~~~~~~~~ Excerpt from the Guide to Platinum-Palladium Photographic Printmaking by Jeffrey D. Mathias ~~~~~~~~
No part of this document may be reproduced in whole or in part without the express written permission of the Author.
created August 1999

PREPARING FERRIC OXALATE POWDER
(by Vicente-M. Vizcay Castro, August/1999)
(American English adaptation by Jeffrey D. Mathias)

 
The process reaction that is used is:
 
2 Fe(NO3)3.9H2O + 3 H2C2O4.2H2O = 1 Fe2(C2O4)3.6H2O + 6 HNO3 + 18 H2O
ferric nitrate oxalic acid ferric oxalate nitric acid water
FW: 403.99734 126.06604 483.84048 63.0129 18.01528
Eq: 807.99468 378.19812 483.84048 378.077 324.275
grams 250. g 117.018 g 149.704 g 116.98 g 100.333 g
calculator  
               
To work the calculator:  Introduce a value in grams in the ferric nitrate calculator box; press the <TAB> key; the necessary amount (in grams) of oxalic acid necessary to react with the ferric nitrate and the amounts (in grams) of the products generated will be calculated and displayed in the appropriate boxes.  (grams are rounded to 0.1 grams)
NOTE:  In this calculation, the Oxalic Acid has been increased by a recommended amount of 2.5% to make sure that all of the ferric nitrate is reacted.

REAGENTS:

The quality of the ferric oxalate obtained depends on the quality on the Reagents (ferric nitrate and oxalic acid) used.
 
 
Ferric nitrate nonahydrate: (Fe(NO3)3.9H2O) Pale-rose-violet deliquesce small crystals.  Turns to brown in air contact.
Dangers:  Fire risk, dangers when in contact with organic materials, strong oxidizer and irritant.
 
Oxalic acid dihydrate: (H2C2O4.2H2O) Transparent acicular small crystals.
Dangers:  Toxic by inhalation and ingestion, irritant.

PRODUCTS:
 
Ferric oxalate hexahydrate: (Fe2(C2O4)3.6H2O) Amorphous powder pale-yellow.
Dangers: Flammable, toxic by ingestion and inhalation.
 
Nitric acid: (HNO3) Diluted in the reaction water. The resulting liquid of the reaction has a concentration of 40-50% nitric acid.
Dangers: Toxic by inhalation, erodes the skin and the mucous membranes. Strong oxidizer agent.
 
Water: (H2O)

All chemicals involved in this procedure are common in alt-photo processes, with the exception of nitric acid (40-50% richness). Take care and consult : MSDS (Material Safety Data Sheets), RTECS (Registry of Toxic Effects of Chemical Substances), LCSS (Lab Chemical Safety Summaries) and "Prudent Practices in the Laboratory: Handling and Disposal of Chemicals." by National Academy of Sciences.
 

Equipment for Safety:

As consequence of the compounds involved in this reaction and during all the phases of this process, the following are recommended:

Basic Equipment List

PROCEDURE

Remember to neutralize used utensils and waste liquids and papers prior to disposal.

WEIGHING
  The following amounts of Reagents with this procedure should produce 125-149 grams of ferric oxalate dry powder.  Proceed as follows:
1: In the "high sided" precipitates beaker (500 ml size), weigh out 250 grams of ferric nitrate nonahydrate (Fe(NO3)3.9H2O).  Break up any lumps that exist.
2: On a paper, weigh out 120 grams of oxalic acid dihydrate (H2C2O4.2H2O).  Preferably the oxalic acid is a powder or small crystals.   [Note that 120 grams are used instead of the 117 predicted so as to provide for an additional amount (2.5%) of Oxalic Acid.]
3: Distribute the oxalic acid into 3 or 5 portions of approximately equivalent size, each one on its own paper.
Illustration: completion of Weighing (step 3)

 
MIXING AND REACTING
From this point the process must be carried out in a well ventilated area and with safe light (normal incandescent light may be OK).  Avoid direct sun, high intensity light, and fluorescent light.
