After cooling, the contents of the crucible are pulverized and heated, with vigorous stirring, to boiling, with one and a half times their weight of water. By this process the potassium chlorid which has formed during the first reaction is dissolved and is thus removed. The residual salt is washed with additional quantities of cold water and finally dried. To remove the potassium salt from the crude potassium perchlorate obtained as above, recourse is had to hydrofluosilicic acid. The reaction is represented by the following formula: 2KClO₄ + H₂SiF₆ = K₂SiF₆ + HClO₄. In order to effect this decomposition the potassium perchlorate is dissolved in seven times its weight of hot water and an excess of hydrofluosilicic added to the boiling solution. The boiling is continued for about an hour until particles of potassium perchlorate can no longer be detected with addition of water to compensate for evaporation.
On cooling the gelatinous potassium silicofluorid is deposited and the perchloric acid separated therefrom as completely as possible by decantation. The residue is again boiled with water and a little hydrofluosilicic acid and the clear liquor thus obtained added to the first lot. Finally, any residual perchloric acid may be removed on an asbestos felt under pressure. The clear liquid thus obtained is evaporated on a steam-bath to the greatest possible degree of concentration and allowed to stand in a cool place for twenty-four hours, whereby is effected the separation of any remaining potassium silicofluorid or potassium perchlorate. The residual liquid when filtered through an asbestos felt should give a perfectly clear filtrate. In order to throw out the last traces of hydrofluosilicic acid and any sulfuric acid present an equal volume of water is added, and while cold small quantities of barium chlorid are successively added until the barium salt is present in a very slight excess. The clear supernatant liquid is poured off after a few hours and evaporated until the hydrochloric acid is all expelled and white fumes of perchloric acid are noticed. Any potassium perchlorate still remaining will now be separated and, in the cold, sodium perchlorate will also be separated in crystals. The clear residue is again diluted with an equal volume of water and any barium salts present carefully removed with sulfuric acid. The mass is allowed to stand for one or two days, and is then filtered through paper and is ready for use. The purity of the acid obtained depends chiefly on the purity of the hydrofluosilicic acid at first used. Hence to get good results this acid must be free from foreign bodies. If an absolutely pure product be desired the acid above obtained must be distilled in a vacuum.
272. Method of Kreider.—Kreider has worked out a simpler method of preparing perchloric acid which will make it easy for every analyst to make and keep a supply of this admirable yet unappreciated reagent. This method is conducted as follows:[227]
A convenient quantity of sodium chlorate, from 100 to 300 grains, is melted in a glass retort or round-bottomed flask and gradually raised to a temperature at which oxygen is freely, but not too rapidly evolved, and kept at this temperature till the fused mass thickens throughout, indicating the complete conversion of the chlorate to the chlorid and perchlorate, which requires from one and one-half to two hours: or the retort may be connected with a gasometer and the end of the reaction determined by the volume of oxygen expelled, according to the equation
2NAClO₃ = NACl + NAClO₄ + O₂.
The product thus obtained is washed from the retort to a capacious evaporating dish where it is treated with sufficient hydrochloric acid to effect the complete reduction of the residual chlorate, which, if the ignition has been carefully conducted with well distributed heat, will be present in but small amount. It is then evaporated to dryness on the steam-bath, or more quickly over a direct flame, and with but little attention until a point near to dryness has been reached, when stirring will be found of great advantage in facilitating the volatilization of the remaining liquid and in breaking up the mass of salt. Otherwise the perchlorate seems to solidify with a certain amount of water and its removal from the dish, without moistening and reheating, is impossible.
