| 234 | 168 | 136 |
| CaH4(PO4)2 + 2 NaHCO3 = CaHPO4 | ||
| 88 | 36 | 142 |
| + 2 CO2 + 2 H2O + Na2HPO4. | ||
Sodium phosphate, according to the United States Dispensatory, is "mildly purgative in doses of from 1 to 2 ounces." The claim is made by the makers of phosphate baking powders that the phosphates of sodium and calcium, products left after the baking, restore the phosphates which have been lost from the flour in the bran. This baking powder residue does not restore the phosphates in the same form in which they are present in grains and it does furnish them in larger amounts—nearly tenfold. However, the residue from these powders is probably less objectionable than that from alum powders. The chief drawback to the phosphate powders is their poor keeping qualities.
198. Alum Baking Powders.—Sulphuric acid is the acid constituent of these powders. The alums are double sulphates of aluminium and an alkali metal, and have the general formula xAl(SO4)2 in which x may be K, Na, or NH4, producing respectively a potash, soda, or ammonia alum. Potash alum is most commonly used, soda and ammonia alums to a less extent. The reaction takes place as follows:
| 475 | 504 | 157 |
| 2 NH4Al(SO4)2 + 6 NaHCO3 = Al2(OH)6 | ||
| 426 | 132 | 264 |
| + 3 Na2SO4 + (NH4)2SO4 + 6 CO2. | ||
If it is a potash or soda alum, simply substitute K or Na for NH4 throughout the equation. The best authorities regard alum baking powders as the most objectionable. Ammonia alum is without doubt the worst form, since all of the ammonium compounds have an extremely irritating effect on animal tissue. Sulphates of sodium and potassium are also objectionable. Aluminium hydroxide is soluble in the slightly acid gastric juice and has an astringent action on animal tissue, hindering digestion in a way similar to the alum itself. Many of the alum powders contain also mono-calcium phosphate; the reaction is as follows:
| 475 | 234 | 336 |
| 2 NH4Al(SO4)2 + CaH4(PO4)2 + 4 NaHCO3 | ||
| 245 | 136 | 132 |
| = Al2(PO4)2 + CaSO4 + (NH4)2SO4 | ||
| 284 | 176 | 72 |
| + 2 Na2SO4 + 4 CO2 + 4 H2O. | ||
These are probably less injurious than the straight alum powders, although the residues are, in general, open to the same objection.
199. Inspection of Baking Powders.—Many of the states have enacted laws seeking to regulate the sale of alum baking powders. Some of these laws simply require the packages to bear a label setting forth the fact that alum is one of the ingredients; others require the baking powder packages to bear a label naming all the ingredients of the powder.
200. Fillers.—All baking powders contain a filler of starch. This is necessary to keep the materials from acting before the powder is used. The amount of filler varies from 15 to 50 per cent; the least is found in the tartrate powders and the most in the phosphate powders. The amount of gas which a powder gives off regulates its value; it should give off at least ⅛ of its weight.
201. Home-made Baking Powders.—Baking powders can be made at home for about one half what they usually cost and they will give equal satisfaction. The following will make a long-keeping powder: cream of tartar, 8 ounces; baking soda, 4 ounces; corn starch, 3 ounces. For a quick-acting powder use but one ounce of starch. The materials should be thoroughly dry. Mix the soda and starch first by shaking well in a glass or tin can. Add the cream of tartar last and shake again. Thorough mixing is essential to good results. Cream of tartar is often adulterated, but it can be obtained pure from a reliable druggist. To insure baking powders remaining perfectly dry, they should always be kept in glass or tin cans, never in paper.