Current mainstream instruments are all designed based on the Karl Fischer coulometric (electrolytic) principle. The reaction mechanism involves the quantitative consumption of water in the sample during the oxidation of sulfur dioxide by iodine, while the consumed iodine is continuously replenished through electrolysis at the anode. When all moisture in the sample has reacted, the platinum electrode indicator system reaches the endpoint. According to Faraday’s law of electrolysis, the amount of electricity consumed during electrolysis is strictly proportional to the mass of water reacted, enabling highly accurate measurement of trace moisture. This method offers significant advantages such as high sensitivity (resolution up to 0.1 μg), wide linear range, and no need for titrant calibration, making it the standard testing method in the power industry.
Preparation and System Equilibration Before Testing
Prior to commencing tests, the system must be thoroughly cleaned, assembled, reagents added, and allowed to stabilize. The quality of this preparation directly determines the reliability of subsequent measurement data.
1. Cleaning and Assembly of the Electrolytic Cell
Before first use or after prolonged disuse, all components of the electrolytic cell must be thoroughly cleaned. Measuring electrodes and the cathode chamber should be cleaned with methanol or acetone; water must never be used, as residual moisture may cause abnormal baseline readings. After cleaning, place the components in a constant-temperature drying oven at approximately 60°C for four hours, then allow them to cool naturally to room temperature. During assembly, apply a thin, uniform layer of vacuum grease to all standard ground glass joints to ensure system airtightness.
2. Reagent Filling
Fresh Karl Fischer electrolyte solution should be carefully filled into both the cathode and anode chambers, ensuring that the liquid levels in both chambers are nearly equal. Install the drying tube containing fresh blue silica gel (color-changing desiccant), and ensure all stoppers are securely tightened.
3. System Equilibration Procedure
Connect the power supply and activate the stirring function to generate a slow, steady vortex in the anode chamber electrolyte. Freshly prepared reagents often appear dark brown due to trace moisture, and the instrument will indicate an “excess iodine” state. At this point, use a microsyringe to slowly inject a small volume of pure water (approximately 20–50 μL for new reagent) into the anode chamber via the sampling valve until the electrolyte changes from deep brown to a clear light yellow. The instrument will then enter the “excess water” state and automatically initiate electrolytic balancing. During equilibration, you may temporarily pause electrolysis, remove the electrolytic cell, and gently shake it to dissolve any trace moisture adhering to the walls, thereby reducing blank current and accelerating the equilibration process. Once the instrument displays a stable “normal” or “balanced” signal, the system is ready for testing.
Calibration Verification: The Core Guarantee of Accuracy
After each instrument startup or electrolyte replacement, accuracy must be verified using pure water as a standard reference—this is a mandatory prerequisite for ensuring valid test results.
Operating Procedures
(1) Accurately draw 0.1 μL (theoretical water mass of 100 μg) of pure water using a clean, thoroughly dried micropipette (capacity 0.5 μL or 1 μL, with accuracy no less than ±0.01 μL). After drawing, expel any air bubbles at the tip and wipe off residual droplets on the outside of the needle with a lint-free cloth.
(2) Press the “Start” or “Sample” button on the instrument panel, immediately insert the needle through the sample inlet stopper, and extend it approximately 2 mm to 3 mm below the surface of the electrolyte in the anode chamber, then quickly inject the pure water. Note: The needle tip must not touch the cell wall or electrode surface; the injection should be rapid to avoid loss due to adhesion.
(3) The instrument automatically performs electrolysis and accumulates counts. Read the displayed value after the endpoint alarm sounds. Using 100 μg of pure water as the calibration standard, the instrument reading is considered accurate if it falls within the range of 90 μg to 110 μg (i.e., ±10% error). For applications requiring higher precision, a tolerance of 100 μg ± 3 μg may be applied. It is recommended to perform 2 to 3 parallel measurements consecutively and take the average as the final evaluation result. If the relative range of measured values exceeds 5%, re-calibration should be performed after checking the sampling procedure or reagent condition.
Operating Precautions and Management Guidelines
To ensure long-term stable operation and extend the instrument’s service life, the following guidelines must be strictly observed during operation:
Electrode Protection: Keep the electrode wire connections clean and dry to prevent oxidation or contamination, which may increase contact resistance and significantly affect measurement stability and endpoint accuracy.
Reagent Management: Replace the electrolyte immediately once it becomes ineffective (turning light brown or reddish-brown); continued use is strictly prohibited. Decomposition byproducts in expired electrolyte can deposit dark brown complexes on the platinum electrodes, which are difficult to remove with routine cleaning and may eventually render the electrodes unusable. When adding samples, strictly control the amount of water introduced—adding more than 0.5 g of water at once will completely degrade 500 mL of electrolyte. When adjusting fresh reagents, follow the principle of “small amounts, multiple additions, slow injection.”
Sampling Procedure: Before aspirating a sample, wipe the outside of the needle shaft and tip thoroughly with filter paper to prevent sweat or external moisture from contaminating the electrolytic cell, which could cause positive measurement bias.
Environmental Control: When replacing reagents or reinstalling cleaned components, perform operations during relatively dry periods and minimize exposure time of parts to ambient air. Use anhydrous ethanol or acetone for cleaning accessories; aqueous solvents are strictly forbidden.
Desiccant Maintenance: Check the color of the silica gel in the drying tube daily. If more than half turns pink or white, replace it immediately with freshly regenerated blue silica gel (dried at 105°C for 2 hours).
Post time: Aug-04-2026