4: The following procedure is designed to mix, as uniformly as possible, the oxalic acid and the ferric nitrate.  To do this:
a) Take one of the portions established in step 3, and pour it into the beaker that contains the ferric nitrate.
b) With a strong glass or plastic rod stir for 1 or 2 minutes.  This will more uniformly mix the oxalic acid and the ferric nitrate in the beaker.  The reaction (endothermic, the beaker cools) begins at the moment of contact between the oxalic acid and the ferric nitrate.  In the contact zones, between both products, colors appear that change from green to yellow (ferric oxalate).  Since the reaction produces water, the mixture is humidified and liquefied during the process which in turn helps facilitate the mixing.
c) Repeat a) and b) until all the oxalic acid established in step 3 has been mixed with the ferric nitrate in the beaker.
At the completion of this step, the temperature of the mixture can decease to 8-9 ºC (for an ambient room temperature of 25-28 ºC).
Illustrations: mixing & checking temperature (step 4)
5: Cover the beaker with plastic film (clear plastic kitchen wrap) and let it rest.

To help homogenize the reactants and products, during the next 2 or 3 hours and  each half an hour, stir the contents of the beaker with the rod. This has to be done gently to avoid splashes, since the content of the beaker already contains nitric acid. Take care to always replace the cover with a new piece of plastic film.

Illustration: covered and resting (step 5)
 
If mixture is uniform the following is observed:
15 minutes
 Illustration		 after step 4 was ended (S4E), the mixture has been liquefied enough, displaying a yellowish brown green coloration. The mixture is quite fluid with bubbles (air), easily moves when inclining the beaker, and has a temperature of 8,5ºC. The part of the beaker's wall over the mixture, appears partially covered with dry yellow particles of FO.
30 minutes
Illustration		 after S4E, the green tonality of the mixture has disappeared and has become a yellowish brown color.
60 minutes
Illustration		 after  S4E, the ferric oxalate begins to precipitate (slowly). The mixture color is very similar to coffee with very little milk.
The mixture temperature has risen to 11 ºC.

It is observed throughout next the 2-3 hours, that the coffee color disperses (as if adding milk to coffee drop by drop).
150 minutes
Illustration		 after S4E, the color is pale-yellow, somewhat brown.
The mixture temperature is 13ºC.
180 minutes after S4E, the color is more yellowish than above.
The mixture temperature is 14ºC.
If mixture is not uniform the following is observed:
The mixture displays a yellowish coloration in the lower part and brown-yellowish in the upper part. At the bottom of the beaker some zones appears that are opened in fan-form of a  yellow-greenish color. This type of structure is a consequence of the fact that the reaction speed is not homogeneous due to the heterogeneous nature of the mixture (the reactants and products concentrations are different at various points of the mixture). The occurrence of the yellow color indicates that, in the points where this color appears, the concentration gradient of ferric oxalate is so high that it supersedes the solubility product of the oxalate and  precipitates.
During this step two layers are observed, one more fluid and light (superior) and the other more compact and granular or earthy texture (inferior). 

It is observed that the beaker externally presents a strong condensation motivated by the low temperature of the beaker. 

At an ambient temperature of 24-28ºC the beaker temperature can go down  to 8-12ºC.

After these 3 hours, observe that the content of the beaker, when stirred, is a yellow mixture of earthy texture due to the initial reactants that didn't react. The external condensation may continue.
6: After 6-8 hours, since the initial mixing process was ended (step 4), observe that the contents of the beaker is of a pale yellow color and has curdled.  That is to say, it resembles cement and is solid so that it does not fall out of the beaker when inverted and  presents  resistance to the penetration of the glass rod. With care (applying gentle force to avoid that the beaker will be broken), penetrate this mass with the glass rod in several points and observe that it is liquefied. Repeat several times in order to liquefy the contents of the beaker as much as possible. Once liquefied, try to dissociate with the rod or spatula, the lumps that exist in order to obtain the most homogeneous fluid that is possible. Stir with the glass rod until the content acquires the appearance of smoothly beaten yogurt or yellow-greenish cream.

