parts the acid properties, and it is
prepared from crude argol, a deposit of grape juice when wine is made.
The residue from this powder is sodium potassium tartrate,
NaKC_{4}H_{4}O_{6}, commonly known as Rochelle salt. This is the active
ingredient of Seidlitz powders and has a purgative effect when taken
into the body. The dose as a purgative is from one half to one ounce. A
loaf of bread as ordinarily made with cream of tartar powder contains
about 160 grains of Rochelle salt, which is 45 grains more than is found
in a Seidlitz powder, but the amount actually eaten at any one time is
small and its physiological effect can probably be disregarded. When a
cream of tartar baking powder is used, the reaction takes place
according to the following equation:

188 84 210 44 18
HKH_{4}C_{4}O_{6} + NaHCO_{3} = KNaC_{4}H_{4}O_{6} + CO_{2} + H_{2}O.

The crystallized Rochelle salt contains four molecules of water, so
that, even allowing for some starch filler, there is very nearly as much
weight of material (Rochelle salt) left in the food as there was of the
original powder. If free tartaric acid were used instead of potassium
acid tartrate, the reaction would be as follows:

150 168 230 88
H_{2}C_{4}H_{4}O{6} + 2NaHCO_{3} = Na_{2}C_{2}H_{4}O_{6}.2 H_{2}O + 2CO_{2}.

But the residue, sodium tartrate, is less in proportion. It has
physiological properties very similar to Rochelle salt. Tartaric acid is
seldom used alone, but very often in combination with cream of tartar.
It is more expensive than cream of tartar; but not so much is required,
and it is more rapid in action.

197. Phosphate Baking Powders.--Here the acid ingredient is phosphoric
acid and the compound usually employed is mono-calcium phosphate,
CaH_{4}(PO-{4})_{2}. This is made by the action of sulphuric acid on
ground bone (Ca_{3}(PO_{4})_{2} + 2 H_{2}SO_{4} = CaH_{4}(PO_{4})_{2} +
2 CaSO_{4}), and it is difficult to free it from the calcium phosphate
formed at the same time; hence such powders contain more or less of this
inert material. The reaction which occurs with a phosphate powder is as
follows:

234 168 136
CaH_{4}(PO_{4})_{2} + 2 NaHCO_{3} = CaHPO_{4}

88 36 142
+ 2 CO_{2} + 2 H_{2}O + Na_{2}HPO_{4}.

Sodium phosphate, according to the United States Dispensator