), front view (sectional); ( b ) , diagrammatic top view; (c), method of retaining electrode tubes Fig.2. Push-button suction control valve “Exposed” electrode A is a 20-mm length of No. 26-gauge platinum wire and has a light flexible copper connecting lead, B, soldered to one end. With 10 mm projecting, the platinum Wire is sealed through one end of a length of glass tubing having an external diameter of 4 mm and58 APPARAT US [Vol. 83 the projecting end is bent hairpin-fashion, as shown. Construction of the “retarded” electrode, C, is similar, but the wire is bent zig-zag and has a 4-mm diameter glass sphere, D, fused on the extremity. The sleeve of glass tubing, E, is about 0.2 mm larger in bore than the diameter of the sphere and slides freely on the electrode tube. To prevent the creeping of solution between electrode tube and sleeve, the glass parts are lightly coated with silicone grease and then wiped with a dry cloth.The collar of rubber tubing, F, rests on a similar collar, G, and prevents the sleeve from falling off during adjustments. The raising of the sleeve allows the approximately 0.1 ml of solution trapped around electrode C to be stirred into the bulk of the solution. Collars of rubber tubing, H, and H,, on the stem of piston I engage collar F and control the rise and fall of the sleeve. The piston is made from glass tubing having an external diameter of 4 mm by blowing on one end a thick-walled bulb about 9 mm in diameter. With use of fine carborundum paste, the piston is ground into cylinder J, which is a 10-mm diameter micro test- tube with a 4.5-mm diameter hole blown centrally through the bottom.This hole forms a guide for the piston stern.’ The piston should slide freely in the cylinder, the travel being limited by collars of rubber tubing, K and L. To prevent its $#ticking to the bottom of the cylinder, collar L rests upon a copper foil washer, M, while K projects slightly from the closed end of suction tube N. The wall of the latter has a 3-mm diameter hole, 0, through which air may be withdrawn from the cylinder. The piston then rises and carries the electrode sleeve with it. 100 mV per ml -A 0.2 0-3 0.4 Sca : for curves and I t 2-0 3.0 4.0 Scale for curves I I I and I V Volume of titrant. ml Fig. 3. Differential titrations: curve I, 2.0 ml of 0.002 N hydrochloric acid with 0.01 N sodium hydroxide; curve 11, 2.55 ml of 0.1 N acetic acid with N sodium hydroxide; curve III, 2.0 ml of 0.1 N sodium acetate with 0.098 N perchloric acid, both in glacial acetic acid solution; curve IV, 3.0 ml of 0.1 N anti- mony trichloride with 0.1 N bromine, both in glacial acetic acid solution Electrodes and cylinder are held by rubber bands stretched over electrode holder, P, a develop- ment of an earlier design,6 which allows the whole assembly to be mounted on a ring stand.Lengths of l-mm diameter brass wire, Ql and Qz, are soldered -Into each of the right-angle bends of the holder. As shown in the diagrammatic top view Fig. 1 ( b ) , the right-hand side of the holder and the wires projecting from this side locate cylinder J parallel to the electrode tubes.Before the cylinder is mounted, the rubber band that secures the electrode tubes in the V-notches is stretched over the wires as shown in Fig. 1 (c). This form of mounting provides firm support, eliminates risk of breakage and allows easy adjustment or replacement of the various components. For semi- permanent assembly, a few drops of Durofix or similar universal cement will reinforce the grip provided by the rubber bands.January, 1958: APPARATUS 59 Gentle suction from a filter-pump or vacuum line is ample for operation. A simple control is a short length of glass tubing with a hole blown midway in the wall. This is inserted in the suction line and, when the hole is closed with the forefinger, the electrode sleeve lifts. More convenient is the brass push-button valve shown in section in Fig.2. Cylinder R carries screwed cap S at one end and has three 3-mm diameter holes drilled through the wall as shown. Over two of these holes are soldered side-tubes T and U, respectively, while the third, V, is open to the atmosphere. A suitably drilled circular plate, W, is symmetrically soldered to the other end of the cylinder. This plate allows the control valve to be panel-mounted and also acts as a stop for the outward movement of piston X, which is a sliding fit in the cylinder and is forced outwards by spring I-. The electrode sleeve lifter, which is connected to side-tube T, is then a t atmospheric pressure. When X is depressed, suction is applied to the lifter by way of the annular groove in the piston.Titrations are performed in the usual way,lJ’ preferably with a concentrated reagent, the electrodes being set just under the surface of the liquid in the titration vessel. In experiments in which appreciable rise of level in the titration vessel occurs, the immersion should be re-adjusted just before the expected end-point. The device is applicable to a variety of acid - base, oxidation - reduction and precipitation titrations. When a 16-rnm diameter micro-beaker with suitable magnetic stirring is used,8 volumes as small as 2 ml can be titrated. Curves I and 11, Fig. 3, are typical; in these experiments the acid solutions were saturated with quinhydrone. Using the chloranil - tetrachlorohydroquinone redox system, Kirrmann and Daune-Dubois have reported the differential macro-titration of organic acids in dimethylformamide s o l ~ t i o n .~ The titrant was, however, aqueous alkali. Curves I11 and IV, Fig. 3, refer to titrations in glacial acetic acid, the titrant being made up in the same solvent. These indicate the possibilities of differential titrimetry in non-aqueous systems. The additives were an equimolar mixture of chloranil and tetrachlorohydroquinone for curve I11 and anhydrous sodium acetatelo for curve IV- This work was carried out with the partial support of the U.S. Atomic Energy Commission. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. MacInnes, D. A., and Jones, P. T., J . Amer. Chem. SOC., 1926, 48, 2831. Hall, K. F., Jensen, M. A., and Baeckstrom, S. A., Ibid., 1928, 50, 2217. MacInnes, D. A., “The Principles of Electrochemistry,” Reinhold Publishing Corporation, New Kirk, P. L., “Quantitative Ultramicroanalysis,” John Wiley & Sons Inc., New York, 1950, p. 47. Stock, J. T., Chem. Age, 1954, 71, 719. Dole, V. I?., and Thorn, N. A.; Anal. Chem., 1955, 27, 1184. Stock, J. T., and Fill, M. A., Analyst, 1946, 71, 536. -,- , Mikrochim. A d a , 1953, 1, 89, Kirrmann, A., and Daune-Dubois, N., Compt. Repad., 1953, 236, 1361. TomiEek, O., and Heyrovskl, A., Coll. Czech. Chena. Comm., 1951, 15, 997. York, 1939, p. 306. DEPARTMENT OF CHEMISTRY UNIVERSITY OF CONNECTICUT JOHN T. STOCK Received J u l y 12th, 1957 STORRS, CONNECTICUT, U.S.A.