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Ferrous salts may be promptly and completely oxidized to ferric salts, even in cold solution, by the addition of potassium bichromate, provided sufficient acid is present to hold in solution the ferric and chromic compounds which are formed. The acid may be either hydrochloric or sulphuric, but the former is usually preferred, since it is by far the best solvent for iron and its compounds.
It is, however, possible to obtain a satisfactory, although somewhat fugitive end-point in the presence of manganous sulphate and phosphoric acid. The explanation of the part played by these reagents is somewhat obscure as yet. It is possible that an intermediate manganic compound is formed which reacts rapidly with the ferrous compounds thus in effect catalyzing the oxidizing process.
The latter object is accomplished by oxidation to stannic chloride by means of mercuric chloride added in excess, as the mercuric salts have no effect upon ferrous iron or the bichromate. The reactions involved are: 2FeCl + SnCl > 2FeCl + SnCl SnCl + 2HgCl > SnCl + 2HgCl The mercurous chloride is precipitated.
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.
The two solutions to be employed in the process under consideration are those of potassium bichromate and ferrous sulphate. As this 48 grams is six times 8 grams, the basis of the system, the normal solution of potassium bichromate should contain per liter one sixth of 294.2 grams or 49.03 grams.
As in the case of the bichromate process, it is necessary to reduce the iron completely to the ferrous condition before titration. The reducing agents available are zinc, sulphurous acid, or sulphureted hydrogen. Stannous chloride may also be used when the titration is made in the presence of hydrochloric acid.
In fact, diny teeth, being organic, seemed to be an especially hard form of boron carbide. Dinies could chew iron. They could masticate steel. They could grind up and swallow anything but tool-steel reinforced with diamond chips. The same amateur chemist worked it out that the surface soil of the planet Eire was deficient in iron and ferrous compounds. The dinies needed iron. They got it.
"Then you have no prejudices as to which is the best developer, pyrogallic acid or ferrous oxalate, not to mention such recent decoctions as eikonogen, quinol, and others?" No, the boy had none. "Well, that's what I like. I like a young man whose mind is open to conviction." The boy was not a conversational success. He evidently did not enter into the spirit of the artist's remarks.
Then run in the permanganate solution from a pipette or burette to determine roughly the amount required. Strong sulphuric acid also decomposes the oxalic acid. The excess of ferrous iron may also be determined by titration with potassium bichromate, if desired. The chlorine generated by the pyrolusite is passed into a solution of potassium iodide.
Those in most common use are iron wire, ferrous ammonium sulphate, sodium oxalate, oxalic acid, and some other derivatives of oxalic acid. With the exception of sodium oxalate, these all contain water of crystallization which may be lost on standing. They should, therefore, be freshly prepared, and with great care.
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