This method is based on that of Dent and Bowman (1996). It is suited to a ‘field laboratory’, requiring only simple equipment and readily available chemicals. Dent and Bowman (1996) saw significant advantages in dealing with soil samples ‘immediately’, as this limited the possibility of drastic changes in the chemistry of actual and potential ASS between sampling and the commencement of critical tests. They also saw difficulties in using results expressed on an oven-dry basis, as highly organic ‘unripe’ ASS may contain as much as 90% water. This can make the volumes that ASS actually occupies hard to assess, unless the dry bulk density of the field sample is known accurately.
The Dent-Bowman Method has three separate components: viz. total (now termed titratable) actual acidity (TAAD-B); total sulfidic acidity (TSAD-B); and residual quick neutralising capacity (RQNCD-B). The method uses a 10 mL, tightly packed field moist soil sample. The TSAD-B test represents the approximate potential acidity that can be released on oxidation and subsequent drainage of the sample material.
Because of the simplicity of the tests and relatively low reagent costs, the operator is encouraged to perform these tests on multiple ‘replicate’ samples, mainly to gain an idea of the point-to-point variability of TAAD-B and TSAD-B, which may be ±30% for closely spaced 10 mL samples of apparently uniform material (Dent and Bowman 1996). Knowledge of this variability has practical importance, as treatments to alleviate acid hazards must deal with the highest concentrations of actual and potential acidity as well as the median. Note that laboratory tests typically deal with sub-samples of profile segments, such as 0–10 cm or 50–60 cm.
Deionised Water
As for Method 3A1. Alternatively, distilled water may be used. If neither of these is available in the field, good quality rainwater (no particulates) may be substituted.
1 M Sodium Chloride
Dissolve 58.44 g LR-grade NaCl in deionised water and make to 1.0 L. If operating remotely, dilute six-fold a saturated solution of NaCl.
Standard 0.1 M Sodium Hydroxide
Prepare by diluting according to instructions an ampoule of standard analytical NaOH concentrate. Alternatively prepare by dissolving 4.10 g of fresh, analytical grade NaOH in deionised water known not to include carbonates. When the covered solution has cooled to room temperature, make to 1.0 L in a volumetric flask. Standardise against a known weight of potassium hydrogen phthalate (KHC8H4O4), previously dried for 2 h at 120°C and subsequently cooled before use in a desiccator. (See Method 4D1 for more details on the use of KHC8H4O4.) For a 5.0 mL volume of exactly 0.1 M NaOH, the equivalent weight of pre-dried KHC8H4O4 is 0.1022 g. Subsequent calculations should use the actual concentration of the NaOH solution.
Standard 0.1 M Hydrochloric Acid
Prepare from a commercially purchased HCl standard solution. Alternatively prepare and standardise as outlined in Method 20H1.
pH Buffers
Obtain commercially or prepare as described in Method 4A1. These should cover the range from ≈ pH 4.0–9.2.
30% Hydrogen Peroxide
Use 30% technical or analytical grade hydrogen peroxide (H2O2). Check the acidity of each new batch of chemical, which should contain less than the equivalent of 6 mol H+/tonne. Adjust, typically with 0.1 M NaOH, to pH 5.5 before use. Handle and store this reagent with caution, as H2O2 is hazardous and is not permitted on domestic airlines. Users should wear PVC gloves and safety glasses when using the chemical. Operations involving H2O2 should be carried out where there is excellent ventilation.
Titratable Actual Acidity (TAAD-B)
Place into a 200–250 mL heat-resistant container a 10 mL volume of sample using a clean, modified plastic syringe (see Note 1). Make this into a 100 mL suspension with 1.0 M NaCl, shake across a period of 2 h then leave the suspension to equilibrate for 24 h. Stir again then, after calibrating a pH meter equipped with a glass/reference electrode combination, measure the pH of the suspension. If pH is ≥5.5, go directly to TSAD-B and undertake that determination. If pH is <5.5, titrate with stirring to pH 5.5 with Standard 0.1 M NaOH solution. This titration is best performed using an auto titrator but may be performed manually. Particularly if performing the titration manually, allow the pH around the end point to stabilise for 1 min, plotting a titration curve to beyond pH 5.5, then reading from the curve the volume of titrant (NaOH) used to reach pH 5.5. Make an appropriate adjustment in the following calculation if the standard NaOH is not exactly 0.1 M.
