Standard solutions should not be kept in a place exposed to direct sunlight. Oxidising and reducing solutions, such as those of permanganate of potash, ferrous sulphate, iodine, hyposulphite of soda, &c., gradually weaken in strength; the solutions of other salts are more stable; while those of potassium bichromate and baric chloride are almost permanent. Solutions of potassium permanganate may be kept for a month or so without much change. The solutions of hyposulphite of soda and of iodine should be examined weekly. Ferrous sulphate solutions, if acidulated with sulphuric acid, may be depended on for two or three weeks without fresh standardising. Before filling the burette, the "Winchester" bottle should be well shaken and a portion of about 50 or 100 c.c. poured into a dry beaker or test-glass. Besides the standard solutions, which are required for titrating an assay, permanent solutions of the metal or acid of equivalent strength are very useful. When the finishing point of a titration has been overstepped (_i.e._, the assay has been "overdone"), a measured volume, say 5 or 10 c.c., of a solution containing the same metal may be added. The titration can then be continued, but more cautiously, and the value in "c.c." for the quantity added be deducted from the final reading. ~Standardising.~--Suppose the object is to standardise a solution of permanganate similar to that referred to above. A convenient quantity of iron (say 0.5 gram) would be weighed out, dissolved in dilute sulphuric acid, and the solution titrated. Suppose 49.6 c.c. of the permanganate solution are required, then 49.6 : 0.5 :: 100 : _x_ _x_ = 1.008 gram. This result, 1.008 gram, is the "_standard_." When a gas is measured, the standard may be calculated in the same way. For example: with 0.224 gram of zinc, 75.8 c.c. of gas were obtained. Then the quantity of zinc equivalent to 100 c.c. of the gas is got by the proportion. 75.8 : 0.224 :: 100 : _x_ _x_ = 0.2955 gram. Using the term "standard" in this sense, the following rules hold good:-- To find the weight of metal in a given substance:--_Multiply the standard by the number of c.c. used and divide by 100._ For example: a piece of zinc was dissolved and the gas evolved measured 73.9 c.c. Then by the rule, 0.2955×73.9/100 should give the weight of the piece of zinc. This gives 0.2184 gram. To find the percentage of metal in a given substance:--_Multiply the standard by the number of c.c. used and divide by the weight of substance taken._ For example: if 2 grams of a mineral were taken, and if on titrating with the permanganate solution (standard 1.008) 60.4 c.c. were required, then (1.008×60.4)/2 = 30.44. This is the percentage. If the standard is exactly 1 gram, and 1 gram of ore is always taken, these calculations become very simple. The "c.c." used give at once the percentage, or divided by 100 give the weight of metal.