PRUSSIATES, in chemistry, salts formed with the prussic acid. These salts have not been attentively examined, on account of their want of permanency, unless they are united with some metallic oxide ; but the prussiate of potash and iron, which is a triple salt, has been used by chemists as the best combination of prussic acid for detecting the existence of iron. In chemistry and mineralogy this is a very important substance, as it is capable of indicating whether most me tallic substances be present in any solu tion whatever, and of pointing out the particular metal, and of ascertaining its quantity. This is done by precipitating the metals from their solution, in conse quence of the insoluble compound which it forms with them. The colour of the precipitate indicates the particular metal, while its quantity enables us to judge of the proportion of metallic oxide con tained in any solution. This salt has obtained, at different times, the names of Prussian alkali, phlogisticated alkali, Prussian test, &c. This salt, though of great importance as a test, is of no use whatever, if it be not quite pure. There are two ways in which this test mdy be rendered impure, besides the introduc tion of foreign ingredients, which it is needless to mention, because it is ob vious that it must be guarded against.
1. There may be a superabundance of alkali present, or, which is the same thing, there may be mixed with the Prussian test a quantity of pure alkali ; or, 2. There may be contained in it a quantity of prussiate of iron, for which prussiate of potash has also a con siderable affinity. If the Prussian test
contain a superabundance of alkali, two inconveniences follow. This superabund ant quantity will precipitate those earthy salts which are liable to contain an ex cess of acid, and which are only soluble by that excess. Hence alumina and bary tes will be precipitated. Another in convenience arising from the supera bundance of alkali in the Prussian test is, that it gradually decomposes the blue prussiate which the test contains, and converts it into a yellow prussiate. In what manner it does this will be under stood, after what has been said, without any explanation. On the other hand, when the Prussian alkali contains a quantity of yellow prussiate of iron, as great inconveniences follow. This yel low prussiate has an affinity for prussic acid, which, though interior to that of the potash, is still considerable ; and, on the other hand, the potash has a stronger affinity for every other acid than for the prussic. When, therefore, the test is exposed to the air, the carbonic acid, which the atmosphere always contains, assisted by the affinity between the yel low prussiate and the prussic acid, de composes the prussiate of potash in the test, and the yellow prussiate is precipi tate.d in the form of Prussian blue, and every other acid produces the same ef fect. A test of this kind would indicate the presence of iron in every mixture which contains an acid (for a precipita tion of Prussian blue would appear), and could not therefore be employed with any confidence.