SOLUTIONS, MAKING UP The method of making up photographic solu tions should not be a haphazard one. The salts, in making up a stock solution, for example, must not be all placed in a dry bottle and the water added, for this conduces to slow and often incomplete solution. Another important point is the quantity of water used. In the majority of the formulae given in this work the quantity of solvent is, where possible, given so as to make a total bulk of either io or 20 oz., or r,000 ccs. Take any formula, such, for instance, as a de veloper containing Pyro . . . i oz. 5o g.
Sodium sulphite . . 4 „ 220 „ Citric acid . . . 1 „ 55 Distilled water to . . 20 „ 1,000 ccs.
If the solids were weighed out and 20 oz. or 1,000 ccs. of water added, the total bulk would be more than 20 oz. or i,000 ccs., and unless the total bulk were measured it would be impossible to determine the exact quantity of solution to use for a given weight of pyro. The correct method is to dissolve the solids in about three fourths of the total bulk of water, and then add sufficient water to make the given quantity. Again, in making developers, it is advisable to dissolve the preservative, the sulphite or meta bisulphite, first and then add the developing agent, except in the case of metol, which should always be dissolved first. When there are several salts given, it is usual in photographic formulae to dissolve them in the order in which they are given, and it is preferable always to use warm or hot water and dissolve each salt separately. In nearly all cases there is considerable lowering of the temperature when salts are dissolved, hence the value of hot water, for most salts are more soluble in hot than in cold water. As all solutions are of greater specific gravity or heavier than the solvents, it is not a good plan to place the salts in a bottle, fill up with water, and allow to stand, as the bottom of the liquid becomes a saturated solution, whilst the top may be nearly pure water. An excellent method is to use linen
or muslin bags containing the salts and suspend them from the top of the bottle or jug, when the solution of the salts rapidly sinks to the bottom, its place being taken by fresh water or weaker solution ; this method has the advantage, too, of straining out any dirt or foreign matter from the solution. The precautions given under the heading " Water " should also be noted.
In a io per cent. solution, io parts by measure contain i part of the dissolved substance. The rough-and-ready method of mixing together i oz. of the salt and 9 oz. of water does not make a true io per cent. solution. Ten fluid ounces contain 4,80o fluid grs., and should therefore contain 48o grs. of the salt. Chemicals are always sold by avoirdupois weight, 1 oz. of which contains 4371- grs., the apothecaries' ounce consisting of 48o grs. The former and lighter ounce therefore needs less water than the latter in order to make a true Io per cent. solu tion ; but, happily, the difference is not so much as to make a very great difference in the working powers of solutions. The correct way to make a So per cent. solution is to place the i oz. (avoirdupois, as bought) of chemical in a measure or bottle, and make up to a total bulk with water, of 9 oz. 55 mins. (9 oz. 1 drill. is near enough). Ten mins. of such a solution will con tain i gr. of the dissolved chemical.
Solutions are not always made up in the To per cent. strength. When the proportion in a formula is given as a percentage, the number of grains of solid per ounce of liquid can be obtained by multiplying the percentage figure by 4 and adding to the result its tenth part. Thus 5 per cent. = 5 x 4 = 20 ; .?5 X 20 = 2 ; 20 -I- 2 = 22 ; that is, 5 per cent. = 22 grs. per ounce. (approx.). The following table, compiled by C. C. Sherrard, shows at a glance the exact com position of "per cent. " and " part " solutions, made by simple multiplication :—