The mixture is a finely divided precipitate of micro crystals which tends to be colloidal and to aggregate with itself, and as a consequence can be  separated by decantation (rest).  The color it becomes is dependent on the size of the groupings: small (pale-yellow-greenish) or large (pale-green). The size of the groupings is dependent on the temperature which the reaction is carried out (>temperature = >size).
 Illustrations: step 6
The reaction can be considered finished when the temperature of the contents of the beaker is equal to the room temperature.

 
DECANTING
7: The product obtained is a fine powder (particle size of 5-15 microns), indicating small groupings, let the beaker rest, covered by the plastic film, during 1-2-3-4... days (the longer, the better) in order to achieve a good separation by gravity.
The separation process by normal gravity (not forced sedimentation) is very slow, the cause of this is the small diameter of the groupings. An approximated estimation of the settling (sedimentation) times can be calculated.
First establish the following hypothesis:
  - Solid density = 2.5 g/ml
- Liquid density = 1 g/ml
- Solid particle size = 10**(- 5) m
- Temperature = 20ºC
- Dynamic viscosity = 10**(- 2) g/cm/s
- Acceleration due to gravity = 980 cm/s2
then obtain:
- Terminal settling Stok velocity:  v = 8.17x10**(- 7) cm/s

- Time for settling (for 8.17 cm):  t = 340 hours.
These results are for an isolated particle, the interactions between particles, the density of the fluid and the real dynamic viscosity (variables in the time) might cause the time to be multiplied by 100 or 1000.
The sedimentation speed may be increased by:
  • increasing acceleration (centrifugation);
  • increasing of the solid particle size [adding 0.5g of aluminum sulfate (it reduces the repulsion forces between particles facilitating flocculation), with further mixing to homogenize its distribution in the mixture];
  • increasing of the solid particle and temperature, and reducing liquid density and dynamic viscosity [by heating the suspension, this causes it to coagulate and facilitates separating the precipitate from the solution (temperature at least 40ºC but not >50ºC)].
    		•
    8: Every day change the plastic film since it becomes etched by the nitric acid.  Observe that, floating on the yellow phase appears a liquid phase, almost colorless, that will increase with time.  It will reach a thickness of several millimeters (it depends of  ferric nitrate humidity), which represents something less than 10% of the H2O and HNO3 produced in the reaction.

    The more days that it is rested will help facilitate the deposit and compacting of the suspension making it easier to eliminate the liquid by pouring off (this liquid has a strength of 40-50% nitric acid).
    9: The liquid that floats over the yellow precipitate (ferric oxalate), it is withdrawn as thoroughly as possible and neutralized.

    Illustrations: decanting & separation

     
    DRYING PHASE I
    The process that now begins has the purpose to remove the maximum amount of acid and water mixture from the precipitate.  This is achieved by capillary action by application of absorbent paper (the white paper towels, or napkins, etc.).

    Attention: the following steps should be accomplished with gloves (always), goggles, mask and in a ventilated place, with normal illumination (as in step 4).  Drying is considered to be eliminating, as thoroughly as possible, a nitric acid dissolution with a strength on the order of 40-50%, that impregnates the ferric oxalate.  Avoid more than 3-4 hours of prolonged contact of the oxalate with the absorbent paper since these are etched by nitric acid and become fragile.
    10: Prepare a plastic container with hermetical cover (at least 30x30 cm square) by placing in its bottom 10 or 20 absorbent papers, and over these papers place the piece of 20x20 cm plastic mesh.  [See Basic Equipment List for descriptions of container (kitchen ware) and  mesh (fiberglass screen).]

    Illustrations: step 10
     
    11: Liquefy and loosen the contents of the beaker (as described in step 6). With the aid of plastic or stainless steel spoons, spatulas or knives, remove the precipitate by loosening little by little and placing it onto the mesh and spreading it, leaving an edge of 1.5-2.0 cm free.  Next, cover the container with its lid.

