Method 59b Determination of extractable phosphorus automated method

An automated method for the Lachat QuikChem Automatic Flow Injection Ion Analyzer is given in Missouri Agricultural Experiment Station (1998), pp. 27-29, and is available free of charge from Lachat (Lachat Instruments, 1988). Sun et al. (1981) describe a method for the Tecator FIAstar® flow injection system. A method for a segmented continuous flow procedure for both phosphate and potassium was devised by Armitage (1965). The parameters for the phosphate analysis using a dilute HCl soil extractant are outlined below. The manifold diagram (Fig. 5.3) has been modified to allow for the fact that Armitage later changed the Sampler I to a Sampler II module.

To waste

To waste

Colorimeter 660 nm

To sampler wash

Colorimeter 660 nm

To chart recorder

Sampler 40 h


2.50 ml min

To sampler wash

0.60 ml min

1.60 ml min

0.80 ml min"

3.40 ml min

1.20 ml min

1.60 ml min sample ascorbic acid molybdate air

0.3 M HCl wash waste flowcell waste

Fig. 5.3. Manifold for the automated determination of phosphorus in soil extracts.


• Hellige-Truog extractant, 0.3 M HCl - dilute 25.8 ml hydrochloric acid, 36% m/m, to 1 l.

• Ammonium molybdate reagent - dissolve 4 g powdered ammonium molyb-date in 1 l of water. Slowly with stirring add 80 ml sulphuric acid, 98% m/m H2SO4, and dilute to 2 l.

• Ascorbic acid, 0.1% (m/v) - prepare fresh daily.

Procedure. The above reagents, together with the segmenting stream of air, are mixed in a double mixing coil, with the sample being introduced halfway along the coil. A further single mixing coil provides a final mixing before the solution passes through a single glass coil in a thermostatically controlled heating bath at 95°C. After development of the colour, the solution passes to a debubbler, whence it proceeds to the colorimeter fitted with a 15-mm flow-cell, and a pair of 660-nm interference filters. The output is to a chart recorder or personal computer. Standards and blanks should be prepared in the extractant solution according to the normal protocol. It is expected that a sampling rate of 40 h-1, with a time ratio of 2:1 sample:wash would be suitable. This may need adjusting with soils of widely differing extractable P concentrations, where large peaks may obscure very small ones.


1. A pump tube has been included for the wash waste for reservoirs that do not have a gravity overflow.

2. The pump tubes in some published manifold diagrams are labelled with the internal diameters of the tubes. We will use the more common convention of labelling with flow rates in ml min-1.

3. Pump tubes are designated by a colour code, which may also be used in the literature. Some common colour codes are listed in Table 5.1.

4. Flow rates are for standard PVC tubing. It is good practice to avoid using the smallest or largest sizes whenever possible.

5. Pump tubes are available in different materials depending on the liquid to be pumped, e.g. for solvents or concentrated acids. Flow rates for these other materials will be different than for standard PVC. Pump tubes may also be available in a specially calibrated or measured flow rate quality at extra cost. Unless specified for medical purposes or to meet regulations, the standard quality is normally adequate. See Chapter 1, 'Peristaltic pumps'.

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