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Measurement of Sodium
and Potassium in Cheese 

Dried milk is dissolved in warm water.  The test solutions and reference solutions are measured directly following dilution with deionised distilled water.    



  1. 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.

  2. 1.25grams of cheese was accurately weighed out on a 5-figure analytical balance and recorded for backtracking the concentration to the sample size.

  3. 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 of both the sample and acid added to avoid boiling sample being spilled as it was heated.

  4. 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.

  5. 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.

  6. 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.

  7. 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.)

  8. The standard was calibrated starting with the lowest concentration standard, working up the concentration gradient over the 5 standards.


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.1 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.


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.

1 AOAC International, ‘AOAC Official Method 990.23: Sodium and Potassium in Dried Milk’, AOAC International (2008), 33.5.12.

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