Determination of Sodium and Potassium using Emulsions and Microemulsions for Sample Preparation
This technique allows use of aqueous standards for calibration rather than relatively unstable organometallic standards and harmful solvents for the extraction.
Microemulsions are thermodynamically stable systems
composed of water, oil and surfactant. In some cases, an
alcohol is added as a co-surfactant.
To prepare a microemulsion of biodiesel or vegetable oil, a sample is first mixed with a surfactant such as Triton X-100 and a surfactant such as propanol, n-butanol or n-pentanol and dilute nitric acid. The emulsion is obtained when a single transparent phase is formed. The composition of the emulsion is 56.7% alcohol, 20% biodiesel or vegetable oil, 14.4% of Triton X-100 and 8% water given
as w/w percentages.
Due to Sodium plating out onto glassware, polypropylene glassware should be used to avoid misreading’s of sodium.
The microemulsion is prepared in 5ml volumetric flasks. To make up a 5ml solution of microemulsion 1ml biodiesel or vegetable oil is added to a mixture of 0.7ml Triton X-100 and 2.83ml propanol with 0.45ml dilute nitric acid and mixed until a single transparent phase was formed.
It is important to note that a concentration of Nitric acid is not given in this method as dilute nitric acid comes from retailers at different concentration levels. Anything between 5-10% is considered dilute nitric acid. It is only added to support emulsion formation and as long as a consistent concentration is used throughout the method, the impact of using 5% nitric in comparison to 10% nitric was negligible.
For the calibration standards, 20% (w/w) of base oil was used in the microemulsion to simulate the biodiesel or vegetable oil phase. The standards were prepared from a NaCl or KCl stock solution in the aqueous phase to give a final concentration range of 0.0 - 4.0mgl-1 in 5ml over 5 standards (0, 1, 2, 3 and 4 mg/l-1 respectively).
If desired, a blank solution made up of the emulsion can be prepared and ran as a blank prior to calibration to reduce interference from the sample matrix.
The LOD (limit of detection) cited for this method, for measurement of sodium and potassium was 0.1μg g-1 and 0.06μg g-1 respectively, as compared to the reference method (BS EN 14108) 0.2μg g-1 for sodium and 0.13μg g-1 for potassium (BS EN 14109).
The LOQ (limit of quantitation) for this method is given as 0.3 μg g-1 and 0.2 μg g-1 respectively and 0.6 μg g-1 for sodium and 0.4 μg g-1 for potassium for the BS methods.
Due to low pH values having an influence on sodium emission levels, where by emission is reduced.  However due to the solution being based on an emulsion, any attempt to increase pH would reduce the oil-water emulsions stability. To counteract this, it is recommended to dose an equivalent
proportion of nitric acid into your standards that was added to the sample.
AA Perkin. (1996). Analytical Methods for Atomic Absorptions Spectroscopy. page 30.
Strassner, J.E. 1968. Effect of pH on Interfacial Films and Stability of Crude Oil-Water Emulsions. J Pet Technol20 (3): 303-312. SPE-1939-PA. http://dx.doi.org/10.2118/1939-PA.