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Application Note

Determination of Ion Concentration in Drinking Water

Drinking water is relatively simple to determine the concentration of ions in via flame photometry. This is due to there being very little contaminants that can cause adverse effects within the overall matrix of the sample solution. In this method the analytes of interest are the full range of ions available to be determined via use of a BWB-Tech Flame photometer, being Sodium, Barium, Calcium, Potassium and Lithium and will be referred to henceforth by their chemical names.

Typical values in drinking water are as follows, Na 20-250mg/l, Ba 0.23-2.5mg/l, Li (Trace) 0.1413mg/l, Ca 30mg/l and K 2.5-5.2mg/l.

For this method you will require:

  1. 1 1L Volumetric Flask

  2. 10 100mL volumetric Flasks

  3. A 10ml pipette and pipette filler

  4. A BWB-Tech Flame photometer

  5. 10,000mg/l stock solutions of Ba, Na, Ca, Li and K

  6. 10% HCl if using glass volumetric flasks

  7. DI water

Initially to determine the concentration of these analytes, a set of standards are required. In this method a stock solution containing all ions of interest will be diluted into a 1L volumetric flask at the concentration of the most concentrated standard.

To the 1L volumetric flask 25ml of 10,000mg/l Na Standard was added, then all remaining ion stock solutions were diluted in a 1:10 ratio (eg: 10ml into a 100ml volumetric flask and 90ml of DI water added) to make 4 1000mg/l solutions. From these solutions 2.5ml of Ba standard, 0.5ml Li standard, 30ml of Ca standard and 6ml of K standard were added to the 1L volumetric flask and if the 1L volumetric flask is made of glass, a few drops of 10% HCl was also added.

The 1L volumetric flask now holds a solution containing 250mg/l Na, 2.5mg/l Ba, 0.5mg/l Li, 30mg/l Ca and 6mg/l K. From this standard 20, 40, 60, 80 and 100ml of the solution was added to the 100ml volumetric flasks as well as a DI water blank solution and was made up to 100ml with DI water if needed. The volumetric flasks were labeled with their concentration with the following labels.

  1. 0mg/l DI water Blank solution

  2. Na 50 mg/l, Ba 0.5 mg/l, Li 0.1 mg/l, Ca 6 mg/L, K 1.2 mg/l

  3. Na 100 mg/l, Ba 1 mg/l, Li 0.2 mg/l Ca 12 mg/l, K 2.4 mg/l

  4. Na 150 mg/l, Ba 1.5 mg/l Li 0.3 mg/l, Ca 18 mg/l, K 3.6 mg/l

  5. Na 200 mg/l, Ba 2 mg/l, Li 0.4 mg/l, Ca 24 mg/l, K 4.8 mg/l

  6. Na 250 mg/l, Ba 2.5 mg/l, Li 0.5 mg/l, Ca 36 mg/l, K 6 mg/l

Using these standards, form a calibration curve on the flame photometer starting from the lowest concentration to the highest concentration standard to avoid residue from previous standards effecting the reading of the next standard. Once the calibration curve is complete run the Tap water sample through the flame photometer in read mode to acquire the full spectra of analytes available to the BWBTech Flame Photometer’s concentration.

If the results from the drinking water is higher than the most concentrated standard, add a small amount extra to a new 1L volumetric stock and repeat the dilutions into the standards accounting for the increase in concentration upon their labeling.

For example, if 300mg/l Na was being read from the flame photometer, adding 30ml of 10,000mg/l stock solution instead of 25ml stock, then adjusting the labels to 60 mg/l, 120 mg/l, 180 mg/l, 240 mg/l and 300 mg/l of Na respectively in their final standards (100ml volumetric flasks) whilst maintaining the levels of the other ions in the standard. Please note that by adding an extra 5ml of 10,000 mg/l to the standard made initially (if the standard is too low in concentration) will dilute the concentration of all the other analytes of interest down rendering the stock solution unusable.

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