CVS – Frequently Asked Questions

A. The reference electrode needs to be maintained. See also Reference electrode.

B. The baseline has not been set correctly. The baseline needs to be defined identically for each addition.

In exceptional cases, the evaluation algorithm cannot calculate results with the "Nonlinear regression technique" curve type. You should be able to solve the problem by selecting the "Linear Interpolation" curve type.

This effect is not uncommon. It is caused by degradation products in the bath sample. With successful determination of recovery in a "check standard", the method can be checked at any time.

A. The electrodes were lifted out of the measuring solution when the sample was added. Please use the pipette opening for sample addition.

B. The temperature is instable.

A. The electrodes are not well conditioned. Please perform a conditioning procedure on the electrodes.

B. The temperature is instable.

A. No sample was added. If requested, add the required sample quantity.

B. The sample does not contain any brightener. Make sure that the sample is fresh. The brightener will volatilize over time.

The working electrode needs to be cleaned. Carry out a conditioning procedure in pure VMS. If applicable, the electrode needs to be cleaned.

The working electrode needs to be cleaned. Carry out a conditioning procedure in pure VMS. If applicable, the electrode needs to be cleaned.

CVS measurements are temperature-dependent. Strong temperature fluctuations (+/- 4 °C) should be avoided to get reliable analysis results.

The CVS measuring instrument used should not be near the air outlet.

The room temperature and/or the solution temperature are not constant: The room and solution temperature should be kept stable during the measurements (± 4 °C).

The reference potential is drifting away: Maintenance of the reference electrode should be carried out in accordance with the electrode leaflet.

The measuring vessel and the electrodes have been contaminated with organic additives. Thoroughly rinse the measuring vessel and the electrodes and use fresh VMS.

No chloride is contained in the VMS: Check the preparation of the VMS and modify if necessary.

One or more electrodes are not connected: Check the electrode cable connections.

One or more electrodes are defective: Carry out maintenance according to the electrode data sheet. The measuring vessel does not contain enough solution or is empty: Check the fill level of the measuring vessel and add solution if required.

The reference potential is offset: Perform maintenance for the reference electrode in accordance with the electrode data sheet.

The reference electrode is not yet equilibrated: After maintenance of the reference electrode, wait for at least 20 min before starting a measurement.

The platinum electrode tip is contaminated: Condition the working electrode in pure VMS.

The platinum electrode tip has been used for a very long time: Immerse the platinum electrode tip in c(NaOH) = 0.5 mol/L for approx. 10 min and then thoroughly rinse with distilled water. Replace the platinum electrode tip if necessary.

The VMS is contaminated: Check whether the reagents are clean. Only reagents with degree of purity "analytical grade" or better should be used.

The water quality is poor: Use distilled water, type-II grade (ASTM D1193-91) or higher.

The measuring solution is contaminated by diffusion: No infeed tubing other than the four-way micro dosing tip (6.1824.000) may be immersed in the measuring solution.

Contact problem at the driving axle (6.1204.x10 or 6.1204.x20): Remove any abrasion residue from the driving axle. If necessary, replace the driving axle.

It is not possible to repair the driving axle. The driving axle is a consumable and needs to be replaced at regular intervals. Driving axles with a mercury contact are supplied in a bag under protective nitrogen atmosphere. We recommend keeping the storage time of driving axles with a mercury contact as short as possible.

The solutions used are pipetted manually: The solution can be added with 800 Dosinos with dosing units to obtain reproducible results.

The solutions are not added through the pipette opening: During a measurement the solutions should only be added through the pipette opening.

The electrode cable has accidentally been jammed in the measuring head cover or a cable contact has been bent: On the 894 Professional CVS you can replace the electrode cable set (see 894 Professional CVS manual). On the 797 VA Computrace this needs to be handled by Metrohm Service.

The measuring head is not correctly locked in place on the measuring head arm: Allow the measuring head to snap into place; you need to hear it click.

The sealing rings have aged or are defective: Please contact us.

The connection between the 800 Dosino and the 894 Professional CVS is either interrupted or the 800 Dosino is showing an error:

• Check the cable connections.
• Disconnect the 894 Professional CVS from the power supply and connect it again.
• Check the dosing and filling rate.
• If necessary, contact Metrohm Service.

The data chip of the dosing unit is mechanically damaged or impaired by chemicals:

• Remove the dosing drive and attach it again.
• Clean the data chip and the contact surfaces.
• Have the data chip replaced by Metrohm Service.

Crystals have formed (in the dosing cylinder, on the valve disk or in the capillary):

• Check the flow path.
• Rinse the dosing unit and the connected tubing and capillaries (prepare function) when the measuring system is not in use.
• Clean the dosing unit at least every two weeks.