After triturating the residue, easily accomplished in a porcelain mortar, an excess of the strongest hydrochloric acid is added to the dry salt, preferably in a tall beaker where there is less surface for the escape of hydrochloric acid and from which the acid can be decanted without disturbing the precipitated chlorid. If the salt has been reduced to a very fine powder, by stirring energetically for a minute, the hydrochloric acid will set free the perchloric acid and precipitate the sodium as chlorid, which in a few minutes settles, leaving a clear solution of the perchloric acid with the excess of hydrochloric acid. The clear supernatant liquid is then decanted upon a gooch, through which it may be rapidly drawn with the aid of suction, and the residue retreated with the strongest hydrochloric acid, settled, and again decanted, the salt being finally brought upon the filter where it is washed with a little strong hydrochloric acid. A large platinum cone will be found more convenient than the crucible, because of its greater capacity and filtering surface. When the filter will not hold all the sodium chlorid, the latter, after washing, may be removed by water or by mechanical means, with precautions not to disturb the felt, which is then ready for the remainder. Of course, if water is used, the felt had better be washed with a little strong hydrochloric acid before receiving another portion of the salt. This residue will be found to contain only an inconsiderable amount of perchlorate, when tested by first heating to expel the free acid and then treating the dry and powdered residue with ninety-seven per cent alcohol, which dissolves the perchlorate of sodium but has little soluble effect on the chlorid.
The filtrate, containing the perchloric acid with the excess of hydrochloric acid and the small per cent of sodium chlorid which is soluble in the latter, is then evaporated over the steam-bath till all hydrochloric acid is expelled and the heavy white fumes of perchloric acid appear, when it is ready for use in potassium determinations. Evidently the acid will not be chemically pure because the sodium chlorid is not absolutely insoluble in hydrochloric acid; but a portion tested with silver nitrate will prove that the sodium, together with any other bases which may have gone through the filter, has been completely converted into perchlorate, and unless the original chlorate contained some potassium or on evaporation the acid was exposed to the fumes of ammonia, the residue of the evaporation of a portion is easily and completely soluble in ninety-seven per cent alcohol and its presence is therefore unobjectionable. One cubic centimeter of the acid thus obtained gives on evaporation a residue of only 0.036 gram, which is completely soluble in ninety-seven per cent alcohol.
Caspari’s acid under similar treatment gave a residue in one case of 0.024 gram and in another 0.047 gram. If, however, a portion of pure acid be required, it may be obtained by distilling this product under diminished pressure and, as Caspari has shown, without great loss providing the heat is regulated according to the fumes in the distilling flask.
Some modification of the above treatment will be found necessary in case the sodium chlorate contains any potassium as an impurity, or if the latter has been introduced from the vessel in which the fusion was made. In these circumstances the hydrochloric acid would not suffice for the removal of potassium, since a trace might also go over with the sodium and thus on evaporation a residue insoluble in ninety-seven per cent alcohol be obtained. To avoid this difficulty, the mixture of sodium perchlorate and chlorid, after treating with hydrochloric acid for the reduction of the residual chlorate, being reduced to a fine powder, is well digested with ninety-seven per cent alcohol, which dissolves the sodium perchlorate but leaves the chlorid, as well as any potassium salt insoluble. By giving the alcohol time to become saturated, which was facilitated by stirring, it was found on filtering and evaporating that an average of about two-tenths of a gram of sodium perchlorate are obtained for every cubic centimeter of alcohol and that the product thus obtained is comparatively free of chlorids, until the perchlorate is nearly all removed, when more of the chlorid seems to dissolve. This treatment with alcohol is continued until on evaporation of a small portion of the latest filtrate, only a small residue is found. The alcoholic solution of the perchlorate is then distilled from a large flask until the perchlorate begins to crystallize, when the heat is removed and the contents quickly emptied into an evaporating dish, the same liquid being used to wash out the remaining portions of the salt. When the distillation is terminated at the point indicated, the distillate will contain most of the alcohol employed, but in a somewhat stronger solution, so that it requires only diluting to ninety-seven per cent to fit it for use in future preparations. The salt is then evaporated to dryness on the steam-bath and subsequently treated with strong hydrochloric acid for the separation of the perchloric acid.