Laboratory tests - Standard test methods

Acid demand value

This test is important for resin coating of sand. Systems using acid catalysts are susceptible to the acidic or basic nature of the sand, which may cause premature polymerisation or retardation of polymerisation of the resin.

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 the nearest ± 0.1 g

50 ml. burette - grade A accuracy (preferably digital)

50 ml. pipette - grade A accuracy

250 ml. beakers - tall form

Reagents

Deionised or dlstllled 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.

0.1 N HCI and 0.1 N NaOH- these can be purchased as certificated standard solutions either at 1OMolar or 1Molar strength and can be used after appropriate dilution. Check equivalence prior to use by performing a blank titration. This should be accurate to within 0.2ml for 50 ml titre volumes.

Method

Weigh into a clean dry 250 ml beaker, 50g ± 0.1 g of sand. Add 50 ml of prepared distilled or deionised water by burette or pipette. Stir the mixture as slowly as possible, whilst maintaining all particles in suspension, and run in 50 ml of acid by burette. Stir for a further 5 minutes and allow to stand for a further 30 minutes.

After 30 minutes replace the beaker on the magnetic stirrer and introduce the clean pH probe carefully to the suspension immediately adjacent to, but not touching, the side of the beaker. Ensure that the porous membrane of the pH probe is consistently below the surface of the suspension as it turns.

Run in alkali, whilst slowly stirring continuously, from a burette to the predetermined set points of pH3, 4 and 5, noting the titre volume in each case. Take precautions outlined in the notes to ensure as much consistency as possible.

Carry out an identical test without the sand to provide a blank correction.

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. In extreme cases this can cause pronounced drift during the titration making it difficult to fix the point at which a measurement should be taken. The rate of stirring should be controlled such that all of the particles are just moving.

Note 2

A blank determination should be carried out: at least once eve!}' twenty tests, daily and as a check of accuracy on the preparation of fresh standard solutions.

Note 3

Sands which contain free carbonates, e.g. calcite or shell fragments or other components that react rapidly with the mineral acid will produce excessive acid demands. Sands with a free calcium carbonate content of more than 0.25% cannot be satisfactorily evaluated using this technique. When mineralogical or chemical information is not available an indication of free carbonate content can be observed by effervescence on addition of acid solution.

Note 4

Results obtained by laboratol}' staff who are only irregularly involved in ADV tests may be misleading. Duplicate testing may be necessal}' to evaluate precision and accuracy. A problem for staff new to this test is a tendency to overshoot the pH 3 point. This is caused by not allowing sufficient time for the solution to equilibrate after addition of the alkali.

Note 5

Inspect the magnetic followers regularly for signs of wear that expose the central metallic core. These should be discarded since acid will react with the metal.

Note 6

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

Note 7

The method text specifies that the pH should be stable for 30 seconds at each set point before a reading is taken. If drift occurs during this time more alkali must be added, dropwise, to the solution.

A further 30 seconds is allowed in which time the pH reading must remain stable, after which the volume of alkali used is recorded.

This procedure must be repeated until a stable reading is obtained. This can become tedious especially at the pH 4 and 5 set points and will be accentuated if there is a vigorous vortex due to high stirring speeds. In these circumstances there is a temptation to accept a reading that is not stable. This can be a significant source of error.

Note 8

The dilute acid and alkali solutions should be

discarded after one month.

Note 9

The pH probe should be stored in electrolyte

solution or a buffer solution (not distilled water).

Calculation of Results

Note 1

The titration to include pH6 and pH7 is normally only used for the blank or for new materials.

Note 2

In unusual circumstances sands that show acidic character due to the presence of minerals such as pyrite may be tested. During the calculation of the results for such sands a negative value for ADV appears. This is a consequence of the test and it is quite correct to report the ADV as negative since, clearly, an acidic sand will not show any acid demand.

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