140 
IOWA ACADEMY OP SCIENCE 
Iron Alum. Five cubic centimeters of a saturated solution, made from good 
clear crystals, were used in each titration. 
Water. The water was purified by the method of Jones and Mackay^; ordi- 
nary distilled water was distilled from a large Jena fiask containing a small 
amount of potassium bichromate and sulphuric acid. This was then doubly 
distilled from a similar fiask and solution through a retort containing barium 
hydroxide and condensed through a block tin tubing. This water was used in 
making up all aqueous solutions. 
Solutions. All solutions were made up to volume at 25° C. Alcoholic solu- 
tions of silver nitrate and ethyl iodide were prepared seven to twenty hours 
previous to using, and of such a strength that when equal volumes of the two 
were mixed the normality desired was obtained. 0.025N silver nitrate and 
ammonium sulphocyanate solutions were used in all titrations, the silver nitrate 
solution was made up by weight and checked against a standard sodium chloride 
solution by the Volhard method. The precipitate of silver chloride was filtered 
off before titrating with ammonium sulphocyanate. Since the results agreed 
very satisfactorily, the other solutions of silver nitrate were made up by weight. 
APPAKATUS AND METHOD. 
In general the method used is similar to that followed by all experimenters 
in the measurement of velocity coefficients of chemical reactions. 
All experiments were performed and solutions made up to volume in a con- 
stant temperature bath at 25°-|-.01. The motor driven stirrer kept the water in 
the large granite tank in constant motion. The heat was supplied by means of 
sixteen candle power electric light bulbs immersed in the bath and electrically 
controlled. The thermometer used was certified and graduated in tenths, such 
that it could easily be estimated to hundredths. 
The bottles were suspended in the bath by means of a copper strip frame- 
work of clamps constructed for this purpose and which would accommodate 
twenty bottles at one time. 
Every precaution was taken to keep the nitric acid, silver nitrate, ethyl 
iodide and solutions from the light. The reaction bottles and all solution 
bottles were thoroughly “seasoned,” glass stoppered, and well protected by 
sacks of heavy black cloth or paper. 
Equal volumes of each solution were quickly poured together, stoppered and 
shaken thoroughly. Then ten cubic centimeters of the mixture, removed by a 
certified pipette previously warmed in the bath, were introduced into 
each of several small dry bottles which had been brought to temperature 
by suspension in the bath. The approximate instant of mixing was taken as 
the initial time. At previously decided instants, a known amount — always an 
excess — of 0.025N ammonium sulphocyanate was quickly poured from an Erlen- 
meyer fiask into the reacting solution to stop the reaction. This mixture was 
then poured back into the flask, the bottle rinsed three times with chlorine-free 
distilled water, diluting the solution to over one hundred cubic centimeters. 
Five cubic centimeters each of a saturated solution of iron alum and of nitric 
acid were then added and the excess of ammonium sulphocyanate determined 
by titrating with 0.025N silver nitrate. From this the amount of silver nitrate 
remaining in the solution at the instant of stopping the reaction was easily 
obtained. 
iAm. Chem. J. 19, 83 (1897). 
