I I/ 



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CHAPTER IV 



THE PRINCIPLES INVOLVED IN THE DETERMINATION OF THE 

 HYDROGEN-ION CONCENTRATION 



TITRABLE ACIDITY AND ALKALINITY 



All acids have one property in common namely, that they contain 

 hydrogen and when the acid becomes neutralized, it is this element 

 which becomes replaced by some other cation. Evidently, then, the 

 strength of an acid is proportional to the number of displaceable hydro- 

 gen atoms which it contains. It may contain other hydrogen atoms 

 which are so bound up in the molecule that they do not become displaced 

 when an alkali is mixed with the acid. For example, in organic acids 

 like acetic, CH 3 COOH, it is only the H atom attached to the COOH 

 group, but not those attached to the CH 3 group, that is replaceable. It 

 must therefore be possible to prepare for every acid a solution having 

 exactly the same neutralizing power as that of any other acid; that is, 

 the same volume of solution must be required in each case to neutralize 

 a given quantity of alkali, the point of neutralization being judged by 

 the change in color of indicators. As a standard a gram-molecular solu- 

 tion of an acid with one displaceable H ion, such as hydrochloric, is 

 chosen. This we call a "normal acid" (N). To prepare a normal solu- 

 tion of acids having two displaceable H atoms, such as H 2 S0 4 , we can not 

 however use a gram-molecular quantity, but must take one-half of it; 

 and similarly in the case of those with three H atoms, such as H 3 P0 4 , 

 a one-third gram-molecular solution will be a normal acid solution. For 

 practical purposes, use is very generally made of solutions that are some 

 fraction of the normal, e. g., tenth or decinormal (written N/10), or hun- 

 dredth or centinormal (N/100). 



In a similar way, alkaline solutions can be prepared, a normal alkali 

 being one which exactly corresponds in strength with a normal acid 

 (i.e., can exactly neutralize it). Now, the characteristic of alkalies is 

 that they produce in solution "OH" or hydroxyl ions; so that the process 

 of neutralization must consist in the union of the H ions of the acid with 

 the OH ions of the alkali to form water: KOH + HC1 = KC1 +H 2 0. We 

 can, therefore, prepare normal solutions of alkalies by dissolving in 1 

 liter of water such quantities of alkali (in grams) as will yield the OH 

 required to react with the available hydrogen in normal acid solutions. 



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