The purpose of this application note is to demonstrate that caustic (NaOH) and sodium carbonate in aqueous process streams can be simultaneously measured. Specificatically that sodium carbonate concentration can be accurately determined at low (<2%) caustic concentrations, using a ClearView db NIR photometer and fiber optic probe.

The dominance of water in the NIR hampers the measurement of caustic. Nevertheless, there are several successful reports of using full-spectrum FT-NIRs and discrete wave-length photometers for moderate caustic concentrations. Watson and Baughman (Spectroscopy, 2(1) 44-48 (1984)) summarized a typical approach using the spectral region from 1700 to 2300 nm and narrow optical cells of <1 mm. Their calibration (Figure 1) is a caustic calibration used when other species are present, such as carbonate and Na2S. Our lower range of interest is circled on their calibration.

How to Measure Caustic & Carbonate Concentrations using NIR Process Spectroscopy

We used the same procedure as described in our article (Spectroscopy, 15 (2) 40-46 (2000)). Samples were thermostated in a sample cell at 35 °C and analyzed with our spectrophotometer. A partial least squares (PLS) analysis of the spectral calibration set was performed. This analysis revealed that several analyzing wavelengths were required in addition to a reference wavelength for both measurements. A ClearView db photometer was configured with those wavelengths. The samples were re-run in the ClearView db and their absorbances were recorded. Multiple linear regression (MLR) coefficients were then calculated.

The resulting MLR calibrations are shown in Figures 1 and 2. Months of online data have demonstrated the linearity of the caustic calibration with a precision of better than ± 0.1. We have extended this approach to another customer’s sample set in the 1-10% caustic range with an R2=0.9938 and an SEE of ± 0.47. It is pointed out that temperature conditioning of the sample in the slipstream to better than 2 °C is important, primarily for caustic. The ClearView db has the capability to correct the calibration for wider temperature fluctuations using the measured sample temperature from an RTD sensor. Nevertheless, this places demands on the sampling system containing the fiber optic probe. Due to the likelihood of cleaning the sapphire windows in the probe with HCl, we recommend a probe with a Hastelloy® body for concentrations over 20%, titanium for concentrations 60-80%, and Kalrez® 6375 wetted o-ring seals.

Guided Wave’s multiple wavelength ClearView db photometer has the sensitivity and stability to measure low levels of caustic in the presence of carbonate.