GILLESPIE: HYDROGEN-ION CONCENTRATION IN SOILS 11 



space over the soil mixture, the current of hydrogen was stopped by- 

 closing the exit, and the vessel was shaken 10 to 20 times. In order 

 to remove residual atmospheric nitrogen another portion of hydrogen 

 was passed through in the same way, and after shaking again 10 to 20 

 times the hydrogen entrance was also closed and the vessel shaken at 

 the rate of 70 to 90 complete swings per minute for five minutes. The 

 motor was stopped, the potassium chloride contact was made in the 

 tube a, the shaking was started again, and the electromotive force of the 

 combination of the electrode with a saturated KC1 calomel electrode 7 

 was determined at once. The vessel was shaken continuously during 

 the measurement, according to the proposal of Hasselbalch for meas- 

 urements on biological fluids, 8 this procedure being especially advan- 

 tageous for work with fluids poor in regulating capacity. From this 

 difference of potential and the temperature the hydrogen-ion exponent 

 was calculated from the figures given by Michaelis. 9 The tempera- 

 ture was observed in a bottle which took the place of the usual potas- 

 sium chloride trough; it remained be- 

 tween 25.5° and 28.5°C. during all the k 33 """ ~ 

 determinations. Tests of the calomel 

 electrode on regulator mixtures of known 

 exponents showed no error due to the pj g 2 . Hydrogen electrode, 

 calomel electrode. The hydrogen elec- 

 trode is shown in figure 2. A large sheet of plati num was used which 

 measured 2.54 cm. by 3.3 cm. It was freshly coated with palladium 

 black for each determination after complete removal of the previous 

 coating. In constructing the hydrogen electrode it was necessary to 

 support the sheet of platinum at both top and bottom, and also to pro- 

 vide leads for the current at two opposite points, in order to secure an 

 even deposit of palladium black. The electrode remained partially 

 (about half) submerged during the shaking. 



The arrangement used for measuring the potentials was that de- 

 scribed by Hildebrand. 10 A capillary electrometer was used as a 

 null-point instrument. A voltmeter was used which read directly to 

 20 millivolts. 11 The potential could be estimated accurately to 2 

 millivolts and often to 1 millivolt. The voltmeter was calibrated 



7 Michaelis and Davidoff, Biochem. Ztschr., 46: 131. 1912. 



8 Biochem. Bulletin, 2: 367. 1913. 



9 Die Wasserstoffionenkonzentration, 157. 1914. 



10 Journ. Am. Chem. Soc, 35: 847. 1913. 



11 For further work a high-grade potentiometer will be available. 



