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
- Corning pH/ion meter or
pH meter with millivolt scale
- Corning fluoride ISE
- Double junction reference electrode
- Lead for reference electrode
- Magnetic stirrer
- Accurate balance weighing to ± O.OOO5g
Reagents Required
- Sodium fluoride, (analar)
- Acetate buffer (1 molar)
- Perchloric acid (30%)
- 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
- Acidify 2 mls of the urine sample to a pH of 2-3 using 30% perchloric acid (approx 6-8 drops).
- Dilute the sample accurately to 10 ml with the acetate buffer solution.
- Fill the outer salt bridge compartment of the double junction reference electrode with 1.0M KNO3.
- Rinse the electrodes in the acetate buffer solution.
- Whilst on a stirrer immerse the electrode into the sample solution and record the stable potential (E1).
- Add a 50 microlitre aliquot of the standard fluoride solution to the sample solution (continue to stir).
- 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.