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Gambica

   

Method F-016G The Determination of Sodium in Fuel Oil

The sample is burned in air in a silica crucible on a hot plate and the residue ashed in a muffle furnace at 525°C. The ash is dissolved in sulphuric and nitric acids and the sodium content is determined using a Sherwood Flame photometer. This method is suitable for concentrations of sodium from 1 ppm upwards.

Equipment Required:

  1. Sherwood Flame photometer
  2. Silica crucible
  3. Hot plate
  4. Muffle furnace
  5. Accurate balance

Reagents:

  1. 1000 ppm sodium stock standard solution: Dissolve 1.5435g anhydrous sodium sulphate (analar) in distilled water and dilute to 500ml. Store in an airtight polythene bottle.
  2. Concentrated sulphuric acid
  3. Concentrated nitric acid

Working Standards Preparation:

Prepare 5 ppm sodium by diluting 5ml of the stock standard to 500ml with delonised water in a 1 litre volumentric flask. Add 20ml of concentrated H2SO4 and 20ml of concentrated HNO3 and mix well. Dilute to the mark with delonised water.

CAUTION: Never add water directly to concentrated acids. HNO3 and H2SO4 must be handled with great caution.

>From the acidified 5 ppm sodium standard prepare a series of diluted standards ensuring that each contains the equivalent of 20ml concentrated HNO3 and 20ml concentrated H2SO4 per litre.

Method:

  1. Heat the sample for 30 minutes at 60°C. Shake well.
  2. Transfer about 10g into a clean, dry vessel and leave to cool to room temperature.
  3. Weigh 10g accurately into a clean, dry silica crucible.
  4. Heat the crucible and contents on a hot plate and ignite the oil. Re-ignite if necessary to produce a solid residue. Perform this step in a fume hood.
  5. Transfer the crucible to a muffle furnace heated to 500°C and maintain the temperature until all of the carbon is burned off.
  6. Allow to cool. Rinse down any ash from the side of the crucible with a few ml of deionised water.
  7. Add 1 ml of sulphuric acid (conc.) and 1 ml of nitric acid (conc.).
  8. Replace on the hot plate and heat until fumes of SO3 evolve.
  9. Allow to cool. Add 10ml of deionised water.
  10. Wash the contents of the crucible into a 50ml volumetric flask and dilute to the mark with deionised water. Carry out the test as soon as possible.
  11. Aspirate the series of standards into the flame photometer in increasing concentrations using delonised water, containing the equivalent of 20mls of concentrated H2SO4 and 20mls concentrated HNO3 per litre, as the blank solution.
  12. Plot the concentration of the standards against the meter reading obtained to attain the calibration curve.
  13. Aspirate the sample solution into the flame and from the meter reading obtained interpolate the sample concentration.
N.B. Dilute the sample if necessary using the blank solution to bring the sodium concentration within the standard range.

Calculation:

To obtain the weight of sodium in mg present in the original 10g of sample multiply the sample concentration reading obtained by 0.05.

12 September, 2008