In this method, F– concentrations in SE are determined using an ion-selective electrode in conjunction with a calomel reference electrode and a high impedance millivoltmeter (APHA-AWWA-WEF 2005b). The electrode measures F– activity rather than concentration, which is influenced by the total ionic strength of the sample. To compensate for variations in matrix ionic strength, measurements of samples and standards are carried out in the presence of a buffer solution. The buffer solution is necessary because the electrode also responds to OH– under alkaline conditions. In an acid medium, F– will complex with H+ ions to form HF complexes. Raising the pH to 5–6 helps to break F– complexes with Al, Fe and Si. A chelating agent is included in the buffer to preferentially complex polyvalent ions and thereby release any bound F–. Nicholson and Duff (1981) report that addition of tri-ammonium citrate to the buffer gives an improved performance of the ion-selective electrode.
The electrode method is suitable for F– concentrations from ≈0.1 to well over 1.0 mg/L. It is not suited for use when aqueous SE contain >10 000 mg/L of dissolved solids. Make appropriate adjustments to volumes of samples and reagents if the quantities of SE for this test are <25 mL.
TISAB Buffer (Total Ionic Strength Adjustment Buffer)
To ≈500 mL deionised water in a 1.0 L beaker, add 57 mL glacial acetic acid (CH3COOH), 58 g sodium chloride (NaCl) and 4.0 g of 1,2 cyclohexylenediaminetetraacetic acid (CDTA; also known as 1,2 cyclohexylenedinitrilotetra-acetic acid). Stir to dissolve. Place beaker in a cold bath and adjust pH to 5.4 by slowly adding 10% sodium hydroxide (NaOH). Transfer to a 1.0 L volumetric flask and make to volume with deionised water.
Fluoride Primary Standard
1 L contains 100 mg F.
Dissolve 0.2210 g anhydrous sodium fluoride (NaF; previously dried at 110°C for 2 h) in deionised water and make to 1.0 L in a volumetric flask. This solution is stable for at least one month when stored at ≈4°C.
Fluoride Intermediate Standard
1 L contains 10 mg F.
Dilute 100 mL of F Primary Standard with deionised water and make to 1.0 L in a volumetric flask.
Fluoride Working Standards
Pipette 0, 2.5, 5.0 and 10.0 mL of F Intermediate Standard to 100 mL volumetric flasks, add 50 mL of TISAB Buffer to each and dilute to 100 mL with deionised water. These Working Standards contain 0, 0.25, 0.50 and 1.0 mg F/L, respectively.
Fluoride specific ion electrode.
Reference calomel electrode (sleeve type) (see Note 1).
Ion-selective meter or digital pH meter.
Set up the ion-selective meter (or equivalent), equipped with a F specific ion electrode and a sleeve-type reference calomel electrode, and associate millivolt values to F– concentrations across the range of Working Standards. When known to be operating within specifications, pipette 25 mL of each SE into separate 50 mL volumetric flasks and dilute each to volume with the TISAB Buffer. Maintain all solutions at the same temperature, preferably 25°C. Transfer these diluted samples individually to clean 100 mL beakers, immerse the electrodes and begin gentle mechanical stirring. After 3 min, read the millivolt meter. Continue stirring until a stable reading is obtained. Record the final value. Rinse the electrodes, blot dry between readings and continue with each sample in turn. Also measure the undiluted F Working Standards.
Prepare a calibration curve (or regression) relating millivolt values to F– concentrations of the Working Standards. Determine the F– concentrations of the test samples from the calibration curve. Frequently, recheck the values for the Working Standards and recalibrate if necessary. Dilute (with deionised water) a sub-sample of any (original) samples that exceed the range of F Working Standards, add TISAB as previously described, mix well, then reanalyse, adjusting for the dilution employed. Make due allowance for any reagent blank.
Report F–/SE (mg/L) with relevant method codes, or divide by 18.998 to convert to cmolcF/L.
1. A ‘sleeve-type’ reference electrode is preferred to a fibre-tip reference electrode, since the latter are inclined to exhibit erratic behaviour when used with very dilute solutions.