EXPERIMENT STATION BULLETINS. 401 



taining distilled water was connected to the jacket of the hydrogen 

 electrode to facilitate the washing of the electrode. The hydrogen 

 and water connection was made by means of a three way, glass stop- 

 cock. The calomel electrode vessel was supplied with a normal solution 

 of potassium chloride, saturated with calomel, from an elevated bottle 

 and the supply regulated with a Hoffman pinch cock attached to the 

 connecting rubber tubing. The electrodes and stirring apparatus were 

 mounted on a large ring support for convenience in operating. 



Preparation of the hydrogen electrode consisted of coating it with 

 platinum black by electrolyzing a dilute platinic chloride solution for 

 about two minutes time and then electrolyzing a dilute solution of sul- 

 furic acid to saturate the platinized electrode with hydrogen. Two dry 

 cells in series were used for this work and the electrode connected to 

 the negative pole. Contact through the liquids to the positive pole 

 of the batteries was made by a small piece of platinum wire attached 

 to a copper wire. When not in use the hydrogen electrode was kept 

 submerged in distilled water. 



Calomel for the calomel electrode was prepared by the chemical 

 method recommended by Ellis, cited by Knight (5), and the cell made 

 up according to standard methods. 



METHOD OP OPERATION 



After considerable experimentation with varying quantities of soils 

 and water and various ways of handling the apparatus to determine the 

 pH value of the solutions, the following method was selected as one 

 designed to cover the desired range of conditions and give the most re- 

 liable results. In the titration work, 10 gms. of soil was placed into each 

 of several tumblers, the number depending on the range to be covered. 

 0.1 N equivalents of the different reagents used were added in steps rang- 

 ing, usually, from one to 10 cc. and sufficient neutral, distilled water 

 was added to each tumbler to raise the liquid volume to 50 cc. Varia- 

 tions in soil quantities and volumes of liquid are noted when they 

 occur, as are other changes from the usual procedure. The pH was 

 determined on these solutions at various times depending on the nature 

 of the titration. 



The hydrogen electrode, after preparation and thorough washing with 

 distilled water, was placed into the soil suspensions always to the same 

 depth, and the flow of hydrogen over it regulated to the rate of about 

 two bubbles a second. The electrode remained in the suspensions con- 

 stantly, except during the change from one to another, and the stirring 

 was continuous. The speed of the stirrer was adjusted to keep all 

 soil particles in motion. At the beginning of a titration, the hydrogen 

 electrode was always brought to equilibrium with an untreated soil 

 suspension. Change to other suspensions was made in order, and with- 

 out washing the electrode. 



The salt arm of the calomel vessel was introduced into the soil sus- 

 pension immediately before reading the voltage and at the same dis- 

 tance from the hydrogen electrode and removed directly after completing 

 the reading. A few drops of the potassium chloride solution were 

 passed through the salt arm of the calomel electrode before and after 



