Radiography in Modern Industry - Kodak

e considered exhausted when that number of films has been processed, or sooner if aprecipitation sludge appears. It must then be replaced, not replenished.After treatment with the Hypo Clearing Agent, films should be washed for 5 minutes, using awater flow which will give a complete change of water 4 to 8 times per hour. However, if watersupplies 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 bereplaced 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 bathmay be checked by testing a processed film for fixer removal as described in the followingsection.Testing For Fixer RemovalFixing chemicals not adequately removed from films by washing will, over a period of time, causestaining of the film and fading of the developed image. When it is known that films must bepreserved indefinitely or when there is doubt as to the adequacy of the washing procedures, theamount of fixing chemicals remaining in the film after washing should be determined. This can bedone in one of two ways.Archival WashingFilm 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 (AmericanNational Standard Practice for Storage of Processed Safety Photographic Film, PH1.43-1979.Published by American National Standards Institute, Inc., New York, NY). Archival washing forthis 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 formeasurement are described in ANSI PH4.8-1971, "Methylene-Blue Method for MeasuringThiosulfate, and Silver Densitometric Method for Measuring Residual Chemicals in Film, Platesand Papers." (Available from American National Standards Institute, Inc., 1430 Broadway, NewYork, NY 10018) The methylene-blue method described in this document measures directly theconcentration of thiosulfate iron. The silver-densitometric method measures thiosulfate as well asother residual chemicals and requires that a calibration curve be used relating the silver densityproduced 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 residualthiosulfate should be chosen from those covered in the ANSI standard mentioned above. Notethat while KODAK Hypo Estimator and KODAK Hypo Test Solution HT-2 (the HT-2 test) provide aquick, convenient means for estimating the amount of hypo (thiosulfate ion) retained in theemulsion, they cannot be used to determine the concentration of residual thiosulfate in terms ofarchival washing standards.The methylene-blue method measures only thiosulfate. The technique is complex, and thesample must be tested within two weeks of processing. The silver densitometric methodmeasures thiosulfate and other residual chemicals. The technique is simpler, and the results arenot 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 sensitiveenough to measure thiosulfate reliably below about 0.9 µg per square centimetre. The twoprocedures for the methylene-blue method described in ANSI PH4.8-1978 cover the range of 0.1to 45 µg of thiosulfate ion (S 2 O 3 --) per square centimetre of the test sample. This is the onlymethod ANSI considers sufficiently reliable for determining such a low concentration as 0.7 µg ofthiosulfate ion per square centimetre.Radiography in Modern Industry 136