AOAC International, ‘AOAC Official Method 990.23: Sodium and Potassium in Dried Milk’, AOAC International (2008), 33.5.12.
Cheese was wet ashed with a combination of acid and a high temperature oven to form a dissolvable product ready for analysis via flame photometry
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 surfaces. If only glass is available, the sodium determination was carried out rapidly.1.25grams of cheese was accurately weighed out on a 5-figure analytical balance and recorded for backtracking the concentration to the sample size.
The sample was then added to a crucible of appropriate size and 10ml of ~30% sulphuric acid. It was ensured that the crucible was less than half full with both the sample and acid added to avoid boiling sample being spilt as it was heated.
The oven was heated to 600 degrees Celsius and the crucible was weighed on the 5-figure analytical balance. The crucible was then added to the oven and left to ash over the course of 2 hours.
The sample was then reweighed and placed back into the oven for 5 minutes. The crucible was then weighed again, if the weight of the crucible has not changed from the previous weight then the sample was finished with the ashing process, if it had significantly changed the process was repeated until all combustible content of the sample had been removed.
The ashed sample was then dissolved in 54ml of DI water with 1ml of 10% HCl. This was then filtered through a funnel and Whatman n.5 filter paper into a 100ml polyethylene volumetric flask and made up to 100ml with DI water.
A minimum of 5 standards were produced with a concentration range of 0-100mg/l potassium and sodium, to each standard 10ml of ~30% sulphuric acid was added and 1ml 10% of HCl was added. (If the sample contained a higher concentration than these standards, the sample solution was further diluted to bring the concentration under 100mg/l.)
The standard was calibrated starting with the lowest concentration standard, working up the concentration gradient over the 5 standards.
If calcium determination was required there will be a certain amount of phosphate interference present, to negate this effect 10ml of 1gram per litre lanthanum chloride solution was dosed into all the samples and standards.