4.2: Measuring Reaction Rates

Introduction

Measuring reaction rates requires measuring the concentration of at least one species in a reaction versus time with a time resolution that is small compared to the rate at which the reaction proceeds. Any method of measuring concentration (nuclear magnetic resonance, mass spectrometry, gas chromatography, titration ...) can be used. However, spectroscopy methods are the most common.

Static Methods

In these cases the reaction is run in a container (ideally with stirring to maintain homogeneity). Some method where data can be collected quickly enough is used to monitor the concentration of species in the reaction. For a slow reaction that takes days this might be as simple as extracting a sample every hour and quantifying the concentrations via a titration or a gas chromatography measurement. For reactions that occur in a few minutes modern spectrometers work well if there is an absorbance that can be associated with a single species in the reaction. In this case the measurement depends on the Beer-Lambert Law to convert the absorbance (A) into the concentration of the species (c): \(A = \epsilon\ell c\), where \(\epsilon\) = the molar absorptivity (units: L/mol/cm) and \(\ell\) = the path length through the sample in cm. A cartoon of such a setup is show in figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Cartoon of a spectrometer monitoring absorbance at a particular wavelength versus time. This can be applied in any wavelength range were the species of interest absorb light (UV, Vis, IR, NIR...).

Some absorbance spectroscopy methods can monitor the absorbance of multiple species simultaneously (e.g. NMR, FT-IR, diode-array based UV-Vis). In that case it is sometimes even possible to monitor the concentration of intermediate species formed and used up during the reaction.

Flow Methods

Continuous Flow

For reactions that take place in milliseconds, the standard approach since the 1950s has been to employ a flow technique of some kind. An early example was used to study fast gas-phase reactions in which one of the reactants is a free radical such as OH that can be produced by an intense microwave discharge acting on a suitable source gas mixture. This gas, along with the other reactant being investigated, is made to flow through a narrow tube at a known velocity.

Figure \(\PageIndex{1}\): A continuous flow fast kinetic system.

If the distance between the point at which the reaction is initiated and the product detector is known, then the time interval can be found from the flow rate. By varying this distance, the time required to obtain the maximum yield can then be determined. Although this method is very simple in principle, it can be complicated in practice.

 Stopped Flow

Owing to the rather large volumes required, continuous flow method is more practical for the study of gas-phase reactions than for solutions, for which the stopped-flow method described below is generally preferred. These are by far the most common means of studying fast solution-phase reactions over time intervals of down to a fraction of a millisecond. The use of reasonably simple devices is now practical even in student laboratory experiments. These techniques make it possible to follow not only changes in the concentrations of reactants and products, but also the buildup and decay of reaction intermediates.

Figure \(\PageIndex{2}\): A stop flow fast kinetic system.

The basic stopped-flow apparatus consists of two or more coupled syringes that rapidly inject the reactants into a small mixing chamber and then through an observation cell that can be coupled to instruments that measure absorption, fluorescence, light scattering, or other optical or electrical properties of the solution. As the solution flows through the cell, it empties into a stopping syringe that, when filled, strikes a backstop that abruptly stops the flow. The volume that the stopping syringe can accept is adjusted so that the mixture in the cell has just become uniform and has reached a steady state; at this point, recording of the cell measurement begins and its change is followed.

Figure \(\PageIndex{3}\): Stop-flow equipment at a biochemistry research laboratory for measuring rapid reactions and properties such as enzyme kinetics. from Wladimir Labeikovsky.

 Quenched Flow

In a quenched-flow instrument, the reaction is stopped after a certain amount of time has passed after mixing. The stopping of the reaction is called quenching and it can be achieved by various means, for example by mixing with another solution, which stops the reaction (chemical quenching), quickly lowering the temperature (freeze quenching) or even by exposing the sample to light of a certain wavelength (optical quenching).

Of course, there are many reactions that cannot be followed by changes in light absorption or other physical properties that are conveniently monitored. In such cases, it is often practical to quench (stop) the reaction after a desired interval by adding an appropriate quenching agent. For example, an enzyme-catalyzed reaction can be stopped by adding an acid, base, or salt solution that denatures (destroys the activity of) the protein enzyme. Once the reaction has been stopped, the mixture is withdrawn and analyzed in an appropriate manner.

Figure \(\PageIndex{4}\): A quench flow fast kinetic system.

The quenched-flow technique works something like the stopped-flow method described above, with a slightly altered plumbing arrangement. The reactants A and B are mixed and fed directly through the diverter valve to the measuring cell, which is not shown in this diagram. After a set interval that can vary from a few milliseconds to 200 sec or more, the controller activates the quenching syringe and diverter valve, flooding the cell with the quenching solution.

 

Contributors and Attributions

• J. Gutow (UW Oshkosh)
• Fast Reactions in Solution by Stephen Lower is licensed CC BY-NC-SA 4.0. Original source: http://www.chem1.com/acad/webtext/virtualtextbook.html