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Updated: May 28, 2025
The copper may be very easily and rapidly separated from the double ammonium oxalate salt, provided a sufficient excess of ammonium oxalate is present. Weak currents cannot be employed for the determination of this element when it is present in large quantities, for under such circumstances the metal does not adhere with sufficient firmness to the electrode.
If the manganese is converted into the soluble double salt, prepared by adding an excess of potassium, and submitted to the electric current, the whole of the manganese will be deposited at the positive electrode. When ammonium oxalate is used, the complete precipitation does not take place.
Boil for two minutes; allow the precipitated calcium oxalate to settle for a half-hour, and decant through a filter. Test the filtrate for complete precipitation by adding a few cubic centimeters of the precipitant, allowing it to stand for fifteen minutes. Redissolve the calcium oxalate in the beaker with warm hydrochloric acid, pouring the acid through the filter.
Calcium oxalate is nearly insoluble in water, and only very slightly soluble in acetic acid, but is readily dissolved by the strong mineral acids. The reactions involved are CaC O > CaCO + CO, CaCO + H SO > CaSO + H O + CO . Dilute the solution cautiously until the salts are brought into solution, adding a little acid if the solution has evaporated to very small volume.
If a solution of ferric oxide and manganese ammonium oxalate is submitted to electrolysis, without the previous addition of ammonium oxalate, the characteristic color of permanganic acid immediately makes its appearance, and the peroxide gradually precipitates itself on the positive, while the iron is deposited on the negative electrode.
Any magnesia present in the filtrate of the oxalate of lime is by the addition of a solution of phosphate of soda separated as phosphate of ammonia and magnesia, after having stood twenty-four hours. The precipitate is filtered off, washed with water to which a little chloride of ammonia is added, dried, and after calcining the fiber and adding the filter ash, glow heated in the crucible.
When the reduction has been completed, the supernatant liquid is poured off, and the residue washed in the ordinary manner. Arsenic cannot be completely separated from either its aqueous hydrochloric acid, or from a solution to which ammonium oxalate has been added in excess.
This advantage becomes particularly apparent with emulsions prepared with ammonia, which frequently show with ammoniacal developer green or red fog, or a fog of clayish color by reflected, and of pale purple by transmitted light. Ferrous oxalate works quite well with plates of that kind; so do soda and potassa developers.
Tin may be easily estimated by electrolysis; it can be separated from its hydrochloric acid solution, or from its double salt with ammonium oxalate, as a beautiful silver gray coating on the platinum. When the ammonium oxalate is substituted by the potassium salt, the operation becomes more difficult, as a basic salt is formed at the opposite pole, and is not easily reduced.
When the duplicate portion of 100 cc. is measured out from the solution, remember that the flask must be rinsed out twice with that solution, as prescribed above, before the measurement is made. Dilute each of the measured portions to 250 cc. with distilled water, heat the whole to boiling, and add ammonium oxalate solution slowly in moderate excess, stirring well.
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