Jtjly 2, 1920] 



SCIENCE 



19 



o£ "standard mixtures" having a known hy- 

 drogen ion concentration or pn value. These 

 investigators established their formulas very 

 precisely by the use of a potentiometer method 

 employing a hydrogen electrode. According 

 to Clark,^ the p'' values in the freshly pre- 

 pared mixtures may be considered reliable to 

 a few hundredths of a Ph unit. Probably the 

 widest application of these standard solutions 

 is, in connection with color indicators, for 

 comparisons with solutions having an un- 

 known hydrogen ion concentration. 



The fact that standard mixtures can readily 

 be prepared, combined with the further fact 

 that the hydrogen electrode is an appliance 

 which is simple and convenient to use, leads 

 to a very obvious suggestion. This is to 

 utilize the hydrogen electrode as a means for 

 comparison of an unknown with a standard 

 solution. It should extend materially the use- 

 fulness of the standard solutions to which 

 reference has been made. 



"We may suppose that we desire to titrate a 

 solution of unknown p'' value to a definite 

 hydrogen ion concentration. From the curves 

 of Sorensen's article, or from the formulas of 

 Clark and Lubs, we select the particular solu- 

 tion having a pn value which corresponds to 

 the point to which we desire to titrate. This 

 solution is placed in one vessel with a hydro- 

 gen electrode, and connection is established 

 between the standard solution and the un- 

 known, in a second vessel, by means of a salt 

 bridge of saturated potassium chloride, so that 

 concentration potentials may be eliminated.* 

 Another hydrogen electrode is placed in the so- 

 lution of unknown concentration, and the two 

 electrodes are connected through a tapping key 

 and a galvanometer of high resistance. Ap- 

 propriate protective resistance may also be put 

 in this circuit. The process of titrating to the 

 desired end point then consists merely of add- 

 ing the titrating solution until, upon tapping 

 the key, no deflection of the galvanometer is 



« Private communication. 



* The suggestion of using an agar-agar salt 

 bridge to minimize diffusion effects (Falles and 

 VoBburgh, J. A. C. S., 40, 1306, 1918) seems a 

 good one. 



observed. The inference is that zero jwten- 

 tial difference between the hydrogen elec- 

 trodes is an indication of equal hydrogen ion 

 concentrations of the two solutions. The 

 supposition may be verified by putting both 

 electrodes into one or the other of the solu- 

 tions and noting whether the galvanometer 

 deflection remains zero. 



It may be pointed out that such a titration 

 can be carried out in any solution in which a 

 hydrogen electrode will maintain its equili- 

 brium, regardless of color, turbidity, or other 

 experimental conditions. The electrolytic 

 portion of the galvanometer circuit will, in 

 most cases, have a low resistance, which 

 insures the desirable condition for sensitive- 

 ness of response of the instrument. The 

 method has most of the advantages of the 

 potentiometer method over the colorimetric 

 methods, with the obvious exception that it 

 can be used only for titrating and that the 

 titration can be carried only to the end point 

 which is determined by the standard solution. 

 With the potentiometer it is i)0ssible, of 

 course, not only to titrate to any end point 

 but also to make a direct measurement, with- 

 out titration, of the p° value, whatever this 

 may be. 



It should be noted, finally, that in the titra- 

 tion described no calomel electrode is used, 

 and that the accuracy with which the titra- 

 tion may be made is limited only by the 

 accuracy with which the pn value of the 

 standard solution is known. 



Paul E. Klopsteg 



Leeds & Nobthrijp Company, 

 Philadelphia, Pa. 



THE AMERICAN SOCIETY OF 

 MAMMALOGISTS 



The second annual meeting of the American So- 

 ciety of Mammalogists was held May 3-5, 1920, in 

 the American Museum of Natural History, New 

 York City. Officers for the coming year are Dr. C. 

 Hart Merriam, president; Mr. E. "W. Nelson and 

 Dr. Wilfred H. Osgood, vice-presidents; Dr. H. H. 

 Lane, recording secretary; Dr. Hartley H. T. Jaxjk- 

 son, corresponding secretary; Mr. J. W. Gidley, 

 treasurer; Mr. N. Hollister, editor; Dr. GUover M. 

 AUen, Dr. E. M. Anderson, Dr. Joseph Grinnell, 



