IAGC and IAEA Interlaboratory Comparisons of Geothermal Water Chemistry: The Propagation of Uncertainty in the Reservoir pH Calculation |
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| Authors: | Mahendra P Verma |
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| Institution: | Geotermia, Instituto de Investigaciones Electricas, Apclo. 1‐475 Cuernavaca, 62490 Morelos, Mexico. e‐mail: mahendra@iie.org.mx |
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| Abstract: | A statistical evaluation of the results of geochemical analyses of geothermal waters during interlaboratory comparison programmes of the International Association of Geochemistry and Cosmochemistry (IAGC) and International Atomic Energy Agency (IAEA) was performed to estimate the uncertainty of measurement of pH, electrical conductivity, Na+, K+, Ca2+, Mg2+, Li+, Cl?, HCO3?, SO42?, SiO2 and B. The uncertainty of measurement was found to increase exponentially with decrease in value (concentration) for all the parameters except for pH, electrical conductivity and SiO2 and was of the same order of magnitude as the concentrations for values of less than 1 μ ml?1. There was an overall uncertainty of ± 2.5% in the measurement of pH and ± 30% in SiO2. For all the other chemical species the uncertainty data were modelled by exponential curves. The sample IAEA14 was prepared by dissolving commercial reagents (i.e., represents a solution of known composition). Thus, the calculated values are considered to be the conventional true values for each chemical parameter. The difference between the measured mean of the data submitted by participating laboratories and the conventional true value for each parameter (i.e., bias of submitted measurements) for the species Na+, K+, Ca2+, Mg2+, Cl? and SO42? was ‐3.5, ‐1.1, ‐13.3, ‐53.6, ‐12.6 and ‐86.6%, respectively. The observed bias was of the same order of magnitude as statistical fluctuations (1s) for Na+ and K+, but significantly higher for Ca2+, Mg2+, Cl? and SO42?. Two methods, uncertainty interval and GUM (“guide to the expression of uncertainty of measurement”) were used to propagate uncertainty in the pH calculation of geothermal reservoir fluid. The application of the methods is illustrated by considering the IAEA10 and IAEA11 samples analysed in the interlaboratory comparisons as separated geothermal waters at atmospheric pressure. |
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| Keywords: | interlaboratory comparison geothermal waters pH calculation bias uncertainty of measurements propagation of uncertainty GUM comparaisons inter‐laboratoires eaux gé othermales calcul du pH biais incertitude cles mesures propagation d'erreur “ GUM” |
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