138 D. L. Randall — Titration of Mercurous Salts. 



vacuum desiccator over sulphuric acid. The method of drying 

 in a vacuum is recommended by Hulett* as much more trust- 

 worthy than the usual one of drying in the air bath at 105°— 

 110° C. where there is danger of volatilizationf, a statement 

 which the experience of the present writer has fully borne out. 

 In every case the standard of the mercurous solution used was 

 fixed by two or more gravimetric determinations made on the 

 same day as the corresponding series of titrations. 



The solutions of permanganate and of ferrous sulphate used 

 were both approximately twentieth normal. The reason why 

 it is desirable to use solutions more dilute than the usual tenth 

 normal strength is obvious when we note that 0"l cm3 of the 

 above twentieth normal permanganate is equivalent to about 

 one milligram of mercury. For the same reason the titrations 

 must be performed with all possible care and accuracy. All 

 the burettes used in the present investigation had been standard- 

 ized by the Berlin Reichsanstalt. 



As the oxidation of mercurous salt by an excess of perman- 

 ganate is very rapid, no great care is necessary in regulating 

 the amount of the excess. It is sufficient in practice to con- 

 tinue the addition of permanganate until the solution, colored 

 brown by the oxides of manganese, takes on a distinctly red 

 tint. Under these circumstances the ferrous sulphate can be 

 added immediately and the titration completed. Table I gives 

 the results obtained with mercurous sulphate. 













Table I. 





















A 











Hg,S0 4 

 sol. 

 cm 3 . 



Dilu- 

 tion 

 at 

 titra- 

 tion 

 cm 3 . 



H 2 S0 4 

 cm 3 . 



KMn0 4 



approx. 



N/20 



cm 3 . 



FeSO, 

 cm 3 . 



[ = KMn0 4 

 cm 3 . 



KMn0 4 

 final 

 cm 3 . 



Hg 

 found 

 grm. 



Hg 

 theory 

 grm. 



Error 

 grm. 



100 



150 



5 



14-70 



lo- 



10-90 



3-80 



•0346 



•0354 



— •0008 



100 



150 



5 



14-72 



io- 



10-90 



3-82 



•0347 



•0354 



— •0007 



100 



150 



5 



14-70 



io- 



10-90 



3-80 



•0346 



•0354 



— •0008 



100 



150 



5 



14-71 



io- 



10-90 



3-81 



•0346 



•0354 



— •0008 



Hg 2 S0 4 

 sol. 











B 











grm. 





















505-8 



500 



5 



29-20 



io- 



10-90 



18-30 



•1668 



•1666 



+ •0002 



500-2 



500 



5 



32-16 



13- 



14-17 



17-99 



•1639 



•1648 



— •0009 



510-1 



500 



5 



29-29 



io- 



10-90 



18-39 



•1676 



•1680 



— •0004 



499-2 



500 



5 



28-95 



10- 



10-90 



18-05 



•1645 



•1644 



+ •0001 



* Zeitschr. f. phys. Chern., xlix, 500 (1904). Hulett, however, employed cal- 

 cium chloride instead of sulphuric acid as the drying agent. 



f Treadwell (Lehrbuch der anal. Chemie, vol. ii, p. 114), states that when 

 the drying is carried out at 105° the results are always about - 4 per cent too 

 low. 



