The purpose of this study was to determine what (if any) effects are related to the amount of Oxalic Acid in the Ferric Oxalate (FO) sensitizer solution.

A low Oxalic Acid solution was needed for the base of this study.   The Vizcay "EDTA Solution" from the Comparison of FO Powders study was modified so as to provide a 30% FO solution.

8.00 grams Vizcay FO Powder (contains 0.15 gram Oxalic Acid & 7.85 grams FO)
1.05 gram EDTA (Na4)
H2O to make 26.2 ml when completely dissolved

The composition of this Base Solution contains concentrations of

30% ferric oxalate
4% EDTA (Na4)
0.587% oxalic acid

The metal solution formulas used with this 30% FO sensitizer can be determined using the Formula Calculator or the tables in Chapter 6, Optimized Formulas for Metal Solutions.  A coating mixture was mixed consisting of:

7 drops sensitizer (described above) (30 % solution)
5 drops K2PdCl4 solution (20.2 % solution, warmed)
2 drops K2PtCl4 solution (25.7 % solution, warmed)

The remaining Base Solution had oxalic acid added, another coating was produced, and this was repeated until the following coating mixtures were made containing the following concentrations of Oxalic Acid.  The pH of the sensitizer solution was measured, but not the coating mixture.  There were concerns that pH measurement of the relatively small amounts of coating mixture might alter the amount of the coating mixture as the probe can carry off several drops.

 

% concentration Oxalic Acid in sensitizer solution	pH (+- 0.1)
0.587	0.6
1	0.3
2	0.1
3	0.0
4	0.0
5	0.0
6	0.0
8	0.0
10	0.0

• Ambient conditions were temperature of 66^oF, relative humidity of 34-36%.
• The paper used was Crane's Parchment Business Card Stock (AKA: Cover-90; CP)
• Coatings were by brush into an area 5 inches by 5 inches (25 inches² or 161 cm²)
• All were dried by the dry method (see Drying the Coating).
• All were exposed with the same negative and a 21-step for 10.5 minutes under UV lamps.
• All were processed the same:
• developed in Potassium Oxalate
• 2 minutes water bath
• 30 minutes total clearing in H3PO4 (3 baths).

Buffered rinse and 10 minute wash.

Observations

At first look all the prints seemed identical.  Closer observation revealed that:

• As Oxalic Acid concentration increased the speed may decrease by about 0.125 stop from 0.587 to 10.  This is not a large change, but noticeable on the 21-step.
• Prints with Oxalic Acid concentrations above 2% seem to have better tonal discrimination and separation (perhaps from an edge effect) and better sharpness.
• Prints with Oxalic Acid concentrations above 5% seem to be fuzzier (perhaps from some increased graininess, increasing with 10 having the most).  The fuzziness offsets (6%) and then more than offsets (8% & 10%) any gain in sharpness.

Conclusions

It seems that an Oxalic Acid concentration from 2% to 5% in the sensitizer solution may be beneficial providing better sharpness and better tonal discrimination with higher concentrations progressively causing graininess.  The resulting effects are very slight.
 

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