APPENDIX. 



HYDROGEN ION CONCENTRATION. 



The reaction of a medium not infrequently is described in 

 terms of the colour change, acid, neutral or alkaline, effected 

 in respect to litmus. In many instances this indication is 

 sufifiicient, but in much biological work a more precise definition 

 of reaction is requisite, and this is made possible by the con- 

 ception known as the hydrogen ion concentration. 



A normal solution of any acid or salt is defined as one 

 containing I gram of hydrogen or its equivalent dissolved 

 in I litre of water. According to this definition, the weights 

 of hydrochloric, nitric, acetic and any other monobasic acid 

 contained in a litre of normal acid would be the respective 

 molecular weights in grams, namely HCl = 36-5, HNO3 = 63, 

 CH3COOH = 60. In the case of a dibasic or tribasic acid, it 

 would be the molecular weight divided by two or by three — 



—£ ? = 49, —i ? = 32-6. 



From this reasoning it follows that whilst normal solutions of 

 all these acids contain in the litre different quantities of acid, 

 they all contain the same quantity of hydrogen, namely, i gram 

 per litre. Whilst, however, they all contain potentially the 

 same amount of hydrogen, it does not follow that the whole of 

 this quantity is ionized ; and inasmuch as the actual acidity of 

 a solution at any given moment is measured by the proportion 

 of ionized hydrogen atoms it contains, it follows that the actual 

 acidity of equinormal solutions of these various acids may be 

 very different. This does not mean that their actual titratable 

 value, as measured by their power of neutraHzing alkali, will 

 be different. Thus for the complete neutralization of the 

 I gram of hydrogen contained in i litre of each of the 



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