Fig. 16.
While of course it is most important, and for accurate work essential, to have as good a balance as possible, much may be done in technical work, even with a good pair of druggists' scales; and most standard solutions may be bought ready made; while from two or three accurately adjusted solutions many others may be made volumetrically.
Preparation of Standard Acid and Alkaline Solutions.—In practice it is very difficult to obtain perfectly pure caustic soda, free from water and carbonic acid, both of which are greedily absorbed by it from the air, so that a standard solution cannot practically be made by directly weighing out the substance as suggested in the introductory paragraph. In sodic carbonate, however, we have a substance which is easily obtained pure and dry, and which may be used for almost all the purposes to which a caustic solution could be applied. A decinormal solution is strong enough for most of the work in a tannery, though it is a convenience to have both normal and decinormal, and a stock of the stronger solution will last a longer time and is readily diluted to decinormal strength by adding 1 part to 9 parts of distilled water. To make a normal solution, about 60 grm. of the purest sodic carbonate are placed in a porcelain basin or platinum crucible and heated over a Bunsen gas-burner or spirit-lamp, nearly to redness, and allowed to cool closely covered up. Of the salt thus dried 53 grm. are accurately weighed into a beaker and dissolved in distilled water. The solution is then poured into a gauged litre flask, and carefully filled up with water at a temperature of 59° F. (15° C.) to the mark on the neck. The whole is then poured into a good-sized stoppered bottle (40 oz.) and vigorously shaken for 5-10 minutes. This thorough shaking is important with all standard solutions, and without experience no one would believe how much shaking is required uniformly to mix a solution. Probably more difficulty to beginners in analysis arises from neglect of this matter than from any other cause. To make a decinormal solution, proceed in precisely the same way, using 5·3 grm. instead of 53; or dilute as above.
Standard Acid Solution.—For this purpose any one of several acids may be used, each of which has its special advantages.
Oxalic acid is the easiest to make of any. A sufficient quantity of pure crystallised oxalic acid is powdered and pressed between filter paper, so as to absorb the moisture which occasionally is retained in cavities of the crystals. 6·3 grm. is then weighed out and dissolved in water, exactly as was done with sodic carbonate, forming a decinormal solution. It is used in Löwenthal's tannin estimation process and may also be employed to determine alkalies, but forms insoluble calcium oxalate with lime salts, and does not give a sharp reaction with methyl orange indicator. Hence litmus must be used, or a few drops of a neutral solution of calcium chloride added to the methyl orange, when hydrochloric acid will be liberated as soon as there is excess of the acid, and the indicator will be promptly reddened. Sulphuric acid is the most permanent of any acid solution, and may be generally employed. It forms insoluble sulphates with lime, baryta, and strontia. To make a normal solution, 35 c.c. of the pure concentrated acid are poured into at least 3 or 4 times as much distilled water, and allowed to cool, and are then made up to about 1 litre and well shaken. The burette is filled with the mixture, 10 c.c. of the standard sodic carbonate are measured into a beaker, 2 or 3 drops of methyl orange solution are added, and the acid is run in with constant stirring till the indicator is just beginning to redden. This must be repeated, and the two titrations should exactly agree. Suppose that 9.5 c.c. are required, then 950 c.c. of the trial acid are equal to 1 litre of the soda. If therefore 950 c.c. be measured into a test mixer, and made up to 1 litre, the solution should be accurately decinormal. Of course great care must be used in the whole process. If a gauged flask only is at hand it will be easier to measure into it the water required to make up the litre, and then fill to the mark with the trial acid. Normal hydrochloric acid may be made exactly as described for sulphuric acid, but using about 100 c.c. of the strongest acid. Decinormal solutions of both these acids may be made by the same methods; using 1 tenth the quantities, or by dilution of the normal solution.
Beside comparison with sodic carbonate solution, hydrochloric acid may also be checked by determining the amount of chlorine present, with silver nitrate (see [p. 98]) 10 c.c. of decinormal acid should of course be equal to 10 c.c. of decinormal silver nitrate.
Table giving the Quantity of the Following Substances contained in or equivalent to 1 litre of Normal or 10 litres of Decinormal Standard Solution.
| Sulphuric acid | 49 | grm. | SO4H2 = 40 grm. SO3 | |
| Hydrochloric acid | 36·5 | " | ClH = 35·5 grm. Cl. | |
| [G] | Oxalic acid | 63·0 | " | C2O4H2 + 2 Aq. |
| Acetic " | 60·0 | " | C2H3O2H. | |
| Soda | 40·0 | " | NaHO. | |
| Sodic carbonate | 53·0 | " | Na2CO3. | |
| [G] | Lime | 28·0 | " | CaO = 37·0 grm. CaH2O2. |
| [G] | Calcic carbonate | 50·0 | " | CaCO3. |
| Ammonia | 17·0 | " | NH3. | |
| [G] | Barium hydrate | 76·5 | " | BaO = 85·5 grm. BaH2O2. |
| Barium chloride | 104·0 | " | BaCl2. | |
| Zinc chloride or sulphate | 32·6 | " | Zn = 16·0 grm. S. as sulphide. | |
| Silver nitrate | 170·0 | " | AgNO3 = 35·5 grm. Cl. | |
| Potassic permanganate | 31·6 | " | K2MnO4. |