What is hypo?

   Hypo (chemical formula, Na2S2O3-5H2O) is the common name for sodium thiosulfate. It is called hypo because it was once incorrectly named sodium hyposulfite. Hypo is used chiefly to "fix" photographic negatives and prints. The English scientist Sir John F. W. Herschel discovered that hypo readily dissolves silver salts. In 1839, he recommended the use of "sodium hypo­sulfite" to dissolve the unexposed silver salts in the talbotype photographic process. The technique has been used since that time.