Method 62b Determination of 2 citric acidsoluble phosphorus method for basic slags Thomas phosphate


• Citric acid monohydrate, crystallized [HOC.COOH(CH2COOH)2.H2O]

• Citric acid extractant - dissolve 10 g citric acid monohydrate in water, make up to 500 ml and adjust to 20°C. This is sufficient for one sample -for a number of samples, increase quantities as appropriate. The concentration may be checked by titrating 10 ml reagent against 0.1 M NaOH using phenolphthalein indicator, when the titre should be 28.55 ml.

Procedure (extraction). Weigh 5.000 g of the finely powdered basic slag sample into a weighing funnel and transfer to a 1-l stoppered bottle. Add 500 ml citric acid extractant at 20°C to the sample while shaking the bottle to avoid caking of the sample. Shake for 30 min; a rotary shaker should be set to 35-40 turns per min. Filter immediately through a dry fluted Whatman No. 4 filter paper; discard the first 20 ml, and collect a sufficient quantity for the analysis in a dry glass receiver. If the basic slag contained 9.2% P2O5 (4% P), the extract will contain 920 pg P2O5 ml-1 (401.5 pg P ml-1).

Procedure (standard curve). Fill a 50-ml burette with the phosphorus intermediate standard solution and dispense 25, 26, 27, 28, 29, 30 and 31 ml into a series of 100-ml volumetric flasks. These will contain 50, 52, 54, 56,

58, 60 and 62 pg P2O5 ml-1 (21.8, 22.7, 23.6, 24.4, 25.3, 26.2 and 27.1 pg P ml-1). Add 25/y ml of 2% citric acid extractant to compensate for that added in the 25 ml sample extract solution, where y is the dilution factor. In the example below, this would be 25/4 = 6.25 ml, which would be most suitably added from a burette. Add 25 ml of the vanadium molybdate reagent and make up to the mark with water (both liquids at 20°C), mix and stand for 10 min.

Using a matched pair of cells, place the lowest standard in the reference beam, and measure the absorbance of the other standards at 420 nm. Plot the standard curve relating absorbance to known concentration.

Procedure (analysis). Dilute the basic slag extract to give (^25 ml) a solution containing 220-248 pg P2O5 ml-1 (96-108 pg P ml-1) at 20°C. Thus, if the basic slag contains 920 pg P2O5 ml-1, a dilution of 25 ml extract to 100 ml will yield a solution of 230 pg P2O5 ml-1 in 0.5% (m/v) citric acid. Pipette 25 ml of this solution into a 100-ml volumetric flask, add 25 ml vanadium molybdate (20°C), make up to the mark with water (20°C), mix and stand for 10 min. Simultaneously prepare a fresh 50 pg P2O5 ml-1 (21.8 pg P ml-1) reference standard containing the same amount of citric acid as the diluted sample extract, against which the absorbance of the samples is measured (see 'Procedure (standard curve)' above). Maintain all solutions at 20°C.

Calculation. Divide the concentration of the sample solution read from the standard curve by 3 (if diluted as in above sample analysis procedure), to obtain the % P, or divide by 1.304 to obtain the % P2O5 in the sample.

Example, if the diluted sample concentration as read from the standard curve was 60 pg P2O5 ml-1, this corresponds to a concentration of:

60 x 100/25 x 100/25 = 960 pg P2O5 ml-1 = 960 x 0.4364 = 418.9 pg P

in the original extract (because 25 ml of citric acid extract was diluted to 100 ml, and 25 ml of this solution was taken and further diluted to 100 ml). There was 500 ml of original citric acid extract, therefore this volume would contain:

500 x 960 pg P2O5 = 480,000/106 g P2O5 = 0.480 g per 5.0 g sample, or 9.60% P2O5 in the original sample.

This is equivalent to 9.60 x 0.4364 = 4.19% P. Since 60 pg P2O5 ml-1 in the final sample solution corresponds to 9.60% P2O5 in the original sample, sample concentrations from the standard curve should be multiplied by 9.60/60 = 0.16 to give the % P2O5, or 0.07 to give the % P in the original sample.

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