Stir the complete reaction mixture in the dark before every illuminated run: same vessel, same catalyst loading, same stirring rate and temperature. Common practice lasts 30–120 min, but the acceptance criterion should be measured, not assumed — sample until concentration drifts below a few percent per hour, then switch on the lamp and start the clock there.
The first conclusion: adsorption–desorption equilibrium must be reached before illumination, otherwise the opening hours of the illuminated run measure adsorption instead of photocatalysis.
When catalyst powder is dispersed in a substrate solution, the surface adsorbs part of the substrate over minutes to hours. Concentration falls with no light involved. If the lamp is switched on during this phase, the apparent disappearance rate combines true photodegradation with continued adsorption, and the fitted rate constant comes out inflated.
In a sealed system the dark hold does more than adsorption work. Dissolved oxygen equilibrates with the headspace, and the liquid and gas phases reach composition balance. In a closed gas-tight setup such as the Labsolar-6A, whose line volume is 65 mL, this equilibration also makes the first gas sample representative of the system rather than of the filling procedure.
Conclusion first: a dark run is only meaningful if every variable except illumination matches the lit run.
Run a no-catalyst blank under identical conditions if substrate stability in the dark is uncertain. If the blank drifts, the substrate is decomposing without light and the dark-equilibration logic still applies, but the data treatment must account for it.
Conclusion first: treat the published ranges as a starting point and replace them with your own measurements before the illuminated run.
| Item | Common practice | Acceptance criterion (self-verify) |
|---|---|---|
| Dark stirring duration | 30–120 min | Consecutive samples differ by less than a few percent per hour |
| Light exclusion | Foil-wrapped vessel or dark cabinet | No measurable drift in a no-catalyst blank |
| Stirring | Same rate and geometry as the lit run | No visible sediment at the bottom of the vessel |
| Sampling | Same volume and filtration as the lit run | Removed volume is small relative to the charge volume |
The table's numbers are conventions, not specifications. A strongly adsorbing substrate on a high-surface-area catalyst may need more than 120 min; a weakly adsorbing one may stabilize within 30 min. Take three or four samples at fixed intervals during the dark phase, quantify each one the same way as the lit run, and extend the hold until the concentration change per hour falls below a few percent. Record the actual duration you accepted — that is the value reviewers and colleagues will ask for.
The first conclusion: the clock must start at lamp switch-on, and the zero-time concentration must be measured after dark equilibration, immediately before illumination.
If time zero is set at lamp-on but the zero-time concentration is taken before adsorption equilibrates, the numerator of every subsequent data point carries the adsorption loss. In a first-order analysis of the natural log of the ratio of concentration to its initial value versus time, this shows up as an inflated rate constant, and the error is largest exactly where kinetics are judged: the early, steepest segment of the curve.
The distortion is worse when catalysts are compared. Two materials with different adsorption capacities will show different dark losses, so a comparison made without a dark run ranks adsorption strength alongside photocatalytic activity. With a completed dark equilibration, both runs start from a measured, stable concentration and the ranking reflects photocatalysis.
Report the dark curve separately rather than deleting it. It documents that equilibrium was reached and quantifies the adsorption contribution, which is itself a material property.
Conclusion first: a dark equilibration that is not documented did not happen, as far as any reader is concerned.
State the dark stirring duration and the measured acceptance result (for example, concentration change below a few percent per hour), the vessel temperature, the stirring rate, the light-exclusion arrangement, and the sampling schedule. Note when the lamp was switched on and how the zero-time sample was taken. These few lines cost little and remove the single most common ambiguity in liquid-phase photocatalysis reports.
No. Thirty to 120 min is common practice, not a specification. Equilibration depends on the substrate, the catalyst surface, and the mixing, so verify it: sample at fixed intervals during the dark phase and extend the hold until the concentration change per hour is below a few percent.
The liquid-phase adsorption hold may not apply, but the analogous practice does: keep the sealed system in the dark at the operating temperature before illumination so gas composition, adsorption, and pressure settle, and verify stability before switching on the lamp.
No. Report the dark curve separately. Once equilibrium is reached, use the concentration measured at lamp switch-on as the initial value; subtracting a reversible adsorption term from the illuminated data distorts the kinetic fit.
