Metrohm reveals sodium determination method

Metrohm has announced a technique to determine the sodium contribution of solvent-extractable organic compounds in Bayer process liquor.

The majority of organic compounds in Bayer process liquor are present as sodium salts.

Knowledge of the sodium contribution of organic compounds in Bayer process liquor is important in seeking causes for organic-related process disturbances, evaluating the effect of processes to remove these compounds and establishing sodium mass balances.

Modern chromatographic methods such as high-performance liquid chromatography (HPLC) and ion chromatography (IC) permit the quantitative determination of lower molecular mass organic compounds and to compute the sodium contribution of these compounds to the total sodium content of the liquor.

High molecular mass organic species have been implicated in a variety of Bayer process disturbances.

However, speciation and quantification of these compounds present greater challenges and it is difficult to apportion the sodium contribution that these compounds make to the total sodium content of the liquor, particularly in a process control environment where simpler, more robust analytical instrumentation is preferred.

It is well-known that a considerable proportion of these higher molecular mass compounds may be extracted into certain non-aqueous solvents after the liquor has been acidified.

It was considered that the gross acid value of the extractable organic compounds could be determined by a non-aqueous titration with standard base in a suitable alcohol.

By knowledge of the amount of base consumed in the neutralisation titration, the total sodium contribution of these extractable organic compounds could be determined.

Potentiometric titrations in non-aqueous media present a number of challenges for the analyst.

These include the dehydration of the necessarily hydrated glass membrane of the pH electrode, the poor electrical conductivity of such non-aqueous solutions, the frequent and rapid fouling of both the glass membrane and the reference electrode and the sensitivity of the very high impedance pH signal to static electrical disturbances.

Thermometric titrimetry offers obvious advantages over potentiometric titrimetry in non-aqueous media, the relatively low impedance sensor requiring no electrical contact with the titration media.

However, the reaction enthalpy of base with many weakly acidic species in non-aqueous media is low and endpoints are often indistinct and imprecise.

Several thermometric titration techniques involving catalytic indicators have been developed to enhance endpoints (Vaughan and Swithenbank in Vaughan, 1973; 2003), but none of these has been proven to have wide ranging industrial application.

A thermometric titrimetric procedure has been developed recently for the determination of free fatty acids (FFA) in edible oils from exotic plants (Carneiro et al, 2002) and this has been extended to cover the determination of FFAs in a range of other plant and animal fats and oils (Metrohm 2006).

The procedure has also been found to be very satisfactory for the determination of weakly acidic species in mineral oils as total acid number, or TAN (Metrohm, 2007).

By using a catalytic endpoint indicator, Carneiro et al showed that sharp, highly reproducible endpoints could be obtained from titrations which otherwise would have revealed little or no inflection at the endpoint.

The catalytic indicator is paraformaldehyde, which undergoes an endothermic depolymerisation with the first trace of excess hydroxyl ions after all acidic species have been neutralised.

This work reports on all successful application of this technique to the determination of extractable weakly acidic species in Bayer process liquor.

Back to top