The standard ferric oxalate solution is also found in commerce. [pg 73] Treated by potassium ferricyanate it should not be colored blue, nor become turpid when diluted with one-tenth part of water and boiled. The former reaction indicates that it contains no ferrous salt, and the latter no basic oxalate.
The authors give the following instructions for preparing the ferric oxalate solution, to which they attach much importance:
Five hundred parts of ferric chloride are dissolved in 5,000 parts of water and heated to boiling, when a solution of soda is added until the liquid becomes alkaline.[19] About 250 parts of caustic soda are generally employed for this purpose. The precipitate—ferric oxide—is now washed in warm water until the last washing water is quite neutral to test paper, then drained and mixed with 200 parts of pure crystallized oxalic acid. The mixture is then allowed to stand in the dark for several days at a temperature not exceeding 30 deg. C. (86 deg. Fahr.) At first the solution from green turns to a yellow green, and finally becomes almost brown. At this moment the excess of ferric oxide is filtered out and the liquor submitted to a quantitative analysis, the result of which leads to ascertain the quantity of ferric oxalate in 100 parts of the solution and the excess of oxalic acid. The solution should then be diluted with distilled water, such as it contains 20 parts of ferric oxalate per 100 parts of water, and oxalic acid must be added in the proportion of from 6 to 8 per 100 of the ferric oxalate, taking into account the quantity of acid the solution already contains. The solution should be kept in the dark. It is altered by light.[20]
| IRON CHLORATE SOLUTION | |
|---|---|
| Ferric oxalate solution | 100 parts |
| Potassium chlorate | 0.4 parts |
This solution is employed to obtain more contrasts.
| PREPARATION OF THE SENSITIZING SOLUTION | |
|---|---|
| Platinum solution | 12 parts |
| Ferric oxalate solution | 11 parts |
| Distilled water | 2 parts |
This solution gives very soft tones with intense black. To obtain more brilliancy we use the following proportions:
| Platinum solution | 12 parts |
| Ferric oxalate solution | 9 parts |
| Chlorate of iron solution | 3 parts |
| Distilled water | 2 parts |
To obtain results comparable to those which the silver printing out process gives, the following mixture is employed:
| Platinum solution | 12 parts |
| Ferric oxalate solution | 8 parts |
| Chlorate of iron solution | 4 parts |
| Distilled water | 8 parts |