Analytical Methods:Field pH

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Questions
Field analysis of pH would usually be carried out to investigate the potential for preservation of particular ecofacts and artefacts, pH can also be used to help confirm the identity of a deposit or inclusion, for example tufa deposits. It is also used in situations where pH data is required quickly to inform the excavation or sampling scheme.

Sampling
Samples should not be taken from the upper 5 cm of the soil profile. Soil pH is highly variable so care should be taken that the sample is representative of the deposit being analysed. If this is not possible replicate samples can be taken.

Analysis
A general assessment of pH can often be made from local vegetation and soil types, however archaeological deposits can have artificially high or low pH as a result of inputs to the soil such as lime mortar, wood ash or coal ash, so care is needed when making such inferences.

More accurate pH measurement requires:
 * pH paper or field pH probe (more sensitive than paper but requires regular calibration). A laboratory pH meter can also be set up in the site hut if necessary
 * distilled water
 * 100 ml glass beakers
 * 25 ml measuring cylinder
 * 10 ml measuring scoop - can be obtained from cook shops or made from translucent film canisters marked at the 10ml level (found by filling with 10ml of water)


 * 1) 10 ml of fresh soil is mixed with 25 ml distilled water and left to stand for 10 minutes.
 * 2) Restir and dip in one end of the pH paper or insert the calibrated pH probe.
 * 3) Compare the colour of the paper with the chart or read of pH from the meter once the value has steadied.

N.b. Do not insert the probe directly into the soil as this can damage the glass bulb.

Data and interpretation
Soil pH is a rapidly adjusting property; pH can only be interpreted in terms of current soil conditions. It is possible that the pH may have changed markedly over the history of the deposit.

More information on the relationship between soil conditions and preservation potential can be found here.

Related techniques
Laboratory pH analysis

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