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Method F-024A Sodium in Straw by Flame Photometry

Sodium hydroxide is added to cellulose animal feeds to improve digestibility. The sodium concentration is determined using a flame photometer.

Equipment Required:

  1. Sherwood Flame photometer
  2. Balance weighing to +/-0.0005g
  3. Filtration apparatus (or centrifuge)
  4. Volumetric glassware

Reagents:

  1. Sherwood 1000 ppm sodium flame photometer standard
  2. 1 Molar ammonium chloride/1 Molar NH4OH solution
  3. Delonised water
Reagent Preparation - Extractant solution

Dissolve 53.49g NH4CI with deionised water in a 1 litre volumetric flask. In a fume cupboard add to this solution 55 mls of ammonia (SG = 0.88). Dilute to the mark with deionised water.

Standard Preparation:

Dilute the 1000 ppm Na standard 1:100 with deionised water giving a 10 ppm top standard. From this prepare a 6, 4 and 2 ppm standard by dilution with deionised water.

Sample Preparation:

  1. Weigh accurately 3g straw and place in a 100 ml volumetric flask.
  2. Add 80 mls of extractant solution and shake vigorously.
  3. Stand for at least 30 minutes intermittently shaking the solution.
  4. Filter or centrifuge to remove solid matter (Note: the solid must be washed with extractant solution and washings collected with supernatent). In a 100 ml volumetric flask dilute the washings and supernatent to the mark with deionised water.
  5. Dilute this solution 1 in 100 with deionised water. The sample is ready for analysis.

Method:

  1. Set up the flame photometer as outlined in its instruction manual.
  2. Aspirate the standards into the flame photometer noting their readings.
  3. Plot meter reding vs. Na concentration.
  4. Aspirate the sample and note the meter reading.
  5. From the graph interpolate the sample concentration.

Calculation:

To obtain the result in % Na in the original straw sample divide the sample concentration in ppm by 3.

NOTE: If a 410 flame photometer is used and the 10 ppm standard set to 10.0, the calibration scale will be linear, ie. 6 ppm standard will read 6.0. Therefore, the ppm Na in the sample can be red directly from the display. If calibration is non linear then to obtain direct readout dilute all standards and samples 1 in 2 with deionised water and multiply the reading by 2.



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