Field measurement of soil pH is recommended practice for soil and land use surveys in Australia. While portable pH meters are now readily available and reliable, there is a long history of use of a pH kit developed by Raupach and Tucker (1959). Its vital components are barium sulfate, a mixed indicator solution and a colour-comparison chart depicting different soil pH values. Measurement precision is considered satisfactory (Aldrick 1988). This is the same mixed indicator/barium sulfate method mentioned in the preamble of Method 4A1.
Neutral barium sulfate (BaSO4) powder
Separately prepare one litre each of half molar aqueous (deionised water) solutions of barium chloride (BaCl2.2H2O; 122.1 g/L) and of sodium sulfate decahydrate (Na2SO4.10H2O; 161.1 g/L).
Heat about 4 litres of deionised water to boiling and add 30 mL of 10 M HCl. Keep this hot, dilute acid stirred continuously and allow the barium and sulfate solutions to run in slowly at equal rates. The addition of the reagents should take an hour or more. When the reaction is complete, allow the barium sulfate precipitate to settle and draw off the supernatant liquor. Wash the product repeatedly, preferably in a vacuum assisted Buchner funnel, until the barium sulfate precipitate has a neutral reaction to the pH indicator, then dry in an oven at around 105°C to constant weight. This product may vary slightly in fineness but should be used as a ‘free-flowing’ powder.
Mixed Indicator Solution
Weigh out good quality reagents of 0.2 g of phenol red – acid free (C19H14O5S), 0.12 g bromocresol purple (C21H16Br2O5S) and 0.08 g bromocresol green (C21H14Br4O5S), and grind together in a glass or agate mortar with 9 mL 0.1 M sodium hydroxide (NaOH; 3.9997/L) until all solid material has dissolved. Dilute this solution with about 70 mL of distilled (preferred) or deionised water and bring to pH 7.0 with dilute HCl or dilute NaOH, assisted by a pH meter and stirring. Make volume to 100 mL and dispense the mixed indicator using a borosilicate or plastic dropping bottle.
Use the example provided on the inside back cover of this book or alternatively construct and photograph a colour scale for the particular mixed indicator across the soil pH range in half-unit intervals from pH 2.0 to pH ≥9.5. For each calibration point on the pH scale, thoroughly mix equal amounts of around ten different soil samples known to have a pH within 0.1 unit of the desired value. The ‘true’ colours given by these mixtures are those obtained within a few minutes of development (see Procedure below for more details).
Take a small quantity of field moist soil (≈0.5 g) and mix it on a white porcelain spot tile with a few drops of the indicator to give a paste. Dust the paste with the neutral, white barium sulfate powder and compare the colour with a standard colour chart after about three min. Avoid inhalation of barium sulfate dust as long term inhalation may lead to deposition in lungs in sufficient quantities to produce baritosis – a benign pneumoconiosis.
Report field pH on ‘field moist’ soil.