Quantitative Analysis of NO3-, SO42-, ClO4- With Water as Internal Standard by Raman Spectroscopy

Internal standard is often required when using Raman spectroscopy for quantitative analysis due to the poor reproducibility of the Raman spectrum. In aqueous solutions, the stretching vibration Raman peak of water at 2700 similar to 3900 cm(-1) has a strong intensity and may be used as an internal standard, but the interaction of water and solute will cause the shape of the water stretching vibration Raman peak to change. In addition, the concentration of water will also change with the change the solute concentration. When the solute concentration is high, the water concentration needs to be corrected. Taking these two factors into consideration, quantitative analyses of NO3, SO42, ClO4 in aqueous solutions with Raman spectroscopy are investigated, focusing on evaluating water as an internal standard. The Raman spectra of different concentrations of NaNO3, Na2SO4, NaClO4 solutions show that with the increase of salt concentration, the Raman peak of water in the range of 2700 similar to 3 900 cm(-1) presents a trend that the left shoulder drops and the right shoulder rise. However, there exists a good linear relationship between A(salt)/AH(2)O and c(sa)(lt)/cH(2)O in NaNO3 Na2SO4, NaClO4 solutions, where A represents the area of the Raman peak and c represents the concentration, and the R(2 )of the three fitting curves are 0. 999 1, 0. 999 1, 0. 999 4, respectively. This indicates that the Raman scattering coefficients of acid ions and water do not change or change in the same proportion. So, although the shape of the water Raman peak having changed, the feasibility of water as an internal standard is not affected. After introducing the correction of the concentration of water, it is theoretically deduced that c(salt) and conform to the relationship: =AR(s)/(1 + BRs), where R-s = A(salt)/AH(2)O. In a wide concentration range from 0. 1 mol . L-1 to near saturation, the standard working curves for NaNO3, Na2SO4, and NaClO4 are obtained to be c(NaNO3) =18. 8R(s)/ (1 0. 6R(s)) (R-2 = 0. 999 1), cNa(2)SO(4) = 20. 2R(s)/(1+ 1. 0R(s)) (R-2 = 0. 998 8) , and cNaClO(4) = 15. 0Rs/(1+ 0. 7R(s)) (R-2 = 0. 999 8) , respectively. The limit of detection (LOD) of NaNO3, Na2SO4 and NaClO4 are found to be 0. 008 0, 0. 005 2 and 0. 007 3 mol . L-1, respectively. On the basis that the shape change of the water Raman peak does not affect its feasibility as an internal standard, when there are two salts in a solution, a water concentration correction for the second salt can be made to improve the quantitative analysis based on the standard curves for the single salt solutions. However, the correction result is limited when the second salt concentration is too large, and the first salt concentration is relatively small because the accuracy of the Raman peak area of the first salt will be affected due to the too large Raman intensity of the second salt.