Use Of Mettler-Toledo Redox Electrode

by

Christian Bogdal

(Laboratory of Joseph W. Stucki, University of Illinois)

1. Read the potential of a certified buffer solution

Manufacturer:	Mettler Toledo GmbH (www.mt.com)
Product designation:	Redox Buffer Solution 468 mV (pH 0.1) / 9883
Order Number:	51 340 066

This value is called: U_{(read buffer)}

2. The internal reference of the electrode is calculated by the following formula:

U_{(int ref calc)} = U_{(buffer theo)} – U_{(read buffer)} .

In this formula, U_{(buffer theo)} stands for the potential of the buffer relative to hydrogen. The values of U_{(buffer theo)} are temperature dependent and listed in table 1. Hence, they have to be chosen according to the measured room temperature.

3. Verify the accuracy of the electrode by:

|U_{(int ref calc)} - U_{(int ref theo)}| < ± 10 mV.

U_{(int ref theo)} symbolizes the theoretical internal reference of the electrode. This value also depends on temperature and is given in table 1.

4. If the electrode presents satisfactory results for its internal reference (i.e., the test of step 3 is valid), measure the potential of the sample solution, which is recorded as U_{(read solution)}.

5. The redox potential of the sample solution is obtained as follows:

U_(solution) = U_{(read solution)} + U_{(int ref calc)} .

Table 1

	Electr.Ag/AgCl	Buffer 230 mV	Buffer 470 mV
T [°C]	U(int ref theo) [mV]	U(buffer theo) [mV]	U(buffer theo) [mV]
10	217.4	471.4	669.0
11	216.7	469.1	669.4
12	216.0	466.8	669.8
13	215.4	464.6	670.2
14	214.7	462.3	670.6
15	214.0	460.0	671.0
16	213.3	457.7	671.4
17	212.6	455.4	671.8
18	211.9	453.1	672.2
19	211.2	450.8	672.6
20	210.5	448.5	673.0
21	209.8	446.2	673.4
22	209.1	443.9	673.8
23	208.4	441.6	674.2
24	207.7	439.3	674.6
25	207.0	437.0	675.0
26	206.3	434.7	675.4
27	205.6	432.4	675.8
28	204.8	430.0	676.2
29	204.1	427.7	676.6
30	203.4	425.4	677.0

Instructions And Hints To Optimize The Method, Increase Accuracy, And Handle The Electrode Appropriately

1. Rinse the electrode between each sample with distilled water (deoxygenated distilled water if working in the glove box).

2. Choose a buffer standard whose value U_{(buffer theo)} is as close as possible to the expected redox potential of the sample U_(solution).

3. The electrode is equipped with a moveable “safe-lock” device. The refilling port is sealed when the hole is visible trough the transparent part of the safe-lock. When the hole is not visible, the electrode is ready to use. For storage the refilling port is closed by turning the moveable safe-lock. For measurements, remove the wetting cap and rinse with distilled water.

4. For storage, rinse or clean the electrode after use and replace the wetting cap, partly filled (1/3) with the reference electrolyte.

5. For refilling: Slip the safe lock approximately 20 mm downwards. Refill the electrode with the recommended reference electrolyte, if necessary. The level of a correctly refilled electrode is approximately 10 mm below the filling hole.

6. Reference electrolyte: KCl 3 M.

7. In case of a slow response/drift check the following actions:

clean and remove deposits from the metal ring with toothpaste or 2 M HCl

rinse with distilled water

change reference electrolyte

clean diaphragm (as described below)

8. The cleaning of the diaphragm involves different steps:

use cleaning solution Pepsin/HCl (to remove proteins)

use cleaning solution Thiourea (to remove sulfides)

rinse with acetone or ethanol (for contamination by oily/organic liquids)

rinse with 0.1 M HCl or 0.1 M NaOH (for contamination by acid/alkaline soluble compounds)