How To Measure Redox Potential(ORP) In Soil

ORP / eh is a comprehensive index of soil conditions, representing the relative level of medium oxidation or reduction. It has an essential impact on soil chemical and biological processes.

Soil ORP meter measurement

The soil ORP meter can be used to test the redox potential of fresh or wet soil on site.

Determination method

Insert the platinum and reference electrodes into the fresh or wet soil. The soluble oxidant or reductant in the soil will receive or give electrons from the platinum electrode until an equilibrium potential builds on the electrode surface. We can measure the difference between the potential and the reference electrode and then add it to the potential value of the reference electrode relative to the hydrogen standard electrode to get the redox potential of the soil.

1) Instrument: soil ORP meter

2) Electrode: 2 pieces of working electrodes and 1 piece of the reference electrode

3) Reagents: potassium ferricyanide ORP calibration solution, pH 4.00 quinone hydroquinone ORP calibration solution, pH 7.00 quinone hydroquinone ORP calibration solution, etc

Step:

clean the electrode with ultrapure water

put the working electrode and reference electrode into potassium ferricyanide ORP calibration solution, pH 4.00 quinone hydroquinone ORP calibration solution and pH 7.00 quinone hydroquinone ORP calibration solution with the constant temperature of 25 ℃ for calibration.

At each measuring point (the soil is fresh or wet, If it is dry soil, we can add an appropriate amount of water for wetting). Drill two holes with a depth less than 2-3cm than the measurement depth in the soil with a stainless steel hollow rod, and then quickly insert the cleaned platinum electrode to the depth.

The distance between the two platinum electrodes is about 30cm, and the insertion depth is the same. Install the reference electrode between the two platinum electrodes, insert the reference into the soil for about 3cm, place the electrode in the soil for at least 30min, set the reading lock is ≤ 2mV, and the reading time is ≥ 30min, ≤ 60min.

5) Record the result after the reading is stable

Note:

1. The measurement error of the soil ORP meter is within ± 15MV, which the solution itself may cause.

2. Measuring the ORP in soil requires the soil is in a fresh or wet state. We should ensure the distance between the two working electrodes and the reference electrode is as consistent as possible. It will lead to unstable electrode reading if there is slight shaking or electrode inclination.

3. When measuring ORP in soil with the soil ORP meter, the reading difference between the two channels is a large range at 21-84mv. We can select multiple points for measurement the calculate the average value as the actual soil ORP.

Potentiometric measurement

In addition to the ORP meter, the common method to determine the redox potential is the direct determination method of the platinum electrode. It is based that the platinum electrode itself is challenging to corrode and dissolve and can be used as an electron conductor.

When the platinum electrode contacts the medium (soil and water), the soluble oxidant or reductant in soil or water will receive or give electrons from the platinum electrode until an equilibrium potential is built on the platinum electrode. Since we can not measure the potential of a single electrode, it must produce a battery with a reference electrode (saturated calomel electrode) with a fixed potential of another electrode. So we can measure the electromotive force of the battery with a potentiometer and then calculate the equilibrium potential established on the platinum electrode, that is, the value of redox potential EH.

Reagent

1) Acid potassium dichromate lotion: weigh 50g of potassium dichromate, add 100ml of water, heat and dissolve, and then slowly add 900ml of sulfuric acid under stirring after cooling（ ρ 1.84g/mL）。

2) Stripping solution: take 8.5ml hydrochloric acid（ ρ 1.19g/ml) in 400ml water, add 2.92g sodium chloride, dissolve and dilute to 500ml with water.

3) Redox standard buffer solution: add a small amount of solid hydroquinone powder into 30ml pH 4.01 buffer solution to make the solution contain insoluble solids.

4) Buffer solution (pH 4.01): weigh 10.21g of potassium hydrogen phthalate (khc8h4o4) baked at 105 ℃, accurate to 0.01g, dissolves with water, and dilute to 1000ml with water.

5) Saturated potassium chloride solution: weigh 35g of potassium chloride and dissolve it in 100ml water.

6) Sodium sulfite.

Instruments

1) Potentiometer (millivoltmeter) or pH meter (ion meter).

