The Use of CO2 Coulometry in Differentiating and Quantifying the Carbonate Phases in the Coal-Bearing Strata of Western Pennsylvania: Its Applicability in Interpreting and Modifying Neutralization Potential (NP) Measurements

Abstract

CO 2 coulometry is a rapid, highly accurate technique which can be used to determine the carbonate carbon contents of coals and associated overburdens. The rate at which the co 2 is evolved during the acid digestion process is a function of which carbonate phases are present. Calcite and dolomite reaction times are approximately six to eight minutes under the conditions described herein, with the maximum change in reaction rates occurring between two and four minutes. Siderite requires a lengthy 10 to 15 minutes to completely react with the maximum change in reaction rate occurring between six and eight minutes. The determination of neutralization potential (NP) in the coal-bearing overburdens of the eastern United States is inaccurate when sideri t$ is pres~nt because of the slow conversion of Fe2 to Fe3. To alleviate the effects of siderite on NP, a 30 wt.% H 2 0 2 solu}ion can be used to accelerate the conversion of Fe2 to Fes+ during the titration process. The addition of H 2 0 2 increases both the accuracy and precision of the NP determination. Despite the addition of H 2 o 2 , many siderite-bearing overburdens still possess NP. This is interpreted as being the result o~: 1) the incomplete oxidation of Fe2+ to Fe3 and the incomplete precipitation of iron hydroxide, and/or 2) the presence of Ca and/or Mg occurring in solid solution in the siderite.

Delineation of Potential Manganese Sources in the Coal Overburdens of Western Pennsylvania

“…The majority of the carbonate present was associated with samples 170 -175. CO 2 coulometry data, as interpreted and discussed by Morrison et al (1990) indicated that the dominant carbonate phase present in these overburdens was siderite. This interpretation was also verified using x-ray diffraction.…”

“…Further data to support the observation that l1n is contained in siderite as a crystal chemical substituent was obtained from a siderite concretion (not from this mine site), {Morrison et al 1990) for which 40 grains of siderite were analyzed for Fe, Mn, Mg and Ca. These data, Table 1, show an average Mn-substitution of 3500 ppm which is equated to a molar substitution of: (Fe.s6Mg.02Ca.1Mn.01)C0 3 • Table 2 further supports the presence of a nearly constant, within experimental error, concentration of Mn in this 13.5 foot sequence of black shales.…”

“…calcite and/or dolomite), CO 2 coulometry was used to quantify the occurrence of siderite. The methodology employed here is described in detail by Morrison et al (1990).…”

“…As the amount of siderite increased, the accuracy of the NP determination became questionable because of the increased Fe in solution Morrison et al (1990).…”

Determination of Calcite and Dolomite Content in Soils and Paleosols by Continuous Coulometric Titration

“…We are aware of only one study that has attempted to quantify various fractions of carbonate using coulometric titration where the CO2 evolved in the reaction with acid was continuously monitored (Morrison et al, 1990). Although Morrison et al (1990) were able to distinguish and quantify siderite from the total carbonate in the rock samples used in their study, they were not successful in distinguishing calcite from dolomite. Our aim was to overcome the limitations in Morrison et al (1990) by grounding the description of the reaction rates of calcite and dolomite in a more theoretically based kinetic model.…”