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When a metal M is immersed in a solution containing its own ion Mⁿ⁺, then  an electrode potential  develops. The value of  the  electrode potential  is given by the Nernst equation.

When a fixed voltage is applied to two electrodes dipping into a solution,a current will flow, depending on the conductivity of the solution. Conductivity measurements do not give information about the nature of the ions in solution. However, they are useful in the quantitative measurement of the ions.

Compounds having one or more asymmetric carbon atoms can rotate the plane of polarized light. Such compounds are called optically active. Measurement   of this property is called polarimetry.
 
The magnitude and direction in which the plane of polarized light is rotated depend on 1.The nature  of the compound, 2. The  concentration of the solution, 3. The path length of light, 4. The solvent, 5. The wave length of light and 
6. The temperature. These variables are related and can be presented in the formula

Mass spectrometry (MS) is a  useful  technique which produces spectra of the masses of the atoms or molecules in a sample.  The spectra are used to determine the elemental or isotopic   details  of a sample. The masses of particles and of molecules, are used to elucidate the chemical structures of molecules, such as peptides and other chemical compounds. Mass spectrometry works by ionizing chemical compounds to generate charged molecules and measuring their mass-to-charge ratios.

A sample, which may be solid, liquid, or gas, is ionized, by bombarding with electrons. This may cause some of the sample’s molecules to break into charged fragments. These ions are then separated according to their mass-to-charge ratio, typically by accelerating them and subjecting them to an electric or magnetic field. Ions of the same mass-to-charge ratio will undergo the same amount of deflection. The ions are detected by a mechanism capable of detecting charged particles, such as an electron multiplier. Results are displayed as spectra of the relative abundance of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses to the identified masses or through a characteristic fragmentation pattern.

In 1896, French physicist Becquerel observed that uranium compounds affected a photographic plate that was wrapped inside a black paper. He found that all uranium compounds gave off penetrating rays. He called the production of radiation by uranium compounds “radioactivity”. Madame and Pierre Curie later identified thorium, polonium, and radium as the other radioactive elements.

Nuclear Magnetic Resonance spectroscopy is a powerful and theoretically complex analytical tool. The nuclei of certain isotopes are continuously spinning with an angular momentum, which can give rise to an associated magnetic field.

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) uses an inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. A plasma is a neutral gas containing a good number of positive and negative ions or free electrons. A plasma can only be created. A combustion flame is one example of a plasma. Advantages of ICP-AES are excellent limit of detection and linear dynamic range, multi-element capability, low chemical interference and a stable and reproducible signal. Disadvantages are spectral interferences (many emission lines), high cost and operating expense and the fact that samples typically must be in solution.

Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and non-uniformity of concentration and path length of analyte atoms (in graphite furnace of AA). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and non-uniformity of concentration and path length of analyte atoms (in graphite furnace of AA). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

When an element burns in a flame it emits radiation characteristic of its identity. Flame photometry is the determination of an element in solution by measurement of emission of light by atoms of the element in flame. Initial tests for identification of a few elements by flame tests are well known. Na burns with yellow flame, K with lilac flame, Ca, with red flame, Cu, with green flame etc. Emission of characteristic radiation by an element and correlation of emission intensity with the concentration of that element form the basis for flame photometry.

The sample to be analyzed is prepared in solution and sprayed under controlled conditions into a flame. The light from the flame enters a monochromator to isolate the desired region of the spectrum. A photocell and an amplifier measure the intensity of the isolated radiation. After proper calibration of the photometer with solutions of known composition and concentration, it is possible to correlate the intensity of a given spectral line with the unknown concentration of that element. A calibration curve prepared using known concentrations is useful. Internal standards and addition of known concentration of sample to be quantified are more useful.

Infrared light (800-1500nm) interacts with molecules at the level of atomic vibrations. An infrared spectrum gives valuable information about the molecular structure. It is rarely used for quantitative purposes and hence concentration measurements are not possible. Infrared light detects vibrations characteristic of functional groups. For example the C=O (carbonyl) group absorbs at 1700 cm-1 (wave number). The main region of interest for analytical purposes is from 2.5- 25µm or 4000- 400 cm-1. At wavelengths below 25µm the radiation has sufficient energy to cause changes in the vibrational energy levels of the molecule, and these are accompanied by changes in the rotational energy levels. The pure rotational spectra of molecules occur in the far infra red region (50-1000 µm) and are used for determining molecular properties.

Spectrophotometry measures how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through the sample solution. Chemical compounds normally absorb or transmit light over a certain range of wavelength. Hence spectrophotometry is used to measure the amount of a known chemical substance. It is one of the most useful methods of quantitative analysis in the laboratory and is widely used.

A  colorimeter or spectrophotometer is an instrument that measures the amount of photons (the intensity of light) absorbed after it passes through the sample solution. With the spectrophotometer, the amount of a known chemical substance (concentrations) can also be determined by measuring the intensity of light detected. Depending on the range of wavelength of light source, it can be classified into two different types, namely,1) UV-visible spectrophotometer which uses light over the ultraviolet range (185-400nm) and visible range (400-700nm) of electromagnetic radiation spectrum and 2) IR spectrophotometer which  uses light over the infrared range (700-15000nm) of electromagnetic radiation spectrum.

