Salt present -- AmCl Am_{2}SO_{4} MgCl_{2} Stannous chloride required 21.6 c.c. 21.6 c.c. 21.6 c.c. 21.6 c.c. Salt present CaCl_{2} FeCl_{2} Al_{2}Cl_{6} Stannous chloride required 21.8 c.c. 21.6 c.c. 21.6 c.c. ~Effect of Varying Iron.~--Titrating a solution (with 20 c.c. of hydrochloric acid) measuring 50 c.c., and kept boiling, the quantity of stannous chloride solution required is practically proportional to the iron present. Ferric chloride added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c. Stannous chloride required 1.1 " 10.5 " 20.6 " 51.4 " 102.6 " The student, having practised some of the above experiments, may proceed to the assay of an iron ore. ~Determination of Iron in Brown Iron Ore.~--Weigh up 1 gram of the dried and powdered ore, calcine in the cover of a platinum crucible, and dissolve up in an evaporating dish[69] with 20 c.c. of strong hydrochloric acid. When solution is complete, dilute to 50 c.c. after replacing any acid that may have been evaporated. Boil, and run in the stannous chloride solution until the colour is faintly yellow; boil again, and continue the addition of the stannous chloride solution, stirring continuously until the solution appears colourless. Note the quantity of the stannous chloride solution required. Suppose this to be 59 c.c. Take 60 c.c. of the standard ferric chloride solution, add 20 c.c. of hydrochloric acid, boil and titrate in the same way as before. Suppose this to require 61 c.c. Then as 61 is equivalent to 60 of the iron solution, 59 is equivalent to 58.13.[70] This gives the percentage. It is not necessary to standardise the stannous chloride solution in this way with each sample assayed, the ratio 61: 60 would serve for a whole batch of samples; but the standardising should be repeated at least once each day. COLORIMETRIC METHOD. This method is valuable for the determination of small quantities of iron present as impurities in other metals or ores. It is based on the red coloration developed by the action of potassic sulphocyanate on acid solutions of ferric salts. _Standard Ferric Chloride Solution._--Take 1 c.c. of the ferric chloride solution used for standardising the stannous chloride solution, add 2 c.c. of dilute hydrochloric acid, and dilute to 1 litre with water. 1 c.c. = 0.01 milligram. _Solution of Potassic Sulphocyanate._--Dissolve 60 grams of the salt in water, and dilute to a litre. It should be colourless. Use 10 c.c. for each test. The quantity of the substance to be weighed for the assay should not contain more than a milligram of iron; consequently, if the ore contain more than 0.1 per cent. of that metal, less than a gram of it must be taken.