In order to achieve accuracy and consistency, it is important to begin with an accurately formulated sensitizer to which the metal solution will be balanced.  The active Ferric Oxalate content needs to be determined.  The Comparison of FO Powders Study demonstrated that FO Powders do not all contain the same amount of or 100% ferric oxalate.  That study estimated the contents of some FO powders.  The purpose of this study is to verify or develop a better estimate for the ferric oxalate composition of those powders.

 

FO Powder	% ferric oxalate by weight, estimated (from Comparison of FO Powders)	% oxalic acid or other material by weight, estimated
Bostic & Sullivan	89.2	10.8
Vizcay	98.08	1.92

Dick Stevens, in his book "Making Kallitypes", presents a method to relate the percent concentration of Ferric Oxalate (FO) solutions with their specific gravity.  From this method, the following formula is derived.

%  = 157.89474 * (SG-1)

where:   % = percent concentration of FO solution
            SG = specific gravity

Expected Accuracy

In order to achieve enough accuracy, a large enough volume of sensitizer must have its specific gravity measured.  The 71 ml of solution measured in this study with dry chemicals weighed at an accuracy of 0.01 grams resulted in a measured Specific Gravity accuracy of +-0.4% and a calculated percent concentration accuracy of +-3%.  This would mean that a calculated percent concentration of 27.0% could range from about 26.2% to 27.8% (or about 26% to 28%).  Any conclusions must consider this range of error, and caution should be exercised whenever differences fall within the error range.
 

Sample preparation

Since the Vizcay FO powder has demonstrated the highest content of ferric oxalate, it was selected for this study.  Several sensitizer solutions were mixed and measured at the following estimated concentrations.  The concentrations were centered around an estimated 26-27% as per the findings of the FO Threshold Study.

The first solution in the following table was mixed and measured; then EDTA was added and the solution measured again; then Oxalic Acid was added and the solution measured again; then more Oxalic Acid was added and the solution measured again.  These steps were to isolate influences of the specific gravity measurement by the Oxalic Acid and EDTA.  Next the concentration of the FO is increased incrementally by adding FO powder and measured as indicated in the following table.  The slight increase in Oxalic Acid from the powder added should be considered negligible.
 

Sample	grams Vizcay FO powder	grams FO powder added to previous sample	total grams EDTA (Na4) (% concentration)	total grams Oxalic Acid [including contents of the FO powder, as above] (% concentration of total Oxalic Acid)	grams Oxalic Acid added to previous sample	amount in ml after dissolving by adding H2O	estimated FO concentration (from content listed above)
A	17.37	0.00	0.00       (0%)	0.33     (.47%)	0.00	71	24%
B	18.10	0.73	0.00       (0%)	0.33     (.47%)	0.00	71	25%
C	18.10	0.00	2.13    (3.0%)	0.33     (.47%)	0.00	71	25%
D	18.10	0.00	2.13    (3.0%)	1.07     (1.5%)	0.74	71	25%
E	18.10	0.00	2.13    (3.0%)	1.78     (2.5%)	0.71	71	25%
F	18.10	0.00	2.13    (3.0%)	2.49     (3.5%)	0.71	71	25%
G	18.82	0.72	2.13    (3.0%)	2.52     (3.5%)	0.014	71	26%
H	19.54	0.72	2.13    (3.0%)	2.53     (3.6%)	0.014	71	27%
I	20.27	0.73	2.13    (3.0%)	2.55     (3.6%)	0.014	71	28%
J	20.99	0.72	2.13    (3.0%)	2.56     (3.6%)	0.014	71	29%

Note:  Extra solution was prepared so as to be available to top off contents of the sample bottle as
           some losses would occur on capping and opening.  No record was kept of any losses or
           displacement of solution due to the addition of solids.  The initial mixture (A) required
           63.2 ml H₂O to make the 71 ml sample bottle volume.
Note:  The addition of EDTA and / or Oxalic Acid is important as the pure FO will typically only
            attain a concentration of less than 26% with typical lab temperature and pressure.
 

Measurement of Specific Gravity (SG)

Equipment:

• balance scale accurate to 0.01 grams
• container filled with water (large enough to completely submerge the sample bottle)
• thin wire
• counter weight to zero scale (as needed)
• sample bottle with cap (small enough to be completely submerged in the water container without touching the container).

Preparation:

• The sample bottle is filled completely so that there is no air trapped inside.
• If the bottle filled with water does not sink in water, then attach a “sinker” (lead) to the bottle for all measurements and consider this part of the bottle weight (the affect is a denser bottle, negative buoyancy).
• The thin wire is tied to the sample bottle with the other end looped over the scale hook.
• The wire is marked at the level of the water in the container when the sample bottle is completely submerged and not in contact with the container.  The wire above the mark is considered to contribute part of the tare weight along with the counter weight.  The wire below the mark is considered to be part of the sample bottle.

