168 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVI. — On a Modification of Pelouze’s Method for determining 
Nitrates. By Professor E. A. Letts and Florence W. Rea. 
(MS. received March 16, 1915. Read June 7, 1915.) 
Pelouze, as early as 1847,* employed a method for determining nitrates 
in the commercial potassium and sodium salts, in which weighed quantities 
of the latter were boiled with ferrous chloride solution (obtained by 
treating a definite weight of iron wire with excess of hydrochloric acid), 
and afterwards the remainder of the ferrous salt was determined by 
titration of the diluted solution with potassium permanganate. 
According to Fresenius,f Pelouze’s method “gives occasionally satis- 
factory results, but can never be relied on.” Fresenius therefore modified 
Pelouze’s method, and obtained very satisfactory figures. Eder,J in his 
turn, modified Fresenius’ method, using first ferrous chloride, later 
ferrous sulphate, and later still a solution of ferrous ammonium 
sulphate, acidulated with sulphuric acid. Both Fresenius and Eder used 
a retort for boiling the mixture of ferrous salt nitrate and sulphuric 
acid, Eder carrying out that operation in a stream of carbonic anhydride, 
while Fresenius employed either that gas or hydrogen. 
It seemed to the authors that by using a retort there was at least a 
possibility of some nitric acid distilling over unchanged, and they therefore 
proceeded as follows. 
An apparatus was employed consisting of a flask of about 200 c.c. 
capacity attached by an india-rubber cork to a Liebig’s condenser in an 
upright position, and so arranged that a stream of carbon dioxide (purified 
by passing it through two wash-bottles, each containing ferrous carbonate 
suspended in water, to retain any traces of oxygen) could be passed into 
the flask and escape from the top of the condenser through a bent tube 
dipping into a vessel containing water. A solution of ferrous sulphate 
containing a molecular weight in grams (277*02) per litre was employed 
and also some free sulphuric acid, to prevent the ferrous salt from being 
oxidised. In some of the experiments a normal solution of potassium 
nitrate was employed, while in the later ones the nitrate solution was 
one-tenth that strength. 
In performing a determination, 10 c.c. of the ferrous solution were 
* Annales d. chim., 1847, 129. 
t Quantitative Analysis^ seventh edition, p. 393. 
I Zeitschr. f. anal. Ghem., xvi, p. 267. 
