THE DETERMINATION OF FLUORIDE IN URINE

 

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

2. Corning fluoride ISE
3. Double junction reference electrode
4. Lead for reference electrode
5. Magnetic stirrer
6. 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.

Method

 

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).

 

7. 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.

Calculation

Calculate DE by E₂ – E₁ (mV).

The sample solution concentration is given by this equation.

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.