LABGUIDE
Determination of Fluoride by Ion Selective Electrode
Equipment:
MV / Ion meter, Fluoride electrode (combination), magnetic stirrer, stir bar, stir bar retriever, stop watch.
Apparatus:
Volumetric flasks 250ml, 100ml; Pipettes 50ml, 100ml; beakers 250ml, 100ml; grad pipettes 1ml, 2ml, 5ml;
Sample: Toothpaste (colgate wintergreen). Use 1.5g per sample.
Chemicals:
Sodium fluoride, NaF, FW = 41.99
Sodium chloride, Na Cl, FW = 58.44
Sodium hydroxide, NaOH, FW = 40
CDTA: trans 1,2, diaminocyclohexane N,N,N1,N1,-tetra acetic acid, (also known as cyclohexylene dinitro tetra acetic acid)
C6H10{N(CH2CO2H)2}2.xH2O, FW = 364.34.
Solutions preparations:
1. 1M NaF. Dissolve 42 g of the salt in 1 liter distilled water, and store in a polythene bottle.
2. TISAB. (Total Ionic Strength Adjuster Buffer)
Dissolve 4g CDTA, 57mls acetic acid, and 58g NaCl in about 500mls distilled water, then adjust to between pH 5 and 5.5 with 5M NaOH (200g in 1 liter soln.) and dilute to 1 liter (use a pH meter)
Analytical notes:
1. Make up sample in triplicate.
2. Do a four-point calibration of meter, and measure from lowest concentration to the highest concentration. eg 10-5M to 10-1M
3. Activity / response measurements.
Make up and measure stds, with TISAB, and without TISAB.
4. Fluoride molarity to ppm:
10-1 = 1900ppm
10-2 = 190ppm
10-3 = 19ppm
10-4 = 1.9ppm
10-5 = 0.19ppm
5. SG Fluoride = 1.545
Sodium fluoride = 0.15% w/v fluoride ion.
Operation of Instrument:
Follow Users Guide: obtain help and advice from Demonstrator or Technician
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