Calculate TAAD-B[molc/m3 of soil] = [Volumeof titrant (mL 0.1 MNaOH) × 10]
Total Sulfidic Acidity (TSAD-B)
Place a 10 mL volume of a second portion of the sample used for TAAD-B into a 200–250 mL heat-resistant container using a clean, modified plastic syringe (see Note 1). Initially make this into a 50 mL suspension with 1.0 M NaCl and stir until the suspension is homogenised, transfer quantitatively to a borosilicate beaker and make volume to 100 mL, then mark the height of the suspension on the outside of the beaker.
Use protective gloves and spectacles and in a ventilated area carefully add 10 mL of H2O2 then transfer the beaker to a water bath set at 50–60°C, or allow the beaker with contents to stand in sunlight until the sample begins to react (bubble). Do not use excessive heat as this can cause the H2O2 to break down and become ineffective. Expect samples high in pyrite and/or related minerals and with elevated levels of OM to froth strongly. Add further 10 mL aliquots of H2O2 until oxidation of the sample is complete, which may take a few hours or days. Up to 200 mL of H2O2 may be needed for peaty soils containing pyritic minerals. At this point, record the total volume of H2O2 used. Oxidation is complete when the addition of further H2O2 produces no further reaction, the mineral soil has become light brown or grey, and the supernatant solution is clear/transparent but not murky. Boil down the mixture to its original volume and wash the internal sides of the beaker to ensure quantitative conditions.
Stir the solution and after calibrating a pH meter equipped with a glass–reference electrode combination, measure pH of the suspension. If pH is <5.5, titrate with Standard 0.1 M NaOH solution to pH 5.5 as described for TAAD-B. Record this titre in mL. Next add a further 10 mL aliquot of H2O2 and stir thoroughly. If the suspension pH falls, resume the NaOH titration back to pH 5.5 and record this additional titre in mL.
Make an appropriate adjustment in the following calculation if the standard NaOH is not exactly 0.1 M. There should be no need to run a blank titration if the deionised water is of good quality and the H2O2 is at pH 5.5 immediately before use.
Calculate TSAD-B [molc/m3 of soil] = [Net volume of titrant (mL 0.1M NaOH) × 10
Residual Quick Neutralising Capacity (RQNCD-B)
Should the suspension pH after H2O2 treatment in the TAAD-B procedure exceed 5.5, titrate the treated suspension with Standard 0.1 M HCl solution to pH 5.5, after double checking the accuracy of pH measurements with appropriate buffer solutions. Record the titre in mL. Repeat the pH measurement 3 h later. If at that time the suspension pH has risen, resume the titration back to pH 5.5 and again record the titre in mL (see Note 2). Make an appropriate adjustment in the following calculation if the standard HCl is not exactly 0.1 M.
Calculate RQNCD-B [molc/m3 of soil] =
[Net volume of titrant (mL of 0.1 MHCl)×10]
Report TAAD-B, TSAD-B and RQNCD-B separately as molc/m3 on a moist soil basis.
1. Dent and Bowman (1996) suggest the use of a large diameter plastic syringe. They suggest that the non-plunger be cut off to give a 10 mL volume (with the plunger drawn fully inside the syringe barrel, with the rubber seal removed if it produces too much friction). It helps to bevel outwards the cut-end of the plastic syringe tube. The 10 mL volume needs to be checked periodically for accuracy. When using the modified plastic syringe to sample soil, sediment or mud, press each sample to remove air bubbles and analyse as soon as possible.
2. Shells and fragments of shell are commonly found in ASS of coastal regions. These shells have neutralising potential but are very slow acting because of their small surface area and coatings of gypsum. For similar reasons, shell can take several days to fully dissolve at pH 5.5. If a measure of the eventual neutralising capacity of coarse shell is required, they may be separated by sieving mechanically and then analysed separately for neutralising capacity.