356 



[Vol. 9 



ANNALS OF THE MISSOURI BOTANICAL GARDEN 



burn (II, '10), Michaelis (II, 14), Clark (II, 15, '20) and 

 Committee on Descriptive Chart for 1918 (II, Conn, 19). 



TABLE II 



RELATION OF P H VALUES TO STRENGTH OF SOLUTION 



Strength 



of 

 solution 



Grams of hydrogen 



per liter 



Expressed 

 logarith- 

 mically 



Ph 



value 





Normal IIC1 



1.0 



io-° 



0.0 



Acid 



1/10NIIC1 



0.1 



10- 1 



1.0 



Acid 



1/100 NIIC1 



0.01 



io-» 



2.0 



Acid 



1/1000 NIIC1 



0.001 



10-* 



3.0 



Acid 



1/10000 NHC1 



0.0001 



io- 4 



4.0 



Acid 



1/100000 N HC1 



0.00001 



io-» 



5.0 



Acid 



1/1000000 N IIC1 



0.000001 



io- 6 



6.0 



Acid 



Pure water 



0.0000001 



io- 1 



7.0 



Neutrality 



1/1000000 NNaOH 



0.00000001 



io- 8 



8.0 



Alkaline 



1/100000 NNaOH 



0.000000001 



io-» 



9.0 



Alkalino 



1/10000 NNaOH 



0.0000000001 



io- 10 



10.0 



Alkaline 



1/1000 NNaOH 



0.00000000001 



io- 11 



11.0 



Alkaline 



1/100 NNaOH 



0.000000000001 



io- 12 



12.0 



Alkalino 



1/10 NNaOH 



0.0000000000001 



io-» 



13.0 



Alkaline 



Normal NaOH 



0.00000000000001 



i o- 14 



14.0 



Alkaline 



METHODS OF MEASURING HYDROGEN-ION CONCENTRATION 



The hydrogen-ion concentration, that is, the true acidity of 



may be measured 



The 



methods used are, first, by measuring the electromotrv 



fo 



of a solution, and second, by the use of certain indicators whose 

 color changes in relation to various levels of hydrogen-ion con- 

 centration have been definitely determined by the first method. 

 The first or electrometric method, with good equipment, is more 

 accurate, while the second or colorimetric method, while not as 

 accurate, is much more available, since the cost is much less, and 



much more sim 



havin 



both in operation is satisfied that for ordinary work, and even for 



special research, the first is not essential for a bacteriologist.) 



For references to methods and apparatus necessary for electro- 

 metric measurements see Clark (II, '20) and Leeds and Northrup 

 Company, Catalogue No. 75 (II, '20). 



Indicators developed by Clark and Lubs (17) for colorimetric 

 work are especially well fitted for bacteriological media since they 

 are as a whole brilliantly colored, their color changes at different 

 levels are marked, and they are readily procurable at reasonable 

 rates. Their color changes with reference to hydrogen-ion con- 



