An in the receiver with potash, separate the salt by crystallization, Class I. Div. II. The taste of pyrotartaric acid is extremely acid. When Properties. heated it melts, and sublimes in a white smoke, without leaving any residue. It dissolves readily in water, and crystallizes again when subjected to spontaneous evaporation. It does not precipitate acetate of lead nor nitrate of silver; but it precipitates nitrate of mercury. Some time after it has been mixed with acetate of lead, needleform crystals make their appearance. When combined in excess with potash, no salt similar to tartar is formed. In this state it instantly forms a precipitate when dropped into acetate of lead. Pyrotartrate of potash is soluble in alcohol. It does not precipitate the salts of barytes and lime, as tartrate of potash does.* SECTION VII.-OF CITRIC ACID. Chemists have always considered the juice of oranges and History. lemons as an acid. This juice contains a quantity of mucilage and water, which renders the acid impure, and subject to spontaneous decomposition. Mr. Georgi took the following method to separate the mucilage: He filled a bottle entirely with lemon-juice, corked it, and placed it in a cellar: in four years the liquid was become as limpid as water, a quantity of mucilage had fallen to the bottom in the form of flakes, and a thick crust had formed under the cork. He exposed this acid to a cold of 23°, which froze a great part of the water, and left behind a strong and pretty pure acid. It was Scheele, how-ever, that first pointed out a method of obtaining this acid perfectly pure, and who demonstrated that it possesses peculiar properties. His process, which is still followed, is this: Saturate lemon juice, while boiling, with powdered chalk. A white powder falls to the bottom, which is lime combined with citric acid. Separate this powder, and wash it with warm water till the * Ann. de Chim. Ixiv. 42. + Kongl. Vetenskaps Academiens Handlingar for 1774, p. 245 Chap. I. Properties. water passes off colourless. Then pour upon it as much sulphuric acid as will saturate the chalk employed, having previously diluted it with six times its weight of water. Boil the mixture for some minutes, and pass it through a filter to separate the sulphate of lime. The liquid is then to be evaporated to the consistence of a syrup, and set aside to cool. A number of crystals form in it: these are citric acid.* Mr. Scheele advises the use of an excess of sulphuric acid in order to ensure the separation of all the lime; but according to Dizé, this excess is necessary for another purpose.+ A quantity of mucilage, he thinks, still adheres to the citric acid in its combination with lime, and sulphuric acid is necessary to decompose this mucilage. Proust, who published a memoir on the preparation of citric acid in the Journal de Physique for 1801, has shown that when too much sulphuric acid is employed, it acts upon the citric acid, chars it, and prevents it from crystallizing. This error is remedied by adding a little chalk. This chemist found, that four parts of chalk require for saturation 94 parts of lemon juice. The citrate of lime obtained amounted to 7 parts. To decompose this, 20 parts of sulphuric acid, of the specific gravity 1·15, were necessary. The crystals of this acid are of a very irregular figure. They are prisms of 6, 8, or 10 sides, terminated at either extremity by an indefinite number of faces. Mr. Brooke has shown that the primary form is a right rhombic prism, whose faces make angles of 101° 30'.§ They are not altered by exposure to the air. Their specific gravity is 1.617. Their taste is exceedingly acid, and even excites pain; but when the acid is properly diluted with water, the acidity is pleasant. These crystals contain combined water; and, therefore, are in the state of a hydrate. According to the analysis of Berzelius,|| they are composed of This water cannot be driven off by heat. When their temperature is raised they melt and then become yellow or brown, and form a thick syrup which cannot be crystallized. 100 parts of the acid, when kept at such a temperature as to be reduced to the state of a white powder without undergoing decomposition, lose about 7 per cent. of water. This approaches to the third part of the whole water contained in the acid. Indeed I have determined by actual experiment, that the loss of weight is just one-third of the water.* Citric acid is exceedingly soluble in water. According to Vauquelin, 100 parts of it require only 75 parts of water. Boiling water dissolves twice its weight of it.