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Standard: |
TAPPI |
Method: |
TAPPI T 406 |
Title: |
Reducible sulfur in paper and paperboard |
TAPPI T 406 Reducible sulfur describes two procedures for the determination of reducible sulfur in paper and paperboard within the context of the given definitions. The quantitative procedure found in Section 9 gives a measure of the reducible sulfur with the precision shown in Section 12. The semi-quantitative test described in section 13 indicates the general level of reducible sulfur with limited accuracy.
The procedures are not necessarily a measure of how much a given test material will tarnish polished metals. When tested by these procedures, paper which has less than 0.0008% reducible sulfur may be assumed to be nontarnishing as far as sulfur is concerned, but if more than 0.0008%, this does not necessarily mean that tarnishing will occur, because sulfur compounds which may not cause staining are reduced by the treatment with the subsequent evolution of hydrogen sulfide. If more than 0.0008% reducible sulfur is found, or if the effect of that or other materials causing stains on silver is desired, the paper should be subjected to an accelerated tarnishing test.
The procedures described do not give the total sulfur content, nor the total reducible sulfur content, nor are they intended to. They are limited to the reducible sulfur content determined under the conditions of the test (i.e., a fairly mild reduction system). Those forms of sulfur believed to be “reducible” under the test conditions are sulfide (released by the acid, not actually reduced), elemental sulfur, thiosulfate, other polythionates or polysulfides, and perhaps sulfite. Sulfate is excluded.
The semi-quantitative method involves the reduction of various forms of sulfur to hydrogen sulfide and the development of a dark spot of lead sulfide on the filter paper impregnated with lead acetate. The intensity of the spot is compared with spots developed from standards and is proportional to the concentration. No colorimeter or spectrophotometer is needed for this method.
The quantitative method uses the identical reduction system to generate H2S, but the sulfide is precipitated by alkaline cadmium sulfate and then converted to methylene blue by reaction with acidic p-aminodimethylaniline in the presence of ferric chloride. The methylene blue is measured spectrophotometrically, and the intensity is compared with standards prepared in a similar manner.
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