The mass spectrometry (MS) is not very well suited to the analysis of mixtures of similar substances because of the multiplicity of fragment ions. The gas chromatography (GC), of course,  has no ability to identify compounds on its own. The GC/MS can easily overcome these defects and can identify the components (fractionated by GC). The combined instrument of GC/MS is an unexcelled instrument that can identify the compounds after their  fractionation.

The GC-MS is composed of two major building blocks: the gas chromatograph and the mass spectrometer. The gas chromatograph utilizes a capillary column which depends on the column’s dimensions as well as the stationary phase properties. The difference in the chemical properties between different molecules in a mixture and their relative affinity for the stationary phase of the column will promote separation of the molecules as the sample travels the length of the column. The molecules are retained by the column and then elute (come off) from the column at different times and this allows the mass spectrometer downstream to capture, ionize, accelerate, deflect, and detect the ionized molecules separately.

High pressure (or performance) liquid chromatography (HPLC) is more versatile than GC since it is not limited to volatile compounds. The choice of mobile and stationary phases is wider. HPLC has high resolving power, separations are speedier, quantifications are more precise and it is in general, more reproducible. Liquid chromatography basically involves separation of components due to differences in the equilibrium distribution of sample components between the mobile (liquid) and stationary phases.

Gas chromatography is one of the most valuable techniques available for organic analysis. It has been applied for the separation, identification and analysis of virtually all volatile compounds. In gas chromatography, the mobile phase is gas. When the stationary phase is liquid , it is called gas liquid chromatography (GLC). If the stationary phase is solid, it is called gas solid chromatography (GSC). The sample is transported through a stationary phase with help of an inert carrier gas, namely the mobile phase. The fractionation of the components in the sample mixture takes place based on the partitioning between the stationary and mobile phases.

When a particle revolves around an axis, a force is developed and acts away from the axis of rotation. This force is called centrifugal force. A particle suspended in a medium takes some time to sediment, which depends upon the size, molecular weight, and viscosity or density of the medium under normal “g” (gravity). To speed up the process of sedimentation, the external field of force is allowed to act on the particle which depends also on the distance of the particle from the axis of rotation in addition to the other factors.

Dialysis
 
The main purpose of dialysis in biological research is the removal of small molecular weight (M.W.) constituents from biological fluids. This is achieved by dialysing the material in a cellulose tubing with a pore size of 40-80 A0 (4-8mm) diameter which allows the passage of compounds with molecular weight less than 10,000. The following is the procedure followed: cut the length of the dialysis tubing depending upon the volume to be processed. Open the sides of the tubing with a flow of distilled water and transfer it to a boiling water bath. Do not allow the tubing to dry as it will distort the pore size. Make a bag by tying a knot at one end of the tubing. Transfer the solution to be dialysed into the bag. Tie another knot at the top of the bag. The bag is then put in a beaker containing the solution against which dialysis is to be carried out. In general, one volume of the desired solution is dialysed against 50-100 volumes of buffer. The total period of dialysis could range from 8-48hr. The efficiency of dialysis could be increased by stirring and changing the dialysate solution at frequent intervals.

Solvent extraction is mainly used to remove dissolved substances from solutions or soluble material from solids /powders/ solid mixtures. Solvent extraction is explained using distribution law or partition law. The law states that if to a system of two liquid layers made up of two immiscible liquids/ slightly miscible liquids is added a third substance soluble in both layers , then the substance distributes itself between the two layers so that the ratio of concentration in one solvent to the concentration in the second solvent remains constant at a constant temperature.

Gel-filtration chromatography or molecular sieve chromatography is a method based on molecular dimension. The gel consists of beads which are composed of dextran polymer (sephadex, agarose or polyacrylamide gels). The separation depends upon the different abilities of the various sample molecules to enter pores which contain the stationary phase. Very large molecules, which never enter the stationary phase, move through the chromatographic bed fastest. Smaller molecules, which can enter the gel pores, move very slowly through the column, because they spend a proportion of their mobility, time in the stationary phase. Molecules are, therefore eluted in the order of their decreasing molecular size.

This technique is used in the separation of ionised and ionisable compounds.It consists of an insoluble matrix to which charged groups are covalently bound. The charged groups are associated with mobile counter ions. These counter ions can be reversibly exchanged with other ions of the same charge without altering the structure of the matrix.

The ion exchanger is complex and polymeric. The polymer carries an electric charge that is exactly neutralized by the charges on the counter ions. These active ions are cations in a cation exchanger and anions in an anion exchanger. A cation exchanger consists of polymeric anions and active cations whereas anion exchanger consists of polymeric cations and active anions. Amberlite IR 120 and Dowex 50 WX are cation exchangers while Amberlite IRA 400 and Dowex 1X are anion exchangers. Ion exchangers should be insoluble in water and hydrophilic to allow the ions to pass through. It should be chemically stable and should be denser than water when swollen.