Metrohm discusses the determination of 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.
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.
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 but none of these has been proven to have wide ranging industrial application.
A thermometric-titrimetric procedure has been developed for the determination of free fatty acids (FFA) in edible oils from exotic plants and this has been extended to cover the determination of FFAs in a range of other plant and animal fats and oils.
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.
By using a catalytic-endpoint indicator, sharp, highly reproducible endpoints could be obtained from titrations that 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.
Allow 10min for pipette to drain properly, due to the viscous nature of the liquor.
Place on a stirrer and, while stirring, carefully neutralise and then acidify with 32 per cent (w/v) hydrochloric acid until the solution is clear.
Cool the solution and transfer to a 250ml separating funnel.
Extract this solution with 5 x 25ml aliquots of cyclohexanol, allowing adequate time after each extraction for a full separation of the two phases.
Return the collected cyclohexanol extract to a clean separating funnel and wash with 10 x 50ml amounts of saturated NaCI solution.
Quantitatively transfer the washed cyclohexanol extract to a 200ml volumetric flask and make to a volume with cyclohexanol.
Dry the solution overnight in a sealed container with 50g anhydrous Na2SO4 freshly dried at 120C using a magnetic stirrer to ensure efficient drying.
Depending on the organic content of the original liquor, transfer 25 to 40ml of the dried cyclohexanol extract into a 200ml volumetric flask and replenish with dry AR grade propan-2-ol.
Allow 10min for the pipette to drain properly.
Pipette a 30ml aliquot into a titration vessel, allowing to drain for a timed 3min.
Add approximately 0.5g of paraformaldehyde and titrate to a thermometric endpoint with 0.1mol/l potassium hydroxide in propan-2-ol.
The analysis has been shown to a highly precise and is suitable for following process trends and for investigations into organic removal processes.
However, this work needs to be followed by an investigation into the range of organic species that can be extracted under these conditions.
A comparison of the two titration plots suggests that the spectrum of organic compounds extracted from the spent liquor from alumina refinery 1 might differ from that extracted from the liquor from refinery 2.
This is suggested by the greater rounding of the endpoint, indicating greater non-equilibrium behaviour between the weakly acidic species and the titrant base at the endpoint.
Such non-equilibrium behaviour does not affect the ability of the thermometric titration technique to accurately and precisely locate an endpoint.
Cyclohexanol was chosen as the most suitable solvent for extracting the organic species present in the acidified Bayer process liquor, after qualitative tests based on the colour of the extract suggested that it was superior to toluene.
A highly reproducible procedure has been developed for the determination of the sodium contribution of acid-extractable organic species in Bayer process liquor.
The precision of the method is estimated to be 0.2 per cent RSD.