Residual thiosulfate tests
From Silvergrain Labs
Processed films and prints must be thoroughly washed to remove the fixing agent from the material. Residual fixer (thiosulfate or hypo) is harmful to the life of the support and the image, and therefore, there is archival standards that set the limit of residual thiosulfate that may remain in the material for archival materials.
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Processing
Adequate fixing and washing are essential. Choose one of the standard washing methods recommended by Ilford, Kodak or Silvergrain.
When the material is toned (recommended), the material should be washed thoroughly after the toning process. Residual toning agent is just as harmful as the residual thiosulfate, and they should be washed out altogether.
Use washing aid for baryta (fiber based) prints.
Testing
If you strictly follow the instructions provided by Kodak, Ilford or Silvergrain, your material most likely meets or exceeds the archival standard for residual thiosulfate. However, if you process important materials, or if your processing and washing procedures deviate from one of the standard recommendations, there is only one way to make sure that your materials meet the standard criteria: testing.
Silver nitrate test
For quality control in darkroom, silver densitometry method (silver nitrate test) is facile and sufficient test. The test was described by Mattey and Henn of Kodak lab in Rochester in 1966. The essense of this research is in Kodak HT-2 hypo test solution and its instruction. This is my recommended test for all practical applications.
- Mattey, D. A. and Henn, R. W. 1966. Determination of thiosulfate and thionates in film with silver nitrate, Photogr. Sci. Engr. 10, 202-8.
- Pope, C. I. 1969. A simplified method for determining residual thiosulfate in processed microfilm. Photogr. Sci. Engr., 13, 278-9.
Methylene blue formation test
This is a more sensitive and accurate test than above, and it is often used for research purposes on freshly processed samples. The processed samples must be tested within 14 days or the results will be inaccurate. (Silver densitometry test above may be used on old samples without losing accuracy.)
- Warburton, C. D. and E. P. Przybylowicz, 1966. A new test method for the measurement of residual thiosulfate in processed film based on borohydride reduction to sulfide and methylene blue formation, Photo. Sci. Engr. 10, 86-92.
- See Kodak Publication H24 Module 3, Processing Kodak motion picture films (This Kodak publication describes the test procedure only. For detailed info, refer to the original paper above.)
- ISO 18917:1999 Photography--Determination of residual thiosulfate and other related chemicals in processed photographic materials--Methods using iodine-amylose, methylene blue and silver sulfide
Iodine-amylose test
This is also a very sensitive and accurate test, which is specified in the recent ISO standard.
- Anselm, C. D. and Nicosia, A. S., 1966. A colorimetric determination of residual thiosulfate, tetrathionate, and pentathionate in film, Photo. Sci. Engr. 10, 53-9.
- Owerbach, D. 1986. A colorimetric determination of residual thiosulfate (hypo) in processed paper, J. Appl. Photogr. Engr., 9, 66-70.
- ISO 417:1993.
Mercuric chloride test
Also known as Crabtree-Ross test. This is an old test, which was specified in ASA PH 4.8-1958, but this procedure has a few serious problems and it is not used recently.
Criteria for archival processing
Safe level of residual thiosulfate varies depending on the type of emulsion. This is because fine grain image has greater surface area to grain volume ratio.
Film
For b&w pictorial films, current ISO 18901:2002 requires that the residual thiosulfate level be below 50mg per square meter for life expectancy (LE) of 100 years, and 14mg per square meter for LE of 500 years. The LE500 value is applicable only to polyester films, because cellulose triacetate film base does not last for 500 years. (That is, the LE500 rating is not applicable to most 35mm and roll formats.)
Note1: older standards (such as ISO 10602:1993) required that the residual thiosulfate be below 7mg per square meter.
Note2: 10mg per square meter is equivalent to 1 microgram per square centimeter.
For b&w pictorial prints, it is common to aim residual thiosulfate level of 10 mg per square meter or below, although 20mg per square meter is adequate.
Effects of residual thiosulfate and thionates
- Kopperl, D. F. and Hutteman, T. J., Jr. 1986. Effects of residual thiosulfate ion on the image stability of microfilms. J. Imaging Tech., 12, 173-180.
- Kopperl, D. F. and Hutteman, T. J. 1988. Effect of residual thiosulphate ion on the image stability of microfilms. J. Photogr. Sci., 36, 98.
