96 PHOTOGRAPHIC FACTS AND FORMULAS 
pared by mixing ferrous sulphate with oxalic acid or a soluble 
oxalate, as follows: 
Ferrous sulphate 278 parts 
Water 500 parts 
Dissolve and add: 
Potassium oxalate, neutral 184 parts 
Water 500 parts 
Mix the two solutions, filter and wash the precipitate with 
four or five lots of distilled water, and dry. The result 
should be 180 parts of ferrous oxalate. Oxalic acid 126 parts 
may be used instead of the potassium salt. Ferrous oxalate 
is soluble in excess of neutral potassium oxalate, or other 
alkaline oxalate. It is obvious that the developer may be pre- 
pared in two ways: either by dissolving the dry salt in oxalate 
solution, or by adding ferrous sulphate solution to excess of 
an alkaline oxalate, when it will remain in solution. Potas- 
sium oxalate is used in preference to the corresponding 
ammonium and sodium oxalates, because it is more soluble. 
Consequently, it will dissolve more of the ferrous oxalate, 
which is the active agent in development. The strongest 
developer can be made as follows: 
Potassium oxalate 500 g 8 oz. 
Water 1000 ccm 16 oz. 
Boil and add: 
Ferrous oxalate, dry 100 g 1 3/5 oz. 
Allow to cool slightly and immediately bottle. This practi- 
cally forms a saturated solution of ferrous oxalate. This 
method is not so convenient as using separate solutions, for 
which the following stock solutions are required: 
A. Ferrous sulphate 330 g 5% oz. 
Water 1000 ccm 16 oz. 
Sulphuric acid l ccm 7 min. 
