594 LEON S. MEDALIA 



referred to. The most important of these was the difficulty 

 experienced in dissolving the powdered indicators as directed for 

 the preparation of the "stock alcoholic solution." Such diffi- 

 culties can easily be overcome by using N/20 NaOH in the 

 amounts recommended by Clark (1920) which are the molecular 

 equivalents of each indicator, for the alcohoUc stock solution 

 (see table 2). 



"stock alcoholic solution" 

 Method of preparation 



The powder (say, 0.1 gram) is weighed out on an analytical 

 balance directly in a clean, dry, heavy-walled test tube, a few 

 drops of the N/20 NaOH is added and the powder is rubbed up 

 with a glass rod; the rest of the required amount of the N/20 

 NaOH is gradually added. The dissolved indicator is now 

 washed into a 50-cc. graduate with 95 per cent ethyl (ordinary) 

 alcohol, and is made up to 50 cc. with the alcohol, making a 0.2 

 per cent "stock alcohol solution." The "indicator watery solu- 

 tion" is prepared from the stock alcoholic solution as already 

 described in the previous article. 



There has recently appeared an exhaustive study on "The 

 Reaction of Culture Media" by Bunker and Schuber (1922). 

 These authors thoroughly discuss the question of H-ion con- 

 centration with particular reference to culture media, and in 

 referring to the author's method, mention that their "indicator 

 watery solution" spoiled in a few days. The "indicator watery 

 solution" has kept well in this laboratory since October, 1919 

 (for nearly two and one-half years) , and is still in good condition. 

 To secure this satisfactory result the author feels that it is 

 necessary to observe a fair amount of aseptic technic. The same 

 sterile pipette can be used for one day for the same indicator, 

 by keeping the pipette upright in a sterile test tube. 



requirements for the preparation of "color 

 standards" 



The requirements for preparing a set of "color standards" 

 for the colorimetric determination of the H-ion concentration 



