After fixation, the excess fixer is removed from the film by a 30-second rinse in water. It is then immersed in KODAK Hypo Clearing Agent solution for 1 to 2 minutes, with agitation. With this procedure, the capacity of the Hypo Clearing Agent bath will be about 750 to 1000 films (8 x l0- inch) or 250 to 330 films (14 x 17-inch) per 5 gallons of solution. If no rinse is used after fixation, the capacity of the bath will be reduced to about 200 to 300 films (8 x 10-inch). The bath should be considered exhausted when that number of films has been processed, or sooner if a precipitation sludge appears. It must then be replaced, not replenished.
After treatment with the Hypo Clearing Agent, films should be washed for 5 minutes, using a water flow which will give a complete change of water 4 to 8 times per hour. However, if water supplies are severely limited, films may be washed in standing water, rather than running water, by soaking for 10 minutes with occasional agitation. The water in the wash tank should be replaced after 10 films (8 x 10-inch) per gallon have been washed.
The effectiveness of the washing procedure and the capacity of the Hypo Clearing Agent bath may be checked by testing a processed film for fixer removal as described in the following section.
Testing For Fixer Removal
Fixing chemicals not adequately removed from films by washing will, over a period of time, cause staining of the film and fading of the developed image. When it is known that films must be preserved indefinitely or when there is doubt as to the adequacy of the washing procedures, the amount of fixing chemicals remaining in the film after washing should be determined. This can be done in one of two ways.
Archival Washing
Film of archival interest--and this includes the majority of industrial radiographs for code work-- should remain unchanged for long periods of time under good storage conditions (American National Standard Practice for Storage of Processed Safety Photographic Film, PH1.43-1979. Published by American National Standards Institute, Inc., New York, NY). Archival washing for this indefinite preservation of films is defined by American National Standards Institute (ANSI) documents in terms of the concentration of residual thiosulfate in the film. Acceptable methods for measurement are described in ANSI PH4.8-1971, "Methylene-Blue Method for Measuring Thiosulfate, and Silver Densitometric Method for Measuring Residual Chemicals in Film, Plates and Papers." (Available from American National Standards Institute, Inc., 1430 Broadway, New York, NY 10018) The methylene-blue method described in this document measures directly the concentration of thiosulfate iron. The silver-densitometric method measures thiosulfate as well as other residual chemicals and requires that a calibration curve be used relating the silver density produced to the thiosulfate content as measured by the methylene-blue method.
For test films or any other films intended for archival keeping, the method for determining residual thiosulfate should be chosen from those covered in the ANSI standard mentioned above. Note that while KODAK Hypo Estimator and KODAK Hypo Test Solution HT-2 (the HT-2 test) provide a quick, convenient means for estimating the amount of hypo (thiosulfate ion) retained in the emulsion, they cannot be used to determine the concentration of residual thiosulfate in terms of archival washing standards.
The methylene-blue method measures only thiosulfate. The technique is complex, and the sample must be tested within two weeks of processing. The silver densitometric method measures thiosulfate and other residual chemicals. The technique is simpler, and the results are not affected as much by the length of time between processing and testing. Like the HT-2 test, the silver densitometric method lacks sensitivity at low levels of thiosulfate. It is not sensitive enough to measure thiosulfate reliably below about 0.9 µg per square centimetre. The two procedures for the methylene-blue method described in ANSI PH4.8-1978 cover the range of 0.1 to 45 µg of thiosulfate ion (S2O3--) per square centimetre of the test sample. This is the only method ANSI considers sufficiently reliable for determining such a low concentration as 0.7 µg of thiosulfate ion per square centimetre.
Methylene-Blue Method
Two variations of this method for measuring the concentration of residual thiosulfate are described in detail in ANSI PH4.8-1978. One covers a range of 0.1 to 0.9 µg of thiosulfate ion per square centimetre; the other, a range of 0.9 to 45 µg of thiosulfate ion per square centimetre. If the film is double coated, the residual thiosulfate ion is assumed to be divided equally between the two sides. Therefore, the concentration per square centimetre of emulsion is one-half of the total determined by either variant of the methylene-blue method. The maximum permissible concentration of residual thiosulfate ion for coarse-grain films, such as industrial x-ray films, is 3 µg of thiosulfate or 2 µg of thiosulfate ion per square centimetre.
Either way, testing must be done within two weeks of processing. Both require several reagents, a photometer or a spectrophotometer, and a calibration curve. Tests are conducted as follows:
Residual thiosulfate is extracted from a test sample and reduced to a sulfide. The sulfide reacts with test reagents to form methylene blue. The absorbance or the transmittance of the blue color is then measured with a photometer or a spectrophotometer, and the thiosulfate level is read from a calibration curve.
The methylene-blue method is a complex multi-step procedure that requires special materials and equipment and specialized analytic techniques not readily available to most industrial radiographers. Complete step-by-step directions for both procedures, including preparation of the test sample (which must be taken from an area of minimum density--preferably an unexposed but processed area), the various reagents, and the calibration curve, as well as information on the interpretation of results, are given in ANSI PH4.8-1978.