The following method using Fluoride Ion Selective electrode allows the measurement of phosphates using a Gran's plot scheme. This method is ideally suited to measurement of phosphate of 0-20 ppm. Lanthanum Nitrate is added to the sample. Lanthanum phosphate is precipitated and the level of excess Lanthanum is titrated with Sodium Fluoride, the level of phosphate is then determined by the difference.

Equipment Required:

  • Gran's plot paper 10% Volume Corrected
  • Digital pH meter
  • Fluoride Electrode
  • Double Junction Reference Electrode
  • (0.1M Sodium Chloride External Filling Solution
  • Magnetic Stirrer
  • 10ml Burette

Reagents Required:

  • 82gm of Sodium Acetate
  • 57m1 Glacial Acetic Acid
  • 4.33gm La (NO3)3. 6H2O
  • 20gm BaCI2 (or 26gm Ba(NO3)2 made up to one liter with distilled water.


  • 0.03M NaF dilute 0.01m Standard Fluoride
  • Phosphate Standard
  • 0.01M Sodium dihydrogen phosphate 1.38gm of Na H2PO4.H20 made up to one litre with distilled water.


Run a calibration curve. Put 100ml of distilled water in a beaker. Using a stirrer to provide mixing add Fluoride in 1ml increments up to 10ml and take readings of observed mi Hi volts.

Scale the vertical axis so that the 10ml point falls near the top of the paper. Draw a straight line through the points it should intercept at the origin. The vertical axis of the paper is now calibrated.


Place 100ml of distilled water in a beaker. Add 2ml reagent using a stirrer to provide mixing record and plot millivolt reading corresponding to additions of 6, 7, 8, 9 and 10ml of titrant. Draw best straight tine between the points it should intercept at 2ml.


Place 100 ml of distilled water in a beaker, add 2ml reagent and lm1 of 0.01M phosphate standard. Use a stirrer to provide mixing, record and plot the millivolt readings obtained by adding 6, 7, 8, 9, 10ml. Draw the best straight line and obtain the intercept - it should fa11 at 1ml.


Place 100ml of sample in beaker add 2ml of reagent, using a stirrer to provide mixing plot millivolt readings corresponding to additions of titrant and extrapolate a straight line through the points to the horizontal axis. Read the unknown phosphate concentrations from the horizontal axis.

Choice of reagent.

Lanthanum has been chosen to precipitate the phosphate since Lanthanum is one of the few cations which will form a very insoluble phosphate precipitate while having a soluble sulfate salt.

Sulfate tends to cause scatter of the points on the Gran's plot decreasing . the precision therefore. Barium is added to the reagent to precipitate the sulfate ion.


Calcium and other divalent ions will interfere if present at sufficiently high levels to form fluoride precipitates.

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