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Time-dependent measurements

Time-dependent measurements of the absorption allow us to follow the chemical reaction kinetic, which causes the absorption. The UV/Vis-spectroscopy is a continuous measuring method, i.e. the reaction can be performed directly without any interruption in the reaction tube (here: cuvette) and the concentration can be detected simultaneously. The measured absorption can be equalized with the concentration of the absorbing substance. With plotting the absorption as function of the measuring time the reaction order and the reaction velocity constant can be determined due adjustment the reaction velocity law to the experimental data.

For a simple reaction of the components A, B, and C to the product D, i.e. without any intermediate step or side-products, we can form following equation. Thereby the coefficient n is the stoichiometrical parameter of the current component.

Reaction equation

The result of it is the velocity law of the temporal decreasing of component A. Thereby k is the reaction velocity constant, which depends on the ambient parameter (e.g. temperature, pressure, catalyst influence etc.).

Velocity law

The sum of the superscripts n is called the reaction order. For the determination of the actual reaction progress, the differential equation has to be solved. By plotting the time flow of the concentration, we can see that for identical velocity constants k the gradient become shallower.

Temporal decrease of the concentration

Temporal decrease of the concentration for different reaction orders with identical velocity constants k

According as the absorbing substance is reacting agent or a product of the investigated reaction, we can see at a time-dependent absorption measurement a decrease or an increase of the intensity.

Time-dependent absorption of the reacting agent

Time-dependent absorption of the reacting agent

Time-dependent absorption of the product

Time-dependent absorption of the product

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