Photographer's Formulary Residual Hypo Test is a simple chemical test that can be used to determine if a print or filmstrip has been adequately washed.
The test is based upon the formation of a yellow stain of silver sulfide - silver thiosulfate on the white of a test print (or the clear area of a negative film strip) when the test solution is applied. This chemical is not sensitive enough to be utilized on archival prints.
UPC: 675151301506