Analytical Methods:Phosphate Qualitative

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Qualitative field phosphate analysis
Based on the methods of Eidt, (1973) and Bjelajac et al. (1996). This is a rapid qualitative analysis for sample based on fresh soil samples. This technique relies on the compariaon of a range of samples from a site to show whether one or more contexts are relatively enhanced in phosphate. Samples may come from contrasting archaeological contexts, a grid across a possible "site" or vertically from contexts forming a section. It is also useful to take a number of reference samples from off-site and / or the underlying geology.

Necessary equipment and chemicals1

 * 90 mm diameter ashless filter paper
 * 5 Molar Hydrochloric acid (HCl)
 * Ammonium molybdate (H24Mo7N6O24)
 * Ascorbic acid (C6H8O6)
 * Distilled or deionised2 water
 * 2 dropping dispensing bottles

2 Deionised water is used in car batteries and can be obtained from most garages.

Laboratory
Before going into the field make the two chamical solutions you will need: Reagent A and Reagent B.

Reagent A Weigh out 5 g of Ammonium Molybdate into a glass beaker, add 30 ml of 5M Hydrochloric Acid and mix. Dissolve in 100ml of distilled water and decant into a dropping dispensing bottle for use in the field.

Reagent B Dissolve 1 g of Ascorbic Acid in 200 ml of distilled water. Decant into a dropping dispensing bottle for use in the field.

In the field

 * Take ca. 50 mg of soil from a depth of at least 30 cm from the modern ground surface.


 * Being careful not to touch the soil place it in the centre of the filter paper.


 * Add two drops of reagent A and leave for 30 seconds.


 * After 30 seconds add a drop of reagent B

Record:
 * 1) The length of time it takes for a blue colour to appear.
 * 2) Whether the colour is light, medium or dark blue.
 * 3) The length of radiating lines of colour.
 * 4) The percent ring around the sample.

Results and Interpretation
This technique is comparative, to be able to interpret the results requires a range of samples from contrasting on-site and off-site contexts.

Bjelajac et al (1996) interpreted the results by calculating a total phosphate score:
 * 1) The length of time for the blue colour to appear should be given a score of 1, 2, or 3. Depending on whether it was short, medium or long.
 * 2) The colour was given a score of 1 - light blue, 2 -  medium blue, 3 - dark blue.
 * 3) Length of radiating lines was given a score based on the length in mm. For example a line 5 mm long is given a score of 5.
 * 4) The percent ring around the sample. Given a score of 1 if less than a 100%, and a score of 2 if the ring around the sample is complete.
 * 5) The 4 scores are added together to give a total phosphate score.

Sample scores are then compared to highlight areas of enhanced phosphate. This can be used to discriminate between on-site and off-site samples in prospection studies, to identify potential buried soil horizons, to highlight areas where cess, dung, bone and other organic remains have accumulated, e.g. cess pits, byres, middens.

This analysis does not allow you to differentiate differentiation between different forms of phosphate, for example those which are naturally or culturally formed, hence follow-on laboratory analysis may be necessary.

Safety
1 Always consult chemical safety data sheets and follow appropriate health and safety protocols before handling or using chemicals.

Many chemical safety data sheetes are available here from this Oxford University based database.

Related techniques

 * Quantitative phosphate analysis

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