14 GILLESPIE: HYDROGEN-ION CONCENTRATION IN SOILS 



Twenty regulator mixtures were prepared for the work, the expo- 

 nents were determined electrometrically and the colorimetric work was 

 done while the solutions were fresh. The reaction varied from one tube 

 to the next in the series by a step of about 0.3 in the value of the expo- 

 nent. From the exponent 3.5 to 5.1 the regulators were prepared from 

 a given quantity of n/10 sodium hydrate and decreasing quantities of 

 n/2 acetic acid and water. The concentration of the sodium acetate 

 formed in the mixing was m/200 in all cases. From 5.3 to 6.8, mixtures 

 of m/10 potassium acid phosphate and m/10 sodium hydrate and water 

 were used which had the same concentration of primary and of secondary 

 phosphate as prescribed by Sorensen 13 and were thus m/15 with phos- 

 phate; and from 7.1 to 9.4, other mixtures of the same two solutions were 

 used (without addition of water), in which the concentration of phos- 

 phate decreased from m/16 to m/20, owing to the dilution involved in 

 the use of sodium hydrate instead of the disodium phosphate used by 

 Sorensen. 



The following six indicators were used: (1) tetrabromphenolsulfone- 

 phthalein (abbreviated in the table to t-br-ph-s-pht) , 3 drops from a 

 capillary pipette of a solution of 0.1 gram in 250 cc. alcohol; (2) methyl 

 red, 2 drops of a solution of 0.1 gram in 300 cc. alcohol and 200 cc. 

 water; (3) dipropyl red, 4 drops from a capillary pipette of a solution 

 made in the same way as the methyl red; (4) bromthymolsulfone- 

 phthalein (br-th-s-pht) , 6 drops from a capillary pipette of a solution of 

 0.1 gram in 200 cc. alcohol; (5) the sodium salt of phenolsulfone- 

 phthalein (ph-s-pht), 1 drop of a 0.06 per cent aqueous solution; and 

 (6) phenolphthalein (ph-pht), 1 drop of a 0.5 per cent solution in 50 

 per cent alcohol. The first, third, and fourth are new indicators re- 

 cently prepared and described by Lubs and Clark. 14 Solutions of these 

 were kindly given to me by Dr. H. A. Lubs. These three new indicators 

 would seem to have considerable value in this type of investigation. 



The colorimetric results are given, together with the electrometric 

 results, in the table herewith (Table I). 



DISCUSSION OF THE RESULTS 



In the table are given the hydrogen-ion exponent for each of the 22 

 soils as determined by the electrometric method and by the different 

 indicators, and also the mean colorimetric result and the mean of the 



13 Biochem. Ztschr., 21: 131. 1909; 22: 352. 1909. 



14 Jour. Wash. Acad. Sci., 5: 009. 1915. 



