VOL.7, No. 7 PREPARATION OF HYDROGEN ELECTRODES
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be established when the electrodes were later saturated with hydrogen in the first pH measurement. After this the electrodes were ready for use, and were transferred to the electrode vessel. Using 0.05 M potassium acid phthalate solution, the following voltages were obtained with 12 electrodes, at 25OC., prepared in two successive amounts of six each, and using a normal potassium chloride-calomel-mercury half cell as the reference electrode. (1) (2) (3) (4)
0.51965 volt 0.5197 volt 0.5197 volt 0.51975 volt
(5) (6) (7) (8)
0.51955 volt 0.5197 volt 0.5196 volt 0.5185 volt
(9) (10) (11) (12)
0.5198 volt 0.5196 volt 0.5196 volt 0.51985 volt
The voltage interpolated from the data of Clark8 for this combination a t 25OC. is 0.5198 volt. Discussion
The method of treatment outlined above has another advantage besides that of saving time. After insertion of the electrodes into the stopper they are not touched by the hands a t any time until ready for use. This eliminates much of the danger of soiling the electrodes, rubbing the foil, or bending it out of shape. If the voltage drop across the electrodes went very much over 2.5 volts. the platinum black deposited in a spongy form that did not adhere at all readily to the electrodes, and for this reason the parallel hook-up was used for platinizing. ? In saturating the electrodes with hydrogen, it was not found necessary to bubble the gas directly on the foil, but anywhere in the solution. Evidently the platinum black adsorbs the gas in solution readily. "lark, "The Determination of Hydrogen I a s , " 2nd edition. The Williams & Wilkins Co., Baltimore, Md., 1923, p. 275.
Find Motor Fuels with Least Knock. Just how much knock automobile fuels used in the manufacture of high-test gasolms do away with has been found. Three engineers of the General Motors Research Laboratories, John M. Campbell, Wheeler G. Lovell, and T. A. Boyd, who reported their findings before the Society of Automotive Engineers recently, said they tried the different fuels on a standard engine and in each case noted the highest compression a t which the engine would mn smoothly. Normal heptane, which knacks more readily than ordinary gasoline, was used as the basis of comparison and varying mixtures of heptane with the more volatile fuels were tested. Heptane, it was found, first caused knocking when a compression ratio of 2.75 had been reached. Benzene proved to be the most efficient anti-knock fuel as a compression ratio greater than 15 was reached before it began to cause noise. The corresponding compression ratio fa toluene is 13.60; for absolute ethyl alcohol, 11.60; iso-octane. 6.90; cydohexane, 4.50; and normal pentane, 3.80.-Science Serdcc
