380 



APPENDIX. 



STANDARD ACIDS AND ALKALIES. 



A normal solution of a substance contains in 1000 cc. that weight in grams, 

 which corresponds to i equivalent in grams, of available hydrogen (1-008 

 grams.) or its equivalent. 



Thus normal hydrochloric acid contains 35-46 + 1-008 = 36-468 grams, 

 of HC1 per litre. 



Normal sulphuric acid contains 

 2-016 + 32-07 + 64 



- - = 49*043 grams, of H 2 SO 4 per litre. 



It is customary to employ normal, half-normal, fifth-normal, etc., solu- 

 tions according to circumstances. But it is often much more convenient to 

 determine the exact strength of a solution than to adjust it to some even 

 fraction. For this reason it is better to express the normality as a decimal 

 coefficient rather than as a fraction. Thus, suppose an acid be found by 

 titration against a known standard to be 0-107 N, it can be labelled as such 

 and used when a solution about one-tenth normal is convenient, the necessary 

 adjustment in the calculation being very simple. The relationship is not 



N 

 so obvious if it be labelled ^ . 



In the author's experience the simplest and most reliable starting point 

 for the preparation of standard acids and alkalies is CO 2 - free, sodium hydroxide, 

 made and stored as described on p. 26. From such a stock it is a simple 

 matter to prepare acids or alkalies of any desired concentration. For 

 further details concerning the preparation and storage of the alkali see 

 p. 322. 



Thus, suppose that o-i N.HC1 be required. Dilute pure concentrated 

 hydrochloric acid about 90 times with distilled water, measure out 25 cc. and 

 titrate it with the standard alkali, using either methyl red or phenol phthalein 

 as the indicator. Suppose that the 25 cc. of dilute acid require 13-8 cc. of 

 alkali which has been found to be 0-1965 N. Then the normality of the 



13-8 

 acid is x 0-1965 = 0-1085 N. 



The acid can be used as such, or if exactly o-i N. be required, then 8-5 cc. 

 of distilled water is added to every 100 cc. of the acid, thus bringing it to the 

 desired concentration. 



It is important to note that acids and alkalies act on glass, and thereby 

 suffer a change in concentration. This is practically avoided by storing in 

 bottles that have been coated internally with a fairly thick layer of paraffin 

 wax. 



