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.
One important factor in purchasing commercial flavorings is their strength, and another one whether they are miscible with aqueous liquids or not. To ascertain their strength and solubility we apply very simple methods.
Proceed as follows: Into a clean half-pint bottle introduce one ounce of syrup, previously acidified and mixed with about 15 minims or grains of citric acid solution. Use a small pipette graduated at 5 or 10 minims (Fig. 420), and add the acid solution with this pipette. Clean this one or take another one and measure 10 minims of the extract or essence into the bottle, which quantity corresponds to about 2 ounces of extract, or essence, per gallon of syrup. Then fill up the bottle at the machine with carbonated water. Now taste and compare the liquid. By testing, its strength can be estimated; by comparing, the solubility is established by a bright appearance.