I’m getting strange results in SO2 mode; how do I know if my instrument is working correctly?

For SO2 measurements with the SC-300 or the SC-100A, there are several quick tests you can do to make sure the instrument is not faulty.

First, if your instrument does not have the latest firmware version 1.1.2, 3.1.2, or 3.2.d, we recommend that you always start with the electrode disconnected. After powering on the instrument (and, on the SC-300, pressing ENTER to confirm SO2 mode), then connect the SO2 electrode.  This avoids incorrect background settings that may make the instrument appear to be insensitive or unresponsive. To determine which firmware version you have, go into Test Mode (see Appendix A in your manual, or above in this section of the Website)

To test the functioning of the instrument:

1.         Be sure the battery is good per the manual’s instructions.

2.         Remove the electrode to expose the BNC connector at the back of the instrument.  Turn on the instrument and select SO2 mode. Short out the terminals on the connector, using a paper clip or similar metal piece to touch the center pin of the connector to its outer metal sheath. The device should indicate “STOP” with its red LED and buzzer or beeper.  If this does not happen there may be a problem with the instrument; contact us for more information.

3.         Connect the electrode and put it in about 20 mL of distilled water; add about 1 ml (half a bulb squeeze) of each of the acid solution and the reactant and swirl in the usual way keeping constant motion. The instrument may or may not indicate STOP as above. If it does not, add a drop of the SO2 Titrant solution. This should make the STOP condition occur. [If it doesn’t you may have an electrode problem read in the next section below how to fix this.] Now add one drop of a concentrated sulfite solution (1-10% is fine) and verify that the STOP signal ends and the PROCEED light illuminates. If this happens, your electrode is probably OK as well.

4.         Finally, you can check your SO2 reagents with the Ascorbic Acid Test.

Preconditioning of the electrode  Note: as of October 2013, all SO2 electrodes are shipped pre-conditioned, so this procedure should not normally be needed.  Sometimes when the SO2 electrode is first shipped it can be shipped “hot”.  A hot electrode is one that is overly sensitive as indicated by high readings (and STOP conditions) when inserted into solutions that should be giving no signal, like pure water or water plus reactant and acid with a drop of 10% KMBS added.  If your electrode shows this behavior, it generally can be fixed by pre-conditioning as follows:

1.         First, turn the power on and press the MODE button until the instrument is in SO2 mode, then press ENTER.  Attach the SO2 electrode to the SC-300 analyzer. Put 20 mL of distilled water (deionized water) in a beaker and add half a bulb squeeze of the acid solution, half a bulb squeeze of the reactant solution and let the SO2 electrode sit in this solution.

2.         Then add one drop of 10% potassium metabisulfite solution (KMBS) to the beaker with the electrode in it. Swirl gently. The instrument LCD should now read 0.0 (in units of nanoamps) or close to it. If it is reading significantly higher than 20 on the screen, let the electrode sit in the solution for half an hour.

3.         After half an hour rinse the electrode with distilled water.  Put it in about 20 mL of distilled water.  Again add half a bulb squeeze of each of the acid solution and the reactant and swirl in the usual way keeping constant motion.  The instrument may or may not indicate STOP as above. If it does not, add a drop of the SO2 Titrant solution. This should make the STOP condition occur. [If it doesn’t you may have an electrode problem]

4.         Now add one drop of KMBS solution (1-10% is fine), swirl and verify that the STOP signal ends and the PROCEED light illuminates. If this is the case, your electrode has been conditioned.  If the electrode is still “hot” and the PROCEED light does not illuminate, let it sit in 20 mL of distilled water with a half bulb full of the acid solution for a few hours. Now repeat the test from step 3.  If it works, great! If not, call us and we will try to troubleshoot or replace your electrode.  Our contact information is at the bottom of the page.

Posted in: SO2 Questions