12
PFP7/REV C/01-08
3.1.1
Sodium
Accurately weigh 0.634g of dry “Analar” quality NaCl, dissolve in pure deionised water and wash
into a 500ml volumetric flask. Fill to the mark with pure deionised water. To prepare the
standard solution for use with the flame photometer, this stock solution should be diluted 1 in
50.
Calculation:
Atomic weight Na = 23.0
Molecular weight NaCl = 58.46
Therefore, 0.634g NaCl contains
0.634 x 23 = 0.25g Na
58.46
Thus in 500ml of solution there is 250mg Na or 50mg Na/100ml.
Diluting 1 in 50 gives a standard of 1mg Na/100ml = 10ppm Na.
3.1.2
Potassium
Accurately weigh 0.477g of dry “Analar” quality KCl, dissolve in pure deionised water and wash
into a 500ml volumetric flask. Fill to the mark with pure deionised water. To prepare the
standard solution for use with the flame photometer, this stock solution should be diluted 1 in
50.
Calculation:
Atomic weight K = 39.1
Molecular weight KCl = 74.56
Therefore, 0.477g KCl contains
0.477 x 39.1 = 0.25g K
74.56
Thus in 500ml of solution there is 250mg K or 50mg K/100ml.
Diluting 1 in 50 gives a standard of 1mg K/100ml = 10ppm K.
3.1.3
Storage
Store solutions away from direct sunlight in a cool place, ideally at temperatures below 25ºC.
Glass containers should not be used for storage as they can affect the sodium concentration
levels. Standards should be stored in sealed, plastic vessels and in high concentrations, (e.g. as
a stock 1000ppm solution) and dilutions prepared as required. The long-term storage of low
concentration standards is not recommended due to degradation of ionic species.
3.2
Sample preparation
There are several practical points regarding sample preparation, which should be adhered to in
order to achieve the required accuracy in your analysis:
1. Avoid handling samples with fingers. This leads to serious contamination, e.g. if a finger
is immersed in 20ml of deionised water the resulting Na concentration will exceed that
of a 10ppm standard.
2. All analyses involve the use of a diluent, which is almost always deionised water. This
should be of the highest quality for accurate flame analysis. Sodium, potassium and
calcium are present in high concentrations in tap water and thus efficient deionisation is
essential.
3. Species that cause interference should be removed from samples or the equivalent
concentration of the interferant should be present in the standards so as to avoid
erroneous results, e.g. if a sample of approximately 10ppm Na contains approximately