Method 710a Determination of total phosphorus in plant material by autoanalysis

Reagents.

• Acid-wash solution - wear PPE. Add 250 ml sulphuric acid (approximately 98% m/m H2SO4) to 250 ml water slowly with stirring. Allow to cool, then make up to 500 ml with water and carefully invert to mix.

• Ammonium hydroxide solution - using a fume cupboard, dilute 158 ml ammonia solution (approximately 35% m/m NH3, 0.880 specific gravity) to 1 l. 3

• Ammonium molybdate solution - add 15 g of ammonium molybdate ((NH4)6Mo7O24.4H2O) to about 900 ml warm water and stir to dissolve. Make up to 1 l in a volumetric flask and filter before use.

• Ammonium vanadate solution - Caution: wear PPE! Add 95.5 ml nitric acid (approximately 70% m/m HNO3) to about 800 ml water in a 1-l volumetric flask. Add 0.5 g ammonium vanadate (NH4VO3), stopper and shake until dissolved, then make up to the mark and mix.

• Vanadate-molybdate reagent - mix the ammonium molybdate solution with the ammonium vanadate solution in the ratio 2:3 molybdate:vana-date. For optimum reproducibility, make up fresh each day just before use.

• Standard solutions - see Method 7.6a.

Procedure. The digestion procedure is given in Method 7.7a. The flow diagram is shown in Fig. 7.4.

To ensure adequate colour development, the flow passes through a double heating bath coil at 80°C. See also Note 2 in Method 7.7a. Procedure (autoanalysis). Switch on the autoanalyser modules and allow pumping of reagents for 30 min to flex the tubes. Carefully pour the sample digest solution from the digestion tube into the sample cup. Use 8.5-ml industrial polystyrene cups or similar size if more than one determination is required on the same sample solution. Load the first sampler tray in the sequence of low to high standards. It is recommended that the lowest standard is in duplicate, and the first peak rejected. After the highest standard, aspirate acid-wash solution for about 5 min to ensure the baseline is reached. Load further trays with about 32 sample cups followed by five standards. The remaining three spare cup positions can be used for higher standards or repeats. Again aspirate acid-wash solution to obtain a baseline between trays; this will enable a correction for any baseli ne drift to be made. If possible, keep samples of similar

Fig. 7.4. Flow diagram for determination of phosphorus in plant digest solutions

analyte concentrations together to avoid interference between adjacent low and high peaks. The sampling rate is set at 40 h-1 with a sample:wash ratio of 2:1. When using a chart-recorder, record the sample number on every tenth peak and label the standard peaks; this makes reading the charts easier and enables the identification of problem peaks so that a repeat can be inserted.

Calculation. Draw a baseline under all the peaks by connecting the baselines obtained at the start, between the tray changes, and at the end. Draw a standard curve using a chart reader (see Chapter 1, 'Chart reader'). If exactly 0.1000 g sample was taken, then divide the concentration corresponding to the sample peak, in pg P ml-1, by 100 to give the % total P in DM.

If y g sample was taken, then (pg P ml-1) x 0.1/100y = % P in DM.

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