M. C. Lea — Relative Affinities of certain Acids. 447 



sodium requisite to saturate a molecule of sulphuric acid and 

 therefore only half a molecule of pyrophosphoric acid is set 

 free. 



In other words : it is found by experiment that the quantity 

 of sodium pyrophosphate necessary to extinguish the reaction 

 for free sulphuric acid with 1000 molecules of that acid is 963 

 molecules ; out of this, 500 molecules of pyrophosphoric acid 

 are set free as just mentioned and there remain 463 molecules 

 of undecomposed pyrophosphate. This number 463 multi- 

 plied by 2 because of the basicity of the acid and divided by 

 1000 to make it correspond to one molecule of sulphuric acid 

 gives *926 as the index of pyrophosphoric acid. 



The state of equilibrium is always conditioned by the degree 

 of concentration. If to any solution of sulphuric acid a salt 

 is added in just sufficient quantity to extinguish the sulphuric 

 acid reaction, it is then only necessary to add a little water and 

 the equilibrium is at once changed : a certain portion of the 

 salt that had been added is re-formed and the sulphuric reac- 

 tion reappears. In order therefore to obtain true comparative 

 results it is necessary to use the sulphuric acid always at exactly 

 the same dilution and to add the dry salt to it. 



The affinity of sulphuric acid for water is a most important 

 factor in all determinations of this nature. Mendeleef indeed 

 expresses the opinion* that most of the affinities hitherto de- 

 termined are unreliable for want of sufficient exactitude in 

 this respect. 



To show how much precaution is needed, the following re- 

 actions may be mentioned. 



When 4 CC of normal sulphuric are added to 40 oc of normal 

 solution of sodium nitrate not a trace of free sulphuric acid 

 can be detected in the liquid. In consequence of the large 

 excess of sodium salt the sulphuric acid has been completely 

 taken up by the sodium with of course an expulsion of an 

 equivalent quantity of nitric acid. 



But when instead of 4 CC of normal sulphuric acid we use 

 40 cc of decinormal acid then although the quantities of acid 

 and of salt are exactly the same the equilibrium is completely 

 changed. The greater quantity of water present by reason of 

 its affinity for sulphuric acid counteracts to some extent the 

 affinity of the sodium. Free sulphuric acid exists in the 

 solution and is abundantly indicated by the herapathite test. 



This difference may be even more strikingly shown in the 

 following manner. Taking the mixture of 4 C0 of normal sul- 

 phuric acid and 40 cc of normal solution of sodium nitrate let a 



* Principles of Chemistry, English ed., vol. i, p. 377, footnote. 



