MICRO-CHEMICAL TESTS.
Oxalic acid is present in the urine in minute quantities in combination with potash, soda, and lime, two centigrams being passed in twenty-four hours (Fiirbringer). When in excess a crystal line deposit of calcium oxalate is precipitated.
Microscopically the crystals are of two kinds, the more com mon being the distinctive and unmistakable, strongly refracting octa hedral crystals (shaped like envelopes). Much less frequently the crystals assume a dumb-bell shape, which is merely an oval or circu lar disc with rounded margins, and a depression in the centre on either face. These are said by Ord to result from slow precipitation in the presence of colloid matter. The crystals are insoluble in vegeta ble acids, alcohol, ether, or water, but they dissolve readily in min eral acids. Oxalic acid is often associated with uric acid and urates (one-third of the cases—Bird) ; urea is also present in greater propor tion than natural (30 per cent. of the cases—Bird). More rarely an excess of phosphates is present.
To estimate the amount of oxalic acid in the urine—for oxaluria as a morbid state cannot be measured by the microscope.alone, since a large proportion of oxalic acid may be in solution—the following method is advised: Quantitative _Estimation of Oxalic Acid (Neubauer' s Method).
The urine passed during twenty-four hours is accurately measured and treated first with calcium chloride and ammonia, then with acetic acid until it has a slightly acid reaction, and afterward a little alcoholic solution of thymol is added to restrain the development of micro organisms. The mixture is allowed to stand for some time, when the white precipitate which forms is separated on a filter, and (to gether with the latter) is placed in hydrochloric acid, gently heated, the fluid filtered off, and the filter washed with water until it no longer has an acid reaction. The collected filtrate is evaporated to a small
bulk in a capsule on the water-bath, then placed in a strong glass cylinder, and the capsule in which it was evaporated is washed with dilute hydrochloric acid and water, the washings being added to the fluid in the cylinder. Ammonia solution is then poured upon the surface of the latter, and the whole is tinted with a few drops of tinc ture of litmus. The mixture is allowed to stand for a considerable time. The precipitate which has formed is obtained on a so-called ash-free filter, the ash constituent of which has been accurately as certained, and the oxalate of lime which adheres to the walls of the cylinder is removed on a glass rod guarded with an india-rubber ring, and is added to the precipitate on the filter. The latter is next freed from chlorine by washing with water and rinsing with acetic acid. The filter is then dried and ignited on a platinum crucible, which is heated to a constant degree in a blowpipe flame. By this means oxalate of lime is changed into lime. Now as 56 parts of lime correspond to 90 parts of oxalic acid, the quantity of the former ob tained when multiplied by 1.6071 shows the quantity of oxalic acid in the urine taken. (Extracted from v. Jaksch, " Clinical Diagnosis," translated by James Cagney.)