November E, 1934
I N D US T R I A L A N D E N G I N E E R I N G C H E M I S TR Y
DISCUSSION The results obtained on a series of pure samples of naphthalene, and on synthetic mixtures containing all the commonly occurring ingredients of commercial poultry lice powders, indicate a high degree of accuracy for the suggested method. These results have been substantiated by analyses of a large number of commercial samples over a period of 3 years. The method of decomposing the picrate by an excess of sodium hydroxide and back-titrating the residual alkali has been found experimentally to offer distinct advantages over the current gas practice of titrating the residual picric acid, and is definitely superior to direct titration of the picrate itself with alkali. It is believed that this method might be advantageously adapted to the determination of naphthalene in
463
coal gas and it should certainly prove satisfactory for the evaluation of commercial samples of naphthalene. ACKNOWLEDGMENT The authors wish to acknowledge the assistance of Louis Sair in performing a number of the analyses reported. LITERATURE CITED (1) -4llen’s Commercial Organic Analysis, 5th ed., Vol. 111, pp. 214-15, P. Blakiston’s Son & Co., Philadelphia, 1925. (2) Ibid., p, 222. (3) Collins, S. H., J. SOC.Chem. Ind., 37, 131R (1918). (4) Colman, H. G., Gas J., 144, 231-2 (1918). (5) Jorissen, W. P., and Rutten, J., Chem. Weebblad, 6, 261-72 (1909).
RJWEIVED June 27, 1934
Simple Apparatus for Photoelectric Titration W. WALKERRUSSELL AND DONALD S. LATHAM, Metcalf Laboratory, Brown University, Providence, R. I.
P
HOTOELECTRIC cells offer a means of ascertaining colorimetric end points in titration analyses which are free from subjective error. Titrations can be made by day or night, independently of lighting conditions and quite as well by a color-blind observer. Furthermore, such a method of analysis lends itself to automatic (electric) recording or control (1). Because of its ruggedness and ease of operation a photovoltaic cell was chosen for the present investigation. Hitherto (2-4) it has been found desirable to employ some type of vernier-controlled shutter, special light filters, and storage battery, or to make use of a differential method involving an additional photoelectric cell in carrying out such titrations. I n the apparatus to be described it has been found possible to dispense with all these items when using bromothymol blue as indicator. A “dead-stop” end point can be obtained accurate to better than 0.05 cc. of 0.1 N alkali, and greater accuracy is obtainable by plotting. This apparatus is largely, if not entirely, composed of items to be found in most laboratories, and is readily assembled. The potentiometric set-up which is shown on the right in Figure 1 comprised a 20-inch (50-cm.) tubular, sliding rheostat, A , having a resistance of 390 ohms, and equipped with a rack and pinion slider which made for easy manipulation. To the slider was attached a pointer, B , which allowed the position of the slider to be read on a centimeter scale held in place by means of the binding posts already on A . A second, shorter sliding rheostat, D, of similar resistance gave the apparatus a wider range, as it allowed readings t o be made upon a convenient portion of scale C. Two dry cells, E and F , furnished the potentiometer current. Two tapping keys, G and H, the former protected by the 10,000-ohm resistance I, were employed. The galvanometer, J,was of the rugged needle type, had a sensitivity of 0.3 microampere per mm., a damping resistance of 2600 ohms, and a coil resistance of 1155 ohms (Land N , type 2320-d). Although the somewhat rough potentiometer just described has been found sufficiently accurate for the present work and those of similar construction have been used in potentiometric titration (5), one of the many types of commercially made potentiometers (an L and N, student type, has been found very suitable) may well prove more convenient. The potentiometer was attached through the switch, K , to the hotovoltaic cell, L, which was a Visitron, Type F2 (manuractured by the G-M Laboratories). The cell housing, M , consisted of a wooden box, about 21 X 7 X 7inches (52.5 X 17.5 X 17.5 cm.), painted flat black on the inside. The end of the box, V , was hinged t o allow light filters to be placed in slot T between L and the titration cell, U . This cell was simply a 2-02. (60 cc.) bottle
with fairly plane, parallel sides. A portion of the light-tight cover of M was removable and notched t o allow for insertion of a small glass tube carrying carbon dioxide-free air for stirring. Through this notch was also inserted a small-bore glass tube which served t o lengthen the buret tip so that it dipped well under the solution in cell U during titration. A cardboard diaphragm, N , with a 1.75-inch (4.2-cm.) circular opening served 10 Pelts
v
FIGURE1. PHOTOELECTRIC TITRATION APPARATUS t o cut out stray light. In order to allow for focusing and variation in illumination, the small double convex lens, 0, and the light bulb, P , were mounted t o slide between Q and R. The light, P, was a 32-candlepower, 6-8 volt automobile headlight bulb which was lighted by means of current from a 6-volt transformer, 8, which was attached directly to the 110-volt lighting circuit.
METHOD The method employs the familiar Poggendorf principle of opposing the potentiometer e. m. f. to that of the photovoltaic cell and balancing by means of a galvanometer as null point instrument. The advantages of this method are well known. For the acid-alkali titrations studied, the procedure is very simple. Having in place the titration cell containing the solution to be analyzed plus 1 cc. of 0.1 per cent bromothymol blue solution, with the air stirring and the reagent buret extension tip dipped well under the solution, the bulb is lighted. The slider is now adjusted t o balance the potentiometer, and this operation is repeated after each addition from the buret. Up t o the immediate
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ANALYTICAL EDITION
Vol. 6, No. 6
vicinity of the end point there should be no change in the e. m. f. of the photovoltaic cell, other than a slow drift or slight variation of the order of a few tenths of scale division (cm.) due to line voltagezfluctuations.