The 800 Dosino was not attached correctly:

• Remove the 800 Dosino and attach it again.
• Check whether the 800 Dosino is correctly attached.
• Disconnect the CVS measuring instrument from the power supply and connect it again.
• If necessary, contact Metrohm Service.

The cables are not connected or not connected correctly:

• Connect the cables in accordance with the instructions in the manual.

The tubing connections are leaking:

• Check the tubing connections and tighten, if necessary.

The rinse and/or waste canister are sealed air-tight:

• Loosen the caps on the canisters a little or remove them.

The contact pressure of the tubing cartridge is set too low: Set the contact pressure of the tubing cartridge in accordance with the sample changer manual.

The pump time of the peristaltic pump is increasing: The pump tubing of the peristaltic pump has aged or is defective and should be replaced.

The PEEK sample needle on the Sample Processor is positioned more than 0.5 mm from the bottom of the sample vial: Position the PEEK sample needle in accordance with the information in the manual.

The dosing units used must be prepared at least once a day before measuring begins. This ensures that fresh solution is used for the analysis.

Port 3 of the dosing unit used can be connected to a waste canister using an outlet tubing. The outlet (port) 3 can be defined for the waste in the software (General settings or Configuration). With these settings, a significantly quicker preparation is ensured.

All dosing units used should be rinsed with distilled water in order to prevent a possible crystallization of metal salts.

The dosing units used should be disassembled and thoroughly cleaned at 14-day intervals.

Auxiliary, reference, and working electrodes should be stored in distilled water. Storage in VMS is not recommended. If the measuring system is not used for an extended period (>24 h), then the reference electrode should be stored separately in c(KCl) = 3 mol/L. The other electrodes can also be stored dry.

It is advisable to condition the electrodes at least once a day before you start work.

Most likely, the VMS in the measuring vessel is contaminated. The contamination might have been caused by the chemicals used but also by organic additives.

The driving axle is defective and needs to be replaced.

No, this is not possible.

The solutions should be protected from direct sunlight. If possible, the solutions should be stored in glass vessels. Unless specified otherwise, additives should be stored at room temperature.

For logistical and technical reasons, Metrohm cannot offer a polishing service for used platinum electrode tips.

A contaminated platinum electrode tip can be conditioned in VMS or diluted H₂SO₄.

To clean a contaminated platinum electrode tip, it can be stored for approx. 60 minutes in c(NaOH) = 1 mol/L. Afterwards, the electrode should be well rinsed with isopropanol and deionized water.

The platinum electrode tip may not be treated in an ultrasonic bath.

The platinum electrode tip can be immersed for a few seconds in concentrated nitric acid to oxidize a metal coating which has accidentally been applied. If after another measurement foreign metal should be found on the surface again, then the method parameters and electrode connectors need to be checked.

Platinum electrode tips with a glass shaft can be operated up to a maximum temperature of 50 °C.

Platinum electrode tips with a PEEK shaft can be operated up to a maximum temperature of 40 °C.

No, the platinum electrode tip may not be treated in an ultrasonic bath. This will irreversibly damage the electrode.

The potentiostat integrated in the Metrohm CVS instruments can easily measure the smallest current with a high degree of precision. For this reason, it is also possible to get precise measurement results with smaller platinum electrode tip diameters. In a few exceptional cases, the method parameters need to be adjusted.

The electrolyte vessel can be filled with any electrolyte that is a good conductor. If KNO₃ interferes with the determination, you can use diluted H₂SO₄, for example, instead of c(KNO₃). However, you should not change the electrolyte in the internal reference system. If another electrolyte is used here instead of c(KCl) = 3 mol/L, then the reference potential changes. This is also the case if you are using a different concentration of KCl.

A correct reference potential of the reference electrode is an important factor for perfect results in CVS analysis. We recommended that you change the reference electrolyte c(KCl) = 3 mol/L at least every three days.

The bridge electrolyte forms an electrical bridge between the measuring solution and the reference electrode. In many cases a strongly acidic measuring solution is used in CVS analysis. Over time, the acid may also reach the bridge electrolyte and thereby change the reference potential. This is why changing the bridge electrolyte every day is recommended.

The reference electrodes filled with KCl gel are maintenance-free, which means that no electrolyte needs to/may be changed.

The auxiliary electrode can be immersed for a few seconds in concentrated nitric acid to oxidize an accidentally applied metal coating. If after another measurement foreign metal should be found on the surface again, then the method parameters and electrode connectors need to be checked.

As a general rule, the auxiliary electrode is maintenance-free and does not require any special cleaning. If you want to mechanically clean the platinum pin, then you can also polish the surface. Make sure that you do not rotate the platinum pin while doing so because otherwise the internal contact may break.