Analytical Methods:Lab pH

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Soil pH
Soil pH is a measure of the concentration of protons (H+); it gives an indication of the acidity or alkalinity of a soil.

Questions
Soil pH is an important property affecting the preservation of many artefacts and ecofacts. Some knowledge of the soils pH is important for understanding why certain materials have been preserved and whether decay processes could account for the absence of others. Soil pH is a key factor in determining the nature of soil development and hence is also used to identify soil type and infer post-depositional processes such as leaching and podzolisation that may have affected a deposit.

Soil pH is also an important variable in the interpretation of many other soil properties - for example, organic matter accumulation, element concentrations (including phosphate), cation exchange capacity, soil corrosivity - and hence is a routine analysis.

Example case studies of where soil pH has been used include:
 * Soil pH, Bone Preservation, and Sampling Bias at Mortuary Sites Claire C. Gordon, Jane E. Buikstra American Antiquity, Vol. 46, No. 3 (Jul., 1981), pp. 566-571
 * Overton down experimental earthworks.
 * Prospection

Samples and storage
Bulk samples are required for pH analysis. Soil pH is a very variable property so it is also worth taking replicate samples, if necessary replicates can be bulked together for the analysis unless you want to be able to distinguish statistically between different contexts.

Whilst samples can be taken from individual contexts to characterise their pH and preservation conditions taking a series of samples vertically through a section can provide more information about the soil processes that have resulted in the current pH conditions. The identification of soil types and post-depositional processes requires sampling of the whole section or soil profile with at least one sample from each context or horizon.

Wet soils left in sealed bags can become more acid as organisms in the soil produce carbon dioxide which builds up and creates carbonic acid. It is important therefore that soils are air-dried before storage.

Analysis
Analysis of soil pH is very simple and straightforward requiring only a pH meter and basic beakers and measuring cylinders.

For a detailed method of pH analysis click here. Soils should be air-dried and sieved to less than 2 mm. These are then wetted with distilled water. The proportion of soil to water is important and should be recorded along with any results. The soil solution is allowed to stand to give it time to reach equilibrium then the pH is read using a calibrated pH meter and probe.

Sometimes the pH is also measured in a calcium chloride (CaCl2) solution. This usually results in a lower pH reading as the calcium ions displace protons from the surface of clays and organic matter putting them into solution. Soil pH can vary quite markedly through the year as salt concentrations in the soil change. The calcium chloride corrects for this seasonal variations and gives a more stable reading if measurements are being taken over a long period of time. Except for monitoring purposes, however, this is probably of little interest to most archaeological projects.

Data and interpretation
Soil pH is a logarithmic measure of proton (H+) concentration and it is recorded on a scale from 1-14, 7 being neutral, 1-6 acid and 8-14 alkaline. The higher the pH the lower the proton concentration and the less acid, or more alkaline, the soil; so a soil of pH 4 contains ten times as many protons as a soil of pH 5 and a hundred times as many protons as a soil of pH 6.

Most natural soils have pHs in the range of 3.5 – 10. Soils with a pH of 6.5 – 7.5 are usually termed neutral, soils with a pH less than 6.5 are increasingly acid soils, and soils with a pH more than 7.5 are increasingly alkaline. Different artefacts and ecofacts require different pH conditions for their preservation. More information on the effect of pH on the preservation of different artefact and ecofact types can be found here.

Soil pH can alter very rapidly in response to changing environmental or burial conditions. Hence, recorded pH is a measure of current acidity / alkalinity; the pH may have altered considerably since the deposits formation. Burial, for example, will often lower pH if anaerobic conditions develop, however if the pH of the burying medium is high, the pH of the underlying deposits can be raised as cations are washed down through the profile and displace protons (H+) which are washed away.

When comparing different pH data sets it is important that they were analysed in the same way. Check the soil:water ratios are the same and also whether they were analysed in water or calcium chloride solution.

Related techniques

 * Field pH analysis
 * Monitoring pH

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