AOAC International, ‘AOAC Official Method 990.23: Sodium and Potassium in Dried Milk’, AOAC International (2008), 33.5.12.
Dried milk was dissolved into an aqueous solution for analysis via flame photometry
The test solutions and reference solutions are measured directly following dilution with deionised distilled water.
Procedure
Thoroughly clean and rinse all glassware with deionised distilled water prior
to use. It is recommended that polyethylene is used to ensure that sodium does not plate out onto glassware. If glass is only available then sodium determinations must be carried out rapidly to avoid inconsistencies.The sample should be weighed to nearest 1mg; 1.25g of test sample was weighed for analysis.
Transfer test sample to 50ml glass beaker and dissolve in approximately
20ml warm deionised water (40º - 50ºC), alternately a steam bath can be used to heat the sample to ensure total dissolution into the solvent.Quantitatively transfer contents of beaker to 500mL volumetric flask, cool to 20ºC and dilute to volume. Mix thoroughly via inversion.
Aspirate solutions into flame, beginning with the standard having the lowest concentration. Typically dried whole milk solution contains approximately 10mg/l sodium and 40mg/l potassium.
The results will not be reliable if this solution contains insoluble particles. If insoluble particulate matter is present in the sample, filter the sample through Whatman No.40 filter paper. The filter paper should be weighed on an analytical balance prior to filtration, and then the solution filtered and the paper dried in an oven at low temperature then reweighed and the difference calculated between the pre and post filter paper. This should then be accounted for when calculating concentrations by weight. If calcium determination is required, 5ml of 1g/l lanthanum chloride solution was added to negate phosphate interference in the sample.
Preparation of Standard Graph
Set the flame photometer in accordance to MultiPoint/Single Ion Calibration found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium emission. Nebulise the working standard solutions and adjust the controls until steady zero and maximum readings are obtained. Nebulise the intermediate working standard solutions and construct a graph relating raw emission data (known as RAW in BWB the flame photometer) to concentration of all the standard solutions.