Procedure

A)  The sample bottle is weighed

B)  The sample bottle is filled with water and weighed submerged in the container of water

C)  The sample bottle is filled with a solution and weighed

D)  The sample bottle still filled with the solution is weighed submerged in the container of water

The Specific Gravity is calculated as:  SG = (C-A) / [ (C-A)-(D-B) ]

 

Measurement and Data

temperature of 62-68^oF

Measured Data and Calculated Specific Gravity (SG) (at 62-68^oF)
[Measurement Accuracy of 0.01 gram]

Sample	Weight of Empty bottle in AIR (grams) (A)	Weight of bottle filled with water submerged in WATER (grams) (B)	Weight of bottle filled with solution in AIR (grams) (C)	Weight of bottle filled with solution submerged in WATER (grams) (D)	SG (C-A) / [ (C-A)-(D-B) ] [ error of +- 0.4% ]	Temperature (oF)
bottle	77.73	44.22	-	-	-	68
A	-	-	157.43	54.55	1.149	65
B	-	-	157.71	54.95	1.155	68
C	-	-	158.29	55.42	1.161	62
D	-	-	158.48	55.61	1.164	62
E	-	-	158.64	55.82	1.167	62
F	-	-	158.68	55.88	1.168	66
G	-	-	158.96	56.11	1.171	64
H	-	-	159.45	56.59	1.178	64
I	-	-	159.96	56.90	1.182	64
J	-	-	160.63	57.56	1.192	64

NOTE:  The accumulated weights of added solids may not be apparent in (C) as some volume
               may have been displaced.  The sample bottle was always completely filled and capped
               so as to prevent any air bubbles.  However no attempt was made to measure changes in
               absolute volume measured or volume displaced.
 
 

Conversion of Specific Gravity into a Solution Concentration Percentage
for Ferric Oxalate using the Dick Stevens' Relationship

Sample	Estimated % concentration	Measured Specific Gravity [ error of +- 0.4% ]	Calculated % concentration [ error of  +- 3% ]
A	24%	1.149	23.5%
B	25%	1.155	24.5%
C	25%	1.161	25.4%
D	25%	1.164	25.9%
E	25%	1.167	26.4%
F	25%	1.168	26.5%
G	26%	1.171	27.0%
H	27%	1.178	28.1%
I	28%	1.182	28.7%
J	29%	1.192	30.3%

Analysis

The Stevens' Calculation of percent concentration seems to not agree with the measurements before the addition of any Oxalic Acid or EDTA, the values are about 0.5% lower than the estimated, and for measurements after all additions of Oxalic Acid and EDTA, the values are about 1% greater than the estimated.  This may be remmidied in one of two ways.

• The Ferric Oxalate made by Vizcay is less pure that estimated.  This could have been caused by a Ferric Oxalate content of 96% by weight instead of the 98% estimated.
• The Ferric Oxalate used by Stevens contained some Oxalic Acid (or other material) which caused his values for concentration to be too high.  This could have been caused by a 1% solution concentration of Oxalic Acid in his FO solution.

From measurments D & E, the original Oxalic Acid contents of .47% solution can be assumed to present a shift in the calculated FO % by about 0.5%.  This indicates a even lower FO composition than first estimated.  But the initial Oxalic (or other) content is likely twice that initially estimated lowering the composition expectation by about 1%.  This combined with the above reduction provides a revised estimate for the Ferric Oxalate composition of about 95%.

With these assumptions, the Vizcay FO powder is estimated to have the following composition.

95 %   ferric oxalate
  5 %   oxalic acid (or other but assumed Oxalic Acid for the moment)

Please note that after further correspondence with Vicente, it seems that this measurement and the Stevens' method may be in doubt.  Vicente has made a convincing argument that his batch of powder made from his method is 98% to 99.5% ferric oxalate.

98% is the value now used in the Sensitizer Formula Calculator.

Currently the Specific Gravity of the B&S and Artcraft FO powders is being measured.  There composition will be refferenced relative to the 95% content of the Vizcay powder and reported here and placed into the calculator.

Please note that the change to 95% will affect the Threshold for FO to a value from 26.0% to 25.2% or from 27.0% to 26.2%.  This will also increase the amount of FO powder weighed out to make the solutions.  The metal solutions will be calculated for the new FO concentrations.  And because of the reduction in metal, the Threshold will be remeasured as this is a function of the amount of metal per area in the print.  At this point, it can not be determined in the amount of necessary metal will push the threshold for the paper/chemistry combination back to 26% or change it to 25%.

 
 
 
 
 
 
 
 
 
 
 

FO means Ferric Oxalate, a light sensitive sensitizing agent for the Pt/Pd process.