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TAPPI T 553 Alkaline Reserve






Alkalinity of paper as calcium carbonate (alkaline reserve of paper)


TAPPI T 553 covers the determination of the alkalinity or alkaline reserve of paper, or both. A qualitative test is described that indicates the presence of carbonate. (The detection limit is approximately 5% calcium carbonate.) A quantitative test is described that determines the alkalinity expressed as percent calcium carbonate or alkaline reserve, or both, expressed as moles per kilogram of paper. (The detection limit is approximately 2% calcium carbonate.)

Summary of test method

Qualitative test, the presence of carbonate is determined by immersing a sample of the paper in hydrochloric acid and observing effervescence. Any carbonate or bicarbonate salt present will produce this effect.

Quantitative determination of carbonate, the paper sample is subjected to digestion in a known quantity of standardized hydrochloric acid. Back titration with standardized sodium hydroxide is used to determine the amount of hydrochloric acid consumed in the digestion process. The subsequent calculation for alkalinity assumes that all of the alkaline material neutralized was calcium carbonate. The calculation of moles per kilogram is independent of the material providing the reserve.

Significance and Use

Growing concern regarding the deterioration of books and various documents in libraries and archives has led to the development of standards by standard-setting bodies for improved permanence in paper. By using alkaline sizing technology, it is possible to manufacture paper at a pH of 7.5 or above and, therefore, incorporate alkaline fillers such as calcium carbonate. Alkaline sizing in itself improves permanence by eliminating acid from the sheet normally associated with a rosin/alum sizing system. The presence of an alkaline filler gives an added measure of permanence because it has the capability of sorbing acidic gases from the environment that might otherwise cause deterioration of the paper.

Various paper product specifications specify an alkaline reserve, frequently at a minimum of 2% calcium carbonate. An alkaline reserve of 0.4 mol/kg is provided by 2% calcium carbonate.

The qualitative test can be used to determine the presence of carbonate, although this may not necessarily confirm that the paper is alkaline-sized or that the filler is calcium carbonate. A paper sized with a rosin/alum system and coated with a coating containing any carbonate salt would give a positive qualitative reaction. Carbonate levels of less than 5% may not show positive results.

The quantitative test assumes that the carbonate is calcium carbonate and reports as such.

If the sample is known to contain no other carbonate or alkaline material, then this test method can be used to determine the calcium carbonate content. If the composition is unknown, or if other materials are known to be present that will react with the acid, the results should not be reported as CaCO3.


This test is performed in duplicate. A sample size of 5 grams of paper is recommended.


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