Fig. 1
Examples of \({}^{1}\)H spectra and \({}^{23}\)Na spectra and temperature measurements. a \({}^{1}\)H and b \({}^{23}\)Na spectra at 6 different temperatures for 3 solutions with different NaCl concentrations: 1, 11, and 23% weight (ppm: parts per million). c Temperature measurements in all the calibration samples containing 0.1–26% NaCl, using frequency shift thermal coefficient difference \(\Delta \alpha\) and intercept difference \(\Delta {\sigma }_{0}\) calculated from the results shown in Fig. 2, as a function of reference temperatures. d Average temperature measurements over all samples as a function of reference temperatures. All temperatures are in \({}^{\circ }\)C. Error bars in d are small and are included within the data solid circles. R\({}_{{\rm{adj}}}^{2}\) adjusted R\({}^{2}\), RMSE root mean square error.
