Laboratory tests - Standard test methods

Determination of pH value

The pH of the sand is a measure of the acidity or alkalinity of the sand. Generally it is thought that pH is an indication of the extent of contamination of the sand surface and the presence of reactive impurity particles.

Apparatus

pH meter-pH probe system:
This should give a reading within ± 30mv of zero in pH 7 buffer solution.

pH probe: this should be of the fast response type, capable of stabilisation from pH 3 to pH 11 in 30 seconds. It should not be a gel-filled type since these are slow to respond. Because of the strong abrasive action of the sand the epoxy-bodied type is favoured for durability.

Magnetic stirrer - preferably non stalling type

Balance - capable of weighing 50g to± 0.1g.

50 ml. pipette - grade A accuracy

250 ml. beakers - tall form

Reagents

Deionised or distilled water - this should be previously prepared by boiling for at least two minutes to expel dissolved gases such as carbon dioxide which affect the pH.

It is important to use this water as soon as possible after cooling. Storage in tightly capped polythene bottles is recommended but for a maximum time of about two hours (check pH before use). Preferably the pH should be greater than 6.3 and ideally between 6.5 and 7.0.

pH buffer solutions - purchased as accurate standard solutions, powders or tablets for calibration of the pH meter according to the manufacturers instructions.

Method

Weigh into a clean dry 250 ml beaker, 50g ± 0.1g of sand.

Add 50 ml by pipette of prepared deionised or distilled water.

Stir for five minutes and measure the pH value whilst stirring. Record the result, when the pH reading is stable, to one decimal place.

Note 1

It is advisable to minimise the build up of a vortex during stirring, since this continuously exposes fresh mineral surfaces due to particle to particle attritioning effects. The rate of stirring should be as slow as possible and controlled such that all of the particles are in motion.

Note 2

Inspect the magnetic followers (stirrers) regularly for signs of wear that expose the central metallic core; discard if the core is exposed.

Note 3

Magnetic followers should be periodically treated with Aqua Regia to remove attached metallic particles and minerals such as magnetite.

Note 4

The pH probe should be stored in electrolyte solution such as 4M potassium chloride or a buffer solution (not distilled water).

Note 5

The measurement of pH alone can be misleading due to the presence of insoluble carbonates, for example. These will cause a significant reaction in an acid environment. It is recommended that Acid Demand Value (ADV, section 3.6) is always conducted in association with pH, especially when new materials are under investigation.

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