This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Although citric acid is soluble in three-fourths of cold water, viz., one pound in twelve ounces, we propose for practical purposes to prepare a weaker solution, as a strong solution separates crystals in the cold. It is recommended to dissolve one pound of the acid in one pint of water, and it is also asserted that such a concentrated solution will keep better than a weaker one; however, we preferred to use two pints of boiling water to one pound of citric acid crystals. On pouring the boiling water over the acid in an earthenware, glass or porcelain pot, and stirring with a glass rod or wooden spatula, it will be quickly dissolved, and a fluid ounce of the solution will represent one-half ounce of citric acid crystals. This solution is filtered through whita filtering paper.
A solution of citric acid, if allowed to stand for a few days, will usually develop fungoid growth. The question of how to suppress it is of the greatest importance. Cleanliness is an important factor for preserving the citric acid solution. Clean and well-stoppered bottles, and distilled or boiled and filtered water only should be employed. Air germs must be carefully kept off, as they contaminate the solution. Dr. Eccles recommends the addition of some calomel (subchloride of mercury), and found that one part in ninety thousand is sufficient to preserve the solution and even stop the decomposition of infected solutions; one grain of calomel in one gallon and one half of citric acid solution will therefore be sufficient to protect against infection, and this quantity is so infinitesmal as to be entirely harmless.
Benzoic acid is another preservative for citric acid solutions; one part in two thousand is the minimum, less will fail. At this rate thirty-two grains are needed for every gallon. Where bottles can be kept closed until required, and made in small quantities, no preservative is needed. Corked bottles containing the solution, if raised to a temperature above 140° F. three times, or kept above this temperature for an hour,will keep indefinitely without change.