The reagents necessary for both quantitative and qualitative testing are as follows:—A standard solution of oxalic acid, of which 100 c.c. equal 1 grm., and a solution of ferrous phosphate, containing about 12·5 per cent. of Fe32PO4, with excess of phosphoric acid.
Into each of two Nessler graduated glasses 7·5 c.c. of the ferrous phosphate solution are run and made up to 50 c.c. with distilled water; both solutions should be colourless; 1, 2, or more c.c. of the solution to be tested are then run into one of the Nessler glasses; if oxalic acid be present, a more or less deep tint is produced; this must be imitated by running the standard solution of oxalic acid into the second Nessler cylinder—the calculation is the same as in other colorimetric estimations. It does not appear to be reliable quantitatively, if alum is present; and it is self-evident that the solution to be tested must be fairly free from colour.
§ 704. Oxalate of Lime in the Urine.—This well-known urinary sediment occurs chiefly as octahedra, but hour-glass, contracted or dumbbell-like bodies, compound octahedra, and small, flattened, bright discs, not unlike blood discs, are frequently seen. It may be usually identified under the field of the microscope by its insolubility in acetic acid, whilst the ammonio mag. phosphate, as well as the carbonate of lime, are both soluble in that acid. From urates it is distinguished by its insolubility in warm water. A chemical method of separation is as follows:—The deposit is freed by subsidence as much as possible from urine, washed with hot water, and then dissolved in hydrochloric acid and filtered; to the filtrate ammonia is added in excess. The precipitate may contain phosphates of iron, magnesia, lime, and oxalate of lime. On treatment of the precipitate by acetic acid, the phosphates of the alkaline earths (if present) dissolve; the insoluble portion will be either phosphate of iron, or oxalate of lime, or both. On igniting the residue in a platinum dish, any oxalate will be changed to carbonate, and the carbonate of lime may be titrated with d. n. HCl acid and cochineal solution, and from the data thus obtained the oxalate estimated. The iron can be tested qualitatively in the acid solution by ferrocyanide of potassium, or it can be determined by the ordinary methods. If the qualitative detection of oxalate of lime in the deposit is alone required, it is quite sufficient evidence should the portion insoluble in acetic acid, on ignition in a platinum dish, give a residue effervescing on the addition of an acid.
§ 705. Estimation of Oxalic Acid.—Oxalic acid is estimated in the free state by direct weighing, or by titration either with alkali or by potassic permanganate, the latter being standardised by oxalic acid. If (as is commonly the case) oxalic acid is precipitated as oxalate of lime, the oxalate may be—
(a) Dried at 100° and weighed directly, having the properties already described.
(b) Titrated with dilate sulphuric acid and permanganate.
(c) Ignited, and the resulting carbonate of lime weighed; or dissolved in standard acid and titrated back—one part of calcic carbonate corresponds to 1·26 part of crystallised oxalic acid, or 0·90 part of H2C2O4; similarly, 1 c.c. of standard acid equals ·05 of calcic carbonate (or ·063 of crystallised oxalic acid).