+ This solution may be kept a long time in close vessels; however, it at last putrefies, and is decomposed. Neither oxygen gas nor the simple combustibles or incombustibles produce any effect upon it. It is capable of oxidizing iron, zinc, tin. It does not act upon gold, silver, platinum, mercury, bismuth, antimony, arsenic. It combines with alkalies, earths, and metallic oxides, and forms salts known by the name of citrates. Sulphuric acid, The action of none of the acids on it has been examined, if we except that of the sulphuric and nitric. when concentrated, converts it into acetic acid. that he could not convert it into oxalic acid by means of nitric acid, as he had done several other acids: but Westrumb affirms, that this conversion may be effected; and thinks that Scheele had probably failed from having used too large a quantity of nitric acid, by which he had proceeded beyond the conversion into oxalic acid, and had changed the citric acid into vinegar; and in support of his opinion, he quotes his own experiments; from which it appeared that, by treating 60 grains of citric acid with different quantities of nitric acid, his products were very different. Thus with 200 grains of nitric acid he got 30 grains of oxalic acid; with 300 grains of nitric acid he obtained only 15 grains of the oxalic acid; and with 600 grains of nitric acid no vestige appeared of the oxalic acid. On distilling the products of these experiments, especially of the last, he obtained vinegar mixed with nitric acid. The experiments of Westrumb have been confirmed by Fourcroy and Vauquelin; who, by treating citric acid with a great quantity of nitric acid, con+ Dizé. Fourcroy, vii. 206. * First Principles, ii. 122. Class I. Div. II. Chap. I Constituents. The proportion of the verted it into oxalic and acetic acids. According to the analysis of Berzelius (corrected a little by theoretic views), citric acid is composed of History, Properties. Now as these atoms give us the true atomic weight of citric acid, there can be no doubt that the result just given represents the true atomic composition of this acid. My own analytical results, so far as they go, coincide with those of Berzelius.† SECTION VIII.—OF PYROCITRIC ACID. This acid was discovered by M. Lassaigne,‡ to whom we are indebted for all the facts respecting it hitherto ascertained. When citric acid is distilled in a glass retort it fuses, water passes over, it then becomes yellow, and the colour deepens more and more, while a white vapour passes over and condenses in the receiver. The product of the distillation is a colourless liquid swimming uppermost, and an amber-coloured oily liquid at the bottom of the receiver. The watery liquid has an acid taste, and the amber liquid contains a good deal of acid which it gives out to water. To obtain the acid contained in the liquid by saturating it with lime, and evaporating the liquid, a solid salt was obtained, which dissolved in water, and gave no smell of acetic acid when treated with sulphuric acid. Two methods of separating the pyrocitric acid from the lime were taken: 1. The lime was thrown down by oxalic acid; 2. The pyrocitrate of lime was decomposed by acetate of lead. An insoluble precipitate of pyrocitrate of lead fell, which was washed and decomposed by sulphuretted hydrogen gas. Pyrocitric acid thus obtained is white, without smell, having * Ann. de Chim, et de Phys. xxi. 100. an acid and slightly bitterish taste, and not easily reduced to regular crystals, and assuming usually the state of a white mass, composed of fine needles interlaced together. When thrown upon a hot body it melts, and is converted into white acrid vapours, leaving a trace of coal. Distilled in a retort it gives off a yellowish sour-tasted oil, and is partly decomposed. It is very soluble in alcohol; and water at the temperature of 50°, dissolves the third of its weight of it. Its aqueous solution reddens litmus paper. It neither precipitates lime-water, nor barytes-water, nor any of the metallic solutions tried, except acetate of lead and nitrate of mercury. With the different bases it forms salts, which have received the name of pyrocitrates. Class I. Div. II. weight, Lassaigne assures us that its saturating power is precisely Atomic that of citric acid. But from the different analyses of the pyrocitrates which he has given, it is difficult to come to any conclusion. The pyrocitrate of barytes he found composed of Pyrocitric acid The pyrocitrate of lead of Pyrocitric acid Oxide of lead While the pyrocitrate of lime was composed of Pyrocitric acid Lime 6.8 3.5* These three salts give us 7.43, 7·01, 6-8, for the atomic weight of tartaric acid; the mean of which is 7.08, which is below the atomic weight of citric acid. If we were to leave out the analysis of pyrocitrate of lime as uncertain, the mean of the other two would give us 7-22 for the atomic weight of this acid, a result very near that of citric acid. But Lassaigne analyzed it by heating pyrocitrate of lead with black oxide of copper, and obtained for its constituents, * In Lassaigne's paper the salt is said to be composed of But I suppose the numbers to have been inverted by the printer. |