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Rapid, accurate analyses for the CO3 content of carbonate rocks may be made by allowing a known amount of the rock to react with a measured quantity of standard H2SO4 and then determining the amount of acid which remains by back-titration with standard NaOH. The technique is faster and more accurate than the leaching method and, more important, allows the analyst to determine directly the CO3 content. Very accurate analyses for CO3 in limestones may be made by potiometric titration methods, but quite satisfactory results are obtained using phenolphthalein as an indicator. As is the case with igneous rocks, the classification of carbonate rocks may be based on modal composition as determined by studies of thin-sections or it may be based on chemical composition. The exact mineralogical composition of a rock cannot be determined from a chemical analysis. The weight percent CO3 is a more meaningful index for the chemical classification of limestones than the percent carbonate mineral or the percent insoluble residue.

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