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 different from chemical thermodynamics, which deals with the direction in which a ...

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

equilibrium Equilibrium may refer to: Film and television * ''Equilibrium'' (film), a 2002 science fiction film * '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film * "Equilibrium" (''seaQuest 2032'') * ''Equilibr ...

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 is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons ...

, 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 atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

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, temperature ...

and ''T'' is the

absolute temperature Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expres ...

. 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 as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...

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 A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda Lambda (; uppe ...

function with the reciprocal

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

(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 is a vector that has its direction unchanged (or reversed) by a given linear transformation. More precisely, an eigenvector \mathbf v of a linear transformation T is scaled by a ...

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