TABLE I. PHOTOELECTRIC TITRATION OF 40-cc. PORTIONS OF 0.01 N HYDROCHLORIC ACIDWITH 0.1 N SODIUM HYDROXIDE
This condition is illustrated by the first portions of the curves in Figure 2. At the end point the addition of 0.002 milliequivalent of sodium hydroxide will cause a wide deflection of the galvanometer needle-i. e., 10 to 15 divisions or more-which requires a 5 to 7 cm. displacement of the
No.
1
2
CHANQEIN SCALB READINGS NPAR E N DPOINT READINQS PPR COMPIJTHID Buret Scale 0.01 cc. OF NaOH END POINT
cc.
Cm.
3.442 3.445 3.464 3.472 3.483 3.429 3.447 3.467 3.483 3.499
24.7 24.65 19.0 17.8 17.8 24.2 24.0 17.5 17.0 16.8
Cm.
.*.
0.2
3.0 1.5 0.0
3.459
...
0.4 6.1 1.0 0.0
4
3.432 3.446 3.455 3.476 3.486
3.458
25.3 25.2 23.4 18.0 18.0
TABLE11. PHOTOELECTRIC TITRATION OF 23-cc. PORTIONS OF 0.01 N SULFURIC ACID WITH 0.01 N SODIUM
HYDROXIDE
No. 5
6
7
I. Titration of 40 cc. of 0.01 N hydrochloric acid with 0.1 N sodium hydroxide (read oc on lower scale). I1 and 111. Titra. tion of 23 ca. of 0.01 N suliuric acid with 0.0: N sodium hydroxide (read cc. on upper scale).
slider along the scale. Such a displacement is equivalent to a change of 15 to 20 millivolts-for example, a drop from around 70 to about 50 millivolts. Where a reproducibility of 0.002 milliequivalent is sufficient,the sodium hydroxide may be added in increments of this order, in the vicinity of the end point, and this single wide galvanometer deflection used as a “dead-stop” end point. Thus any scale reading or plotting of results can be avoided and titration made very rapid. When seeking agreement of 0.0005 milliequivalent or better, resort is had to recording scale readings and then to computation or plotting, as shown in Tables I and I1 and in Figure 2.
RESULTS Data obtained in titrations of approximately 0.01 N hydrochloric and sulfuric acid, respectively, with 0.1098 N and 0.00926 N sodium hydroxide (carbonate-free) are presented in Tables I and 11. In the first case a microburet was used and in the latter an ordinary high-grade buret. In these titrations bromothymol blue was employed as indicator with no light filter, and the apparatus attached directly to the 110-volt lighting circuit. The same is true of the titration graphs shown in Figure 2. The order of agreement obtainable in successive analyses is readily apparent.
RRADINGS NEAR END POINT Buret Scale
cc.
Cm.
24.60 24.69 24.80 24.90 25.00 24.61 24.69 24.80 24.90 25.01 24.59 24.67 24.73 24.79 24.84 24.89 24.61 24.64 24.71 24.78 24.85 24.91
24.5 24.25 20.1 18.6 l S , 50 24.15 23.85 20.0 17.9 17.5 24.5 24.0 21.9 19.2 18.1 17.9 23.5 23.45 22.0 18.5 17.4 17.4
CHANQP I N SCALE RDADINGS PPR CALCULAT~D 0.05cc. OF NaOH ENDPOINT Cm.
...
0.1
1.9 0.8
0.1
24.77
0.2 1.8 1.1 0.2
24.78
...
...
The accuracy of the method was examined by comparing the color of the titration solution in the vicinity of the end point with a series of bromothymol blue standards, after each addition of alkali. It is evident from the plot of pH against solution added, in Figure 2, that the end point comes too early by about 0.3 to 0.4 pH. However, because of the steepness of the titration curves this fact causes a discrepancy of only 0.05 cc. of 0.01 N alkali. A correction would hardly be required except for microtitration. For the determination of very small amounts of acid or where line voltage fluctuations are troublesome, the bulb filament should be lighted from a constant wattage transformer or an accumulator. P A S a further application of the apparatus, iodine solution was titrated with sodium arsenite using no added indicator but employing a blue filter (Wratten No. 78). I n one case 25 cc. of 0.04 N iodine required 4.400 cc. of roughly 0.2 N arsenite, while a duplicate visual titration employing starch as indicator required 4.403 cc. of the arsenite. Other applications will immediately present themselves to anyone interested in photoelectric analysis. ’
LITERATURE CITED
(1) Hickman, K., and Sanford, C. R., IND. ENQ.CHEM.,Anal. Ed., 5, 65-8 (1933). (2) Muller, F., 2.Elektrochem., 40,46-51 (1934). This paper contains
a considerable bibliography. (3) Partridge, H. M., IXD.ENQ.CHEM.,Anal. Ed., 4, 315-17 (1932). (4) Partridge, H. M., and Smith, R. A., Mikrochemie, 11, 311-26 (1932). (5) Roberts, H. S., J . A m . Chem. Soc., 41, 1358-62 (1919). R~CRIVE D 28, 1934. July
