The technique used in this procedure is that of a single known addition to minimize sample matrix effects which cause problems with the direct potentiometric approach.

Equipment Required

  1. Corning pH/ion meter or

pH meter with millivolt scale

  1. Corning fluoride ISE
  2. Double junction reference electrode
  3. Lead for reference electrode
  4. Magnetic stirrer
  5. Accurate balance weighing to ± O.OOO5g

Reagents Required

  1. Sodium fluoride, (analar)
  2. Acetate buffer (1 molar)
  3. Perchloric acid (30%)
  4. 1,0M KNO3 (outer reference filling solution)

Standard Preparation                                        

To 2.210g of dessicated sodium fluoride in a 1 litre volumetric flask add deionised water to the mark. This is 1000 ppm F- stock solution.

Reagent Preparation

Add 57 ml of glacial acetic acid to 500 ml deionised water in a 1 litre volumetric flask. Adjust the pH to 5.2 with sodium hydroxide solution and dilute to the mark with deionised water. This is 1 molar acetate buffer. A ten fold dilution of this buffer is used for analysis.



  1. Acidify 2 mls of the urine sample to a pH of 2-3 using 30% perchloric acid (approx 6-8 drops).
  2. Dilute the sample accurately to 10 ml with the acetate buffer solution.
  3. Fill the outer salt bridge compartment of the double junction reference electrode with 1.0M KNO3.
  4. Rinse the electrodes in the acetate buffer solution.
  5. Whilst on a stirrer immerse the electrode into the sample solution and record the stable potential (E1).
  6. Add a 50 microlitre aliquot of the standard fluoride solution to the sample solution (continue to stir).


  1. After the standard addition, record the new stable potential (E2).

NOTE: If a Corning pH/ion meter is used which has a K mode then the sample

concentration is automatically displayed in direct concentration units. If a pH meter with millivolt scale is used, perform the following concentration.


Calculate DE by E2 – E1 (mV).

The sample solution concentration is given by this equation.

Fluoride Ion Selective Electrode

Cu = unknown solution concentration

Cs = standard concentration

Vs = volume of standard

 Vu = volume of unknown

 DE = change in potential (mV)

S = slope of .electrode (mV)

To attain the concentration of fluoride in the original urine sample, multiply the result calculated by 5.

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