The purity of a calcium carbonate sample is examined using two analytical methods available in our laboratories. First, we will illustrate the use of thermogravimetric analysis (also called thermal gravimetric analysis or TGA) to determine the calcium carbonate purity. Then we will perform an elemental analysis of the sample using our wavelength-dispersive x-ray fluorescence spectrometer (WD-XRF). The information provided by either of the two analytical techniques tells us less than the total story, but together the two analysis techniques give us a more comprehensive understanding of the sample composition. We will discuss these issues.
Thermogravimetric Analysis (TGA) for Purity
The quantitative measurement of the thermal decomposition of CaCO3 in a N2 atmosphere while heating a sample of calcium carbonate was used to assess its purity. The CaCO3 sample was heated in N2 (10 cc/min) from room temperature to 900°C at a heating rate of 10 °C/min in our ISI TGA-1000 instrument.
The weight loss due to the decomposition of CaCO3 to form CaO and CO2 starts around 615°C and the decomposition is complete around 777°C. The decomposition of a pure CaCO3 sample would produce a weight loss of 43.971% due to the evolution of CO2. Figure 1 shows the TGA thermogram for the thermal decomposition of the tested CaCO3 sample. The blue curve is the first derivative of the sample weight curve with respect to the temperature. From room temperature to 615°C, the weight loss was 0.488%. A great rate of increase in weight loss starts to occur after 615°C. Between 615°C and 776.6°C, the weight loss was 42.639 %. The maximum rate of weight loss is at 761.53°C. The sample further continued to lose weight from 776.6°C to 899.7°C, where the interval weight loss was 0.618%.
Compared to the 43.971% weight loss for a pure CaCO3 sample, this sample exhibited a weight loss of 42.639%. This implies that the sample was no more than 96.97% pure. Of course, it might be somewhat less pure than that if some of the impurities also decompose in the temperature range from 615 °C to 777 °C.
Wavelength-Dispersive X-Ray Fluorescence Analysis
To identify the elemental impurities in the CaCO3 sample, the sample was analyzed using our ThermoFisher ARL PERFORM’X Sequential X-Ray Fluorescence Spectrometer in a He atmosphere. This analysis does not detect the elements lighter than fluorine when a helium atmosphere is used, as it must be when particulate samples are analyzed. Consequently, carbon and oxygen are among the elements not detected and a pure calcium carbonate sample should be 100.00% Ca.
The elemental composition of CaCO3 is shown in Table 1. As expected from TGA results, apart from calcium, other elements including sodium and magnesium were present in appreciable concentrations. The Ca concentration was 96.38 wt.%, not 96.97 wt.% as implied by the TGA analysis. Apparently, 0.59 wt.% of the XRF detectable elements in the sample do consist of impurities that decomposed in the same temperature range that the calcium carbonate decomposed in. Note that sodium and strontium carbonate are stable to temperatures above 777 °C, but magnesium carbonate can decompose in the same temperature range as calcium carbonate does. Consequently, most of the additional 0.59 wt.% impurity implied by XRF compared to that implied by TGA may well be due to the magnesium carbonate impurity, which is a common impurity in calcium carbonates.
Table 1: Elemental composition of the CaCO3 sample.
| Element | Wt% | Est.Error |
|---|---|---|
| Ca | 96.38 | 0.18 |
| Na | 2.99 | 0.19 |
| Mg | 0.493 | 0.025 |
| Sr | 0.0426 | 0.0021 |
| Cl | 0.035 | 0.0021 |
| I | 0.0165 | 0.0061 |
| Fe | 0.0131 | 0.0026 |
| Ag | 0.0128 | 0.0033 |
| Sc | 0.0061 | 0.002 |
| U | 0.0041 | 0.0019 |
| K | 0.0038 | 0.001 |
| Rb | 0.0029 | 0.001 |
Conclusions
The TGA analysis shows our calcium carbonate sample to have a calcium carbonate purity of as much as 96.97% by weight. The XRF analysis says the purity cannot be greater than 96.38 wt.%, but the XRF does not tell us what portion of the 96.38 wt.% of the Ca is in the form of calcium carbonate. Some of the calcium might be in the form of CaO or CaCl2, for example. The TGA result tells us that almost all of the Ca is in the form of CaCO3 and that since the primary impurities revealed by XRF are Na, Mg, and Sr, then really only the magnesium in the form of its carbonate can be reducing the calcium carbonate from its upper limit of 96.97 wt.%. The maximum weight percent of magnesium carbonate is 0.493 wt.%. Consequently, the calcium carbonate weight percent is almost certainly between 96.48 and 96.97 wt.%.
