5A1 Chloride – 1:5 soil/water extract, potentiometric titration

This method carries the same code in Rayment and Higginson (1992). Chloride ions in clarified soil/water extracts are determined by potentiometric titration with AgNO₃ in conjunction with an Ag/AgNO₃ electrode array. For routine soil testing of non-contaminated rural lands, it is usual to assume no interfering ions of consequence are present. I^– and Br^– are titrated as Cl^–, however, so may need to be considered when soil samples are derived from mine spoil, contaminated land and the like. The same applies to ferricyanide, which causes high results and should be destroyed if known to be present in measurable concentrations. Interference from chromate and dichromate ions can be avoided by reducing them to the chromic state, or by their removal. Ferric iron also interferes when present in substantially greater amounts than Cl^– (APHA 1998).

Table 5.1. Summary details of method codes, method titles, technologies and expected reliability of soil tests described in this chapter.

This titration-based method has long been used for assessing the Cl^– salinity of Australian soils, but it offers few analytical advantages over an alternative (not included) based on a mercurimetric titration, except when soil extracts are deeply coloured.

Apparatus

Silver/Silver Chloride Electrode

Silver (Ag)/silver chloride (AgCl) electrodes can be obtained commercially or manufactured from a length (≈15 cm × ≈1.6 mm dia) of Ag wire attached to an electrical connection of a millivolt meter or equivalent. The clean Ag wire (preclean with 30% HNO₃ until effervescence just commences) should be coated with a thin layer of AgCl by electrolysis. Pre-clean the Ag wire with fine emery paper followed by 30% HNO₃ until effervescence just commences, then immerse (to about 1 cm) in 0.1M HCl and connect to the positive terminal of a 2 volt battery. Connect a Pt electrode to the negative pole through a variable resistance and ammeter. Pass a current of 3 to 4 milliamps until an even covering of AgCl deposits on the Ag wire. This should take from 1 to 30 min. For convenience, prepare several electrodes in the session and store in an aqueous NaCl solution of about 2000 mg/L. When not in use, store the electrodes in water. It is necessary to clean and recoat the Ag/AgCl electrodes occasionally but, with care, they should last for several years (Piper 1944).

Double Junction Calomel Electrode

Use a calomel electrode with a secondary salt bridge of saturated potassium nitrate to prevent leakage of Cl^– into the test solutions. Alternatively, a mercury/mercurous sulfate electrode, such as Radiometer type K601, may be substituted (IR Wilson, pers.comm.).

Reagents

0.01128 M Potassium Chloride Standard

1 mL contains 0.4 mg Cl.

Dissolve 0.8411 g potassium chloride (KCl; previously dried at 110°C for 2 h) and dilute to 1.0 L with deionised water.

Silver Nitrate Standard A

1 mL 0.011283 M AgNO₃ contains 1.22 mg Ag, equivalent to 0.4 mg Cl.

Dissolve 1.9166 g silver nitrate (AgNO₃) and dilute to 1.0 L with deionised water to prepare a solution of ≈0.01128 M AgNO₃. Store in a brown glass bottle, away from light.

Silver Nitrate Standard B

1 mL 0.0564 M AgNO₃ contains 6.08 mg Ag, equivalent to 2 mg of Cl.

Dissolve 9.583 g AgNO₃ and dilute to 1.0 L with deionised water to prepare a solution of ≈0.0564 M AgNO₃. Store in a brown glass bottle, away from light.

1.5 M Nitric Acid

Procedure

Standardisation of AgNO₃ solution

For the AgNO₃ Standard A, pipette 20.0 mL KCl Standard into a 250 mL beaker, add about 100 mL deionised water, 2 mL 1.5 M HNO₃ (dip pipette) and place on a magnetic stirrer. Insert the Ag/AgCl electrode and the preferred reference electrode connected to an automatic titrator or a millivolt meter. Commence stirring and titrate with AgNO₃ solution. When using a millivolt meter, record the millivolt readings for each addition of AgNO₃. The end-point is indicated when the largest voltage change occurs for a small constant addition of AgNO₃. This should occur at ≈267 mV with a calomel reference electrode.

Repeat for the AgNO₃ Standard B but use a 100 mL aliquot of KCl standard.

where t = titre (mL) of the particular AgNO₃ solution

These standardisations should be repeated on completion of each batch.

Soil Extract

Prepare 1:5 soil/water suspensions as described for Method 3A1 and clarify by centrifugation or filtration. Take a suitable aliquot of known volume into a 250 mL beaker. If there is evidence of appreciable interfering ions, these should be removed/destroyed at this stage; see APHA (1998) for details. Otherwise add 100–200 mL deionised water, add 2 mL 1.5 M HNO₃ and introduce a mechanical or magnetic stirrer. Insert electrodes and titrate with the appropriate AgNO₃ Standard (depending on the expected level of Cl^–) to the endpoint.

Calculation

For exactly 0.011283 M AgNO₃ solution:

For exactly 0.0564 M AgNO₃ solution:

Make appropriate adjustments to calculations if AgNO₃ Standards are not exactly the concentrations shown.

Report water-soluble Cl^– (mg Cl/kg) on an air-dry soil basis.