LABORATORY
1.Pipette 50.0 mL or an appropriate amount diluted to 50 mL of digested sample into an acid cleaned, dry 125 mL Erlenmeyer flask.
2.Add 1 drop of phenolphthalein indicator. If a red color develops, add 5 N sulfuric acid until the color disappears.
3.Add 8.0 mL of combined reagent and mix thoroughly.
4.Allow at least 10 minutes (but not more than 30 minutes) for color development.
5.Measure absorbance at 880 nm using a reagent blank to zero the spectrophotometer.
The reagent blank is made using 50 mL of distilled water carried through the digestion step and ascorbic acid procedure.
In highly colored or turbid samples, prepare a sample blank by adding all the reagents to the sample except the ascorbic acid and potassium antimonyl tartrate. Subtract the absorbance of this blank from the absorbance of the sample.
6.Check the sample’s absorbance against the calibration curve and determine the concentration. Correct for dilution.
Blank: A preliminary analysis omitting only the sample to provide an unbiased reference point or baseline for comparison. The Blank is usually run on distilled water.
CALIBRATION CURVE
Since the phosphate concentration is measured as a function of absorbance, a standard curve of absorbance versus known phosphate concentrations must be prepared. Six standard phosphorus concentrations and a distilled water blank are treated with the same digestion procedures as the samples. These 6 values are used to plot absorbance versus phosphate concentration to give a straight line passing through the origin.