Magnetic Susceptibility is defined as:
"The ratio of the intensity of magnetism induced in a substance to the magnetising force or intensity of field to which it is subject."
Sherwood manufactures two Magnetic Susceptibility Balances;
The Mk1 and The AUTO.
Basic Principles of Magnetic behaviour:
Based on their magnetic properties, all substances can be classified into one of three groups, those attracted by a strong magnetic field, known as paramagnetic, those repelled, designated diamagnetic, and, finally, the most recognised class, ferromagnetic, unique in their ability to retain their own magnetic field. Ferromagnetics are able to retain a permanent magnetic field since their free electrons are in close proximity and remain aligned even after the external magnetic field is removed.
Unlike the ferromagnets, the magnetic properties of the diamagnetic or paramagnetic materials could only be observed and measured when these samples are held within a magnetic field applied externally.
Uses for The Sherwood Scientific Magnetic Susceptibility Balance's:
- Assignment of the oxidation state - for the metal in complexes of the transition, lanthanide and actinide elements.
- Stereo chemical information - for example , square planar Ni2+ complexes are diamagnetic, while octahedral Ni2+ complexes are paramagnetic with two unpaired electrons.
- Information concerning ligand field strength - transition metal complexes can be high-spin or low-spin, depending, in part on the field strength of the ligands. Therefore the calculation of magnetic moments and hence the number of unpaired electrons can be used to assess ligand field strength.
- Antiferromagnetic interactions in dimers and polymers - antiferromagnetic interactions between neighbouring metal atoms or ions in dimeric or polymeric complexes (e.g. cupric acetate) will lead to magnetic moments which are smaller than expected.
- Complexation by ligands - for a number of transition metal species, complexation by ligands alters the magnetic behaviour. Thus, a number of square planar Ni2+ complexes are diamagnetic when dissolved in non-coordinating solvents such as benzene and chloroform but paramagnetic in coordination of two ligand pyridine. This is due to axial coordination of two ligand molecules to give a 6-coordinate Ni2+ complex.
- Criterion of purity - pure Y203 is diamagnetic- but contamination with lanthanides such as erbium or dysprosium can cause samples to be paramagnetic.
- Measurement on air-unstable compounds - the balance is ideally suited for compounds which decompose when exposed to the air since almost anything that is not ferromagnetic or too bulky or heavy can be sealed into the sample tubes. For example, a glass B10 socket and a stopper can be used to seal the end of a sample tube.
- Measurement of solutions - liquid samples are readily handled and magnetic titrations can be performed.
Sherwood Scientific's Magnetic Susceptibility Balances:
Sherwood Scientific's Magnetic Susceptibility Balances are recognised in hundreds of teaching and research laboratories throughout the world.
Based on a design by the Late Professor Evans of Imperial College London they offer a number of significant advantages over traditional methods.
The Mk1 balance adheres closely to Evan's original design.
The new Magnetic Susceptibility Balance - AUTO is a microprocessor controlled, state of the are balance for detecting the magnetic properties of gases, liquids and solids. The improved sensitivity, versatility and overall performance make it ideally suited for new analytical applications in the research laboratory and industrial quality control.
Both the Mk1 and the AUTO balances are exclusively manufactured by Sherwood Scientific Ltd in its development and manufacturing facility in Cambridge, UK.
Magnetic Susceptibility at the molecular level:
The nature of the electrons within a sample determine the magnetic properties. The magnetic forces that are generated are more or less neutralised when two electrons become paired. Free unpaired electrons give rise to magnetic forces which are attracted to a strong magnetic field, and the strength of these attractive forces are in direct proportion to the number of free electrons. The presence of free electrons results in materials being classified as paramagnetic and the lack of them results in compounds being diamagnetic. Crystallinity, chemical reactions, oxidation states, and virtually anything that can alter the electronic configuration of a compound, may also change the magnetic properties. Analogous to spectral measurements, magnetic susceptibility measurements are both qualitative and quantitative in nature.
Basic Design Principle:
The traditional technique, developed by Gouy, employs a conventional laboratory balance and large permanent magnets. The magnets remain stationary while the sample is caused to move, giving apparent gain or loss in sample weight.
Both the Magnetic Susceptibility Balance MK1 and the Magnetic Susceptibility Balance AUTO work on the basis of a stationary sample and moving magnets. Two pairs of magnets are placed at opposite ends of a beam making a balanced system having a magnetic field at each end. Introduction of the sample into the magnetic field attempts to deflect the beam and the movement is optically detected. A compensating force is applied by introducing a current through a coil between the other pair of magnets. The current required to maintain the original postition of the balance beam is proportional to the force exerted by the sample; and the direction that the beam (magnetic field) moves indicates whether the sample is paramagnetic or diamagnetic which is shown by a plus or minus indication on the display.
Calculation of Magnetic Susceptibility
|in this table X = Chi
|The volume susceptibility (Xv) = I / H
I = Intensity of magnetism produced in a substance
H = Intensity of magnetic field applied
|The mass susceptibility (Xg) = Xv / d
d = density of substance
|The calculation of Xg from the readings on the Magnetic Susceptibility Balance is simple
|Xg = Cl(R-Ro) / 109m
C = calibration constant of the balance
I = length of sample in cm (1>1.5cm.)
m = mass of sample in gm
R = balance reading for sample in tube
Ro = balance reading for empty tube