2) The electrode rack

3) Platinum electrode: seal the platinum wire with a diameter of 0.5mm ~ 1mm and length of 10mm ~ 13mm at one end of a glass tube with an inner diameter of 3mm ~ 4mm, length of 10cm ~ 15cm and its thermal expansion coefficient close to platinum, and expose it for 5mm ~ 10mm. Before using a platinum electrode, check whether there is a crack at the sealing point between the platinum wire and glass tube, and use a stripping solution for surface treatment.

4) Saturated calomel electrode.

5) Thermometer

Note :

1) surface treatment method of platinum electrode:

immerse the platinum electrode in 25ml stripping solution, heat it to slight boiling, add a small amount of sodium sulfite (about 0.2g in 100ml solution), continue to heat and keep the solution volume unchanged for about 30min, and wash the electrode with water after cooling.

If it tests at room temperature, it needs to be soaked for more than half a day, and add the same amount of sodium sulfite 2 ~ to 3 times. It should be soaked in acidic potassium dichromate lotion for 30min if dirty or long-used platinum electrodes before stripping treatment. After the surface treatment, the platinum electrode must check to see whether the electrode potential is accurate in the redox standard buffer solution.

Insert the platinum electrode and calomel electrode into the redox standard buffer solution, measure the electromotive force of the battery, and then calculate the pH value at this temperature (T). The difference between the calculated pH value after the measured electromotive force E (MV) and the pH value of the buffer solution at this temperature should be less than 0.04ph. If the difference is too significant, the platinum electrode must be treated again with stripping solution and re-inspected.

PH value of potassium hydrogen phthalate buffer solution at different temperatures

2) After a few measurements of the saturated calomel electrode and wiping the front end of the saturated calomel electrode, it is better to soak it in potassium chloride saturated solution to restore the salt bridge liquid connection state. Suppose it determines some polluted soil (such as soil containing a large amount of sulfide). In that case, we should use a double liquid salt bridge method and pour the saturated potassium chloride solution into the outer tube.

Steps

1)Before measurement, first turn the potentiometer (millivoltmeter) or pH meter (ion meter) selection switch to the “MV” gear, connect the platinum electrode to the positive position and the saturated calomel electrode to the negative position, and turn on the power switch, adjust the zero button to the zero position, and turn off the power switch.

2) Insert the two electrodes into the soil, turn on the power switch, balance for 2min or 10min, record the positive or negative potential value (MV), and turn off the power switch.

3) In order to convert and pH correction, we need to measure the temperature and pH value at the same time.

Note

1), The two electrodes can be directly inserted into the soil, making the distance between them as close as possible when measuring the field. Read the value after keeping the balance time for about 10min.

2) The repetition times of measuring points are determined according to the representation and the uniformity of soil. Generally, repeat the measurement 5 times. During repeated determination, When the platinum electrode is taken out, it should wash with water, sucked dry with filter paper, and then used. When the saturated calomel electrode shifts, the front salt bridge (referring to the front sand core in contact with the soil) should be cleaned and slightly soaked in the saturated potassium chloride solution.

Calculation

When the MV value is positive, calculate the redox potential according to formula (1);

when the MV value is negative, calculate the redox potential according to formula (2):

Eh=Ee+Ed……（1）

Eh=Ee-Ed……（2）

Where:

EH — soil redox potential, MV;

EE — the standard potential value of saturated calomel electrode at different temperatures, MV;

ED — measured potential value, MV.

Standard potential values of saturated calomel electrode at different temperatures

Since H + is involved in many redox reactions, there is a corresponding relationship between EH and pH values. When the EH value of soil measured at different pH values is to be converted into Eh value at the same pH value for comparison, it must be corrected according to the corresponding change of EH value caused by the change of pH value Δ Eh/ Δ PH value is used as the correction factor.

Although the actual value of this correction factor varies greatly due to the different types, under the common soil fertility and water conditions and when the soil is not in a strong reduction condition:

EH value decrease 60mV (30 ℃) when pH value increase each unit as the correction factor, i.e Δ Eh/ Δ PH = – 60mV, and vice versa.

For example, when soil pH = 5, Ed = 300mV, and when converted to pH = 7, Eh of soil is 300mV – (7-5) × 60mV=180mV。 If it is not corrected, we must mark the pH value of the soil at the time of measurement.

difference

Multiple samples are measured in parallel and take the arithmetic mean value as an integer. The allowable difference of multiple parallel determination results is 1mV.