Pressure jump is a technique used in the study of

chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...

. It involves making rapid changes to the pressure of an experimental system and observing the return to equilibrium or

steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...

. This allows the study of the shift in equilibrium of reactions that equilibrate in periods between milliseconds to hours (or longer), these changes often being observed using

absorption spectroscopy Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating fi ...

, or

fluorescence spectroscopy Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electro ...

though other spectroscopic techniques such as CD, FTIR or

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...

can also be used. Historically, pressure jumps were limited to one direction. Most commonly fast drops in pressure were achieved by using a quick release valve or a fast burst membrane. Modern equipment can achieve pressure changes in both directions using either double reservoir arrangements (good for large changes in pressure) or pistons operated by piezoelectric actuators (often faster than valve based approaches). Ultra fast pressure drops can be achieved using electrically disintegrated burst membranes. The ability to automatically repeat measurements and average the results is useful since the reaction amplitudes are often small. The fractional extent of the reaction (''i.e.'' the percentage change in concentration of a measurable species) depends on the molar volume change (Δ''V''°) between the reactants and products and the equilibrium position. If ''K'' is the equilibrium constant and ''P'' is the pressure then the volume change is given by: :

\Delta V^o = -RT \left(\frac \right)_T

where ''R'' is the

universal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...

and ''T'' is the

absolute temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics. Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic w ...

. The volume change can thus be understood to be the pressure dependency of the change in

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...

associated with the reaction. When a single step in a reaction is perturbed in a pressure jump experiment, the reaction follows a single exponential decay function with the reciprocal

time constant In physics and engineering, the time constant, usually denoted by the Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system.Concretely, a first-order LTI system is a s ...

(1/τ) equal to the sum of the forward and reverse intrinsic rate constants. In more complex reaction networks, when multiple reaction steps are perturbed, then the reciprocal time constants are given by the

eigenvalue In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denoted ...

s of the characteristic rate equations. The ability to observe intermediate steps in a reaction pathway is one of the attractive features of this technology.

References

{{Reflist Chemical kinetics