Method F-015G The Flame Photometric Determination of SuIphate
The sulphate present in the sample is dissolved in water and precipitated out of solution using an excess of barium chloride. The precipitate is removed by centrifugation and is then dissolved in ammonium Ethylyenediaminetetra-acetate (EDTA). The
Ba2+ complex formed is then aspirated directly into the flame and the sulphate content of the sample calculated from the barium concentration recorded. Samples must be
free of anions that produce insoluble barium salts other than sulphate.
Equipment Required:
A Sherwood Model 410 Flame photometer with barium interference filter.
An accurate balance weighing to +/-0.0005g
Centrifuge
Reagents:
Sodium sulphate (analar)
Concentrated hydrochloric acid
Barium chloride solution (1 % W/V)
Ammonium EDTA: Dissolve 5g of EDTA in 100ml deionised water, add 50ml of ammonia solution (sp gr 0.88) and dilute to 500ml with deionised water.
Preparation of Stock Standard:
Dissolve 0.6713 sodium sulphate (analar) in deionised water and dilute to 100mls.
This is 2000 ppm S042- stock standard.
Method:
Accurately weigh a sample containing not more than 0.2g of sulphate as S042-. Dissolve in deionised water and transfer the solution to a 100ml volumetric flask.
Add 10ml of concentrated HCI. Mix thoroughly and dilute to the mark with deionised water.
Pipette 5ml of the sample solution into a 10ml calibrated centrifuge tube and add 5ml 0.1 % BaCI2. Mix the solution well and spin in a centrifuge for three minutes at 4000 rpm.
Discard the supernatant liquid.
Re-suspend the pellet in 10ml of deionised water and spin again for 3 minutes at 4000 rpm.
Discard the supernatant liquid.
Add ammonium EDTA solution and stir until the pellet completely dissolves. Use gentle heat if necessary.
Dilute exactly up to the 10ml calibrated mark with the ammonium EDTA solution.
Subject a 5ml portion of the stock sulphate standard to identical treatment (ie. stages 1 -9). This yields a final solution containing 1000 ppm S042-.
Using the ammonium EDTA solution as diluent prepare additional standards of 600, 300 and 100 ppm S042- from the stock standard.
With the barium filter in place aspirate the Top standard (1000 ppm) into the flame and set the display to read 1000
Without touching the controls aspirate other standards to check linearity.
Aspirate the sample into the flame and read the Displayed Barium, hence sulphate, concentration in the sample solution.
Calculation:
To obtain the mass of sulphate in the original sample in mg divide the sample sulphate concentration by 5.
