Orthophosphoric acid has a pleasant acid taste and a distinctly acid reaction; it is used as a medicine, and is not poisonous (phosphorous acid is poisonous). Alkalis, like sodium, potassium, and ammonium hydroxides, saturate the acid properties of phosphoric acid when taken in the ratio 2NaHO : H_{3}PO_{4}--that is, when salts of the composition HNa_{2}PO_{4} are formed. When taken in the ratio NaHO : H_{3}PO_{4}, a solution having an acid reaction is obtained, and when 3NaHO : H_{3}PO_{4}--that is, when the salt Na_{3}PO_{4} is formed--an alkaline reaction is obtained. Hence many chemists (Berzelius) even regarded the salts of composition R_{2}HPO_{4} as normal, and considered phosphoric acid to be bibasic. But the salt Na_{2}HPO_{4} also shows a feeble alkaline reaction, so that it is impossible to judge the characteristic peculiarities of acids by the reactions on litmus paper, as we already know from many examples. Orthophosphoric acid is tribasic, because it contains three equivalents of hydrogen replaceable by metals, forming salts, such as NaH_{2}PO_{4}, Na_{2}HPO_{4}, and Na_{3}PO_{4}. It is also tribasic, because with silver nitrate its soluble salts always give Ag_{3}PO_{4},[15] a salt with three equivalents of silver, and because by double decomposition with barium chloride it forms a salt of the composition Ba_{3}(PO_{4})_{2}, and silver and barium hardly ever give basic salts. With the metals of the alkalis, phosphoric acid forms soluble salts, but the normal salts of the metals of the alkaline earths, R_{3}(PO_{4})_{2} and even R_{2}H_{2}(PO_{4}), are insoluble in water, but dissolve in feeble acids, such as phosphoric and acetic, because they then form soluble acid salts, especially RH_{4}(PO_{4})_{2}.[16] [15] Silver orthophosphate, Ag_{3}PO_{4}, is yellow, sp. gr. 7·32, and insoluble in water. When heated it fuses like silver chloride, and if kept fused for some length of time it gives a white pyrophosphate (the decomposition which causes this is not known). It is soluble in aqueous solutions of phosphoric, nitric, and even acetic acids, of ammonia, and many of its salts. If silver nitrate acts on a dimetallic orthophosphate--for instance, Na_{2}HPO_{4}--it still gives Ag_{3}PO_{4}, nitric acid being disengaged: Na_{2}HPO_{4} + 3AgNO_{3} = Ag_{3}PO_{4} + 2NaNO_{3} + HNO_{3}. When alcohol is added to silver orthophosphate, Ag_{3}PO_{4}, dissolved in syrupy phosphoric acid, it precipitates a white salt (the alcohol takes up the free phosphoric acid) having the composition Ag_{2}HPO_{4}, which is immediately decomposed by water into the normal salt and phosphoric acid.