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Gambica

   

Method F-001G Determination of Alkalis in Cement

This method describes how soda and potash may be estimated in cement using a Sherwood Flame Photometer.

The addition of aluminium nitrate solution suppresses the interference caused by concentrations of lime up to 350mg/litre which is the equivalent concentration for cement, allowing for the effect of silica and a alumina already present in solution.

For materials other than cement, the addition of aluminium nitrate can be adjusted according to the lime concentration expected.30ml of the solution is sufficient for a Ca0 concentration of 1 g/litre. Excess of aluminium nitrate is not harmful.

Equipment Required:

  1. Sherwood Flame Photometer
  2. Balance weighing +-0.0005g
  3. Whatman No.40 filter paper
  4. Sodium chloride (analar)
  5. Potassium chloride (analar)
  6. Concentrated HCI
  7. Aluminium (high grade)
  8. Concentrated HN03

Reagent Preparation:

Standard Solution

This contains 10mg/litre K20 and 5mg/litre Na20. Dissolve 0.7915g KCI (analar) and 0.4715g NaCI with distilled water in the same 1 litre volumetric flask. Dilute to the mark with distilled water and mix well by inversion. This solution contains 500mg/ litre K2O. Dilute exactly 20mls of this solution in a 1 litre volumetric flask with distilled water and add 10ml of concentrated HCI very slowly. Dilute to the mark with distilled water.

This is the universal standard containing 10mg/litre K20 and 5mg/litre Na20 and can be used for all purposes.

Aluminium Nitrate Solution:

Dissolve 30g of aluminium known to have a low alkali content, (high grade) in 400ml of 50% HN03. Add a few drops of mercuric chloride to aid solution of aluminium. When the aluminium has dissolved and the solution cooled dilute to 1 litre with distilled water.

Method:

  1. Mix 0.5g cement with 20-30ml of distilled water, add 5ml of concentrated HCI and dilute to about 50ml.
  2. Heat the solution to boiling.
  3. Filter the solution through a 9cm No.40 Whatman filter paper into a 500ml volumetric flask.
  4. Wash the filter paper 6 times with hot water.
  5. Add 10ml of the aluminium nitrate solution.
  6. Cool the solution and dilute to the mark with distilled water.
  7. Prepare a series of standards by dilution of the universal standard ensuring that each prepared standard contains the equivalent of 1 ml of concentrated HCI per 100mls.
  8. Aspirate the set of standards directly into the Flame Photometer and plot a calibration curve of Na20 concentration vs meter reading.
  9. Aspirate the cement sample solution and read off the Na20 concentration from the calibration curve.
  10. Repeat stages 8 and 9 using the potassium interference filter to determine the K20 concentration in the cement sample.
  11. Calculate the alkali content of the cement by multiplying the mg/l K20 and Na20 present in the sample solution by the dilution factor.

12 September, 2008