    Illustrations: step 11
     
    12: After 3-4 hours open the container and observe where moisture has penetrated the papers.
    a) Remove the humid papers and replace with new papers.
    b) The consistency of the precipitate at this time will be quite firm.  Place 3 or 4 papers on top of the mass and strike on top with the fist several times (this is to seat and loosen the mass and at the same time to extract more liquid from the upper part of the mass.)
    c) Repeat a) and b), at intervals of several hours (2-3 hours), until it is observed that the moisture that impregnates the papers is insignificant, indicating that this phase of drying has ended.
    Dispose of papers by diluting in a bucket of water and neutralizing to pH 7, then pouring liquid down the drain with water and placing fully neutralized papers with other trash.
    DRYING PHASE II
    13: Move the precipitate along with the mesh onto new absorbent papers (5 or 10 layers) which are placed into the container (The mesh can be removed after the third or fourth change which will be made in this phase, leaving the precipitate directly on the absorbent papers)

    The container should be left open to facilitate the drying and elimination of the nitric acid and water.  The container should be located in a place without light which is well ventilated. The air can be forced to circulate over the container as a soft current (that does not blow away the ferric oxalate powder).

    Take into account that the gases eliminated in the process of drying are very corrosive and strong oxidizers.  Any iron or steel present will be oxidized.

    With the spatula or the knife, break up, as much as practical, the precipitate chunks that have been deposited on the absorbent paper.  Spread it most uniformly over the mesh-paper.

    With each paper change, reduce the size of the chunks with the aid of spatula or knife.

    Illustrations: during Drying Phase II
     
    14: Repeat step 13, 5-8 times each 2 or 3 hours.  When nearly all of the material on the paper is pale-yellow powder and small chunks, this phase can be considered completed.

    Illustrations: ending of Drying Phase II
     
    Dispose of papers by diluting in a bucket of water and neutralizing to pH 7, then pouring liquid down the drain with water and placing fully neutralized papers with other trash.
    DRYING PHASE III
    15: To dry further, the ferric oxalate is moved to a plastic or glass desiccator.  This may be constructed from  a container without a cover which is placed into a larger plastic container (large enough to hold two of the smaller containers) with a hermetical cover.  A drying agent (dry calcium oxide or hydroxide, zinc oxide, etc.) is placed in a similar plastic container without a cover which is also placed into the larger plastic container.  In this manner the drying agent will not come into contact with the ferric oxalate.
    [See Basic Equipment List for a description of a desiccator container.]

    50 grams of CaO or similar agent should be enough to dry 150 grams of FO.

    Illustrations: Drying Phase III using a glass desiccator with drying agent in lid
     
    16: Keep the oxalate in the container until it is considered sufficiently dry, then move it to the final container, an amber glass bottle with hermetical cover.
    Dispose of spent drying agent by diluting in a bucket of water and neutralizing to pH 7, then pouring liquid down the drain with water.
    DRYING OPTIONS
  • If one is impatient they can skip the decanting phase and directly go from the conclusion of

    •    the reaction (completion of step 6) to DRYING PHASE I.
     
  • Some of the drying steps can be obviated or simplified if the liquefied precipitate obtained at

    •    the conclusion of the reaction (completion of step 6) is submitted to filtration (vacuum filtration
       better).  For this use filters Watman 50 or Millipore type 4.
    CONCLUSION
    The theoretical weight of the ferric oxalate obtained in this reaction is about 149.7 grams. There are losses of weight as consequence of the numerous raking of the oxalate as well as the portion dissolved in the eliminated liquid.  With care, somewhat more than 125 grams of dry ferric oxalate is obtainable.

    The obtained ferric oxalate contains some residual oxalic acid and nitric acid, which helps prevent ferric oxalate reduction with time.
     

    Back to Table of Contents      Home    Back to Introduction & Tests
    ---------------------------------------------------------  IMPORTANT  ---------------------------------------------------------
    These instructions are only for personal use.  No commercial or any derived advantage of them are permitted.  No express or implied license for use exists.

    No part of these instructions may be distributed, reproduced, republished, or stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the express written permission of the author, Vicente-M. Vizcay Castro.
    ---------------------------------------------------------------------------------------------------------------------------------------

     
     

    page visit counter