Time Exposed. 50 per cent. Acid. 10 per cent. Acid. 1 per cent. Acid. 1 hour 33.2 c.c. 34.4 c.c. 34.5 c.c. 1 day 5.0 " 24.0 " 27.6 " 2 days 3.0 " 14.5 " 21.3 " These indicate very clearly the increased susceptibility to oxidation in strongly acid solutions. _A standard solution of ferric chloride_ is prepared in the same manner as that described under the experiments on the methods of reduction; but it should be of twice the strength, so that 100 c.c. may contain 1 gram of iron. This solution is used for standardising the stannous chloride when required; and must be carefully prepared; and tested for the presence of nitric acid. The titration is more limited in its application than either of the oxidising processes because of the restrictions as to bulk, quality and quantity of free acid present, and other conditions of the solution to be assayed. The following experiments show the conditions necessary for a successful titration. ~Effect of Varying Temperature.~--Twenty c.c. of ferric chloride solution with 20 c.c. of strong hydrochloric acid, diluted to 50 c.c., gave the following results when titrated:-- Temperature 15° 30° 70° 100° Stannous chloride required 22.8 c.c. 22.0 c.c. 22.1 c.c. 22.0 c.c. The finishing point, however, is more distinct the hotter the solution; so that it is best in all cases to run the standard into the boiling solution. ~Effect of Varying Bulk.~--Solutions containing the same quantity of iron and acid as the last, but diluted to various bulks, and titrated while boiling, gave the following results:-- Bulk 30 c.c. 100 c.c. 500 c.c. Stannous chloride required 21.5 " 21.7 " 24.3 " ~Effect of Varying Quantities of Hydrochloric Acid.~--In these experiments the bulk before titration was 50 c.c. except in the last, in which it was 70 c.c. With less than 5 c.c. of strong hydrochloric acid the finishing point is indistinct and prolonged. Strong hydrochloric acid present 5 c.c. 10 c.c. 20 c.c. 30 c.c. 50 c.c. Stannous chloride required 21.1 " 21.1 " 21.2 " 21.8 " 22.2 " ~Effect of Free Sulphuric Acid.~--In these experiments 20 c.c. of hydrochloric acid were present, and the bulk was 50 c.c. Strong sulphuric acid present -- c.c. 3 c.c. 5 c.c. 10 c.c. Stannous chloride required 21.6 " 22.3 " 22.9 " 23.1 " This interference of strong sulphuric acid may be completely counteracted by somewhat modifying the mode of working. Another experiment, like the last of this series, required 21.6 c.c. ~Effect of Foreign Salts.~--Experiments in which 10 grams of various salts were added showed them to be without effect. The results were as follows:--