Analytical Chemistry notes

MDCAT Chemistry

Analytical chemistry deals with the separation, identification and measurement of substances present in a sample. This chapter covers errors in measurement, chemical tests, chromatography, spectroscopy and instrumental methods such as conductometry, polarography and polarimetry.

Introduction to Analytical Chemistry

Analytical chemistry is the branch of chemistry concerned with finding what substances are present in a sample and how much of each substance is present. The identification of components is called qualitative analysis, while the measurement of their amounts is called quantitative analysis.

A sample may contain a major constituent, a minor constituent or a trace constituent. Analytical methods may be based on chemical reactions or on the measurement of physical properties such as absorption, emission, electrical conductance or rotation of light.

Qualitative analysis identifies the ions, elements or compounds present in a sample.
Quantitative analysis determines the amount or concentration of a constituent.
A major constituent is present in a relatively large amount.
A trace constituent is present in an amount less than 0.01%.
Separation, identification and measurement are important steps in chemical analysis.
Analytical methods may be chemical, physical or instrumental in nature.

Accuracy, Errors and Deviations

Repeated measurements of the same quantity do not always give exactly the same result. The difference between an observed value and the accepted or true value is called an error. Errors may arise from the instrument, the method, the observer or environmental conditions.

The deviation of an individual result is the difference between that result and the average value. The arithmetic mean of the different deviations observed in several measurements of the same quantity is called the average deviation. Careful measurements reduce random error, but systematic errors require correction of the method or instrument.

Arithmetic mean = sum of all readings divided by the number of readings.
Deviation is the difference between an individual reading and the mean or accepted value.
Average deviation is the arithmetic mean of the deviations of repeated readings.
Accuracy describes closeness to the true value.
Precision describes closeness of repeated readings to one another.
A complete chemical analysis does not normally include separating a sample in its original mixture form as the final analytical step. Separation is used to prepare components for identification or measurement.

Chemical Tests and Identification

Qualitative analysis uses characteristic chemical reactions to identify ions and elements. The observation may be a colour change, precipitate, gas evolution or formation of a coloured complex.

In the detection of nitrogen in an organic compound, nitrogen is converted into cyanide during sodium fusion. The cyanide reacts with iron salts to form ferrocyanide and ferric ferrocyanide. The ferric ferrocyanide gives the characteristic Prussian blue colour.

Ammonium ion, NH4+, is tested with Nessler's reagent.
Nessler's reagent gives a yellow to brown colour or precipitate with NH4+ depending on its concentration.
The Prussian blue compound formed in the nitrogen test is ferric ferrocyanide, Fe4[Fe(CN)6]3.
The Prussian blue colour confirms the presence of nitrogen when the sodium fusion test is properly performed.
Chemical identification depends on a reaction that is sufficiently specific for the ion or element being tested.
Sodium thiosulphate, Na2S2O3, is used as the fixing solution in photographic processing.

Chromatography

Chromatography is a separation technique based on the different distribution of components between a stationary phase and a mobile phase. Components that have greater attraction for the stationary phase move more slowly. Components that dissolve more readily in the mobile phase move more rapidly.

Paper chromatography is used to separate coloured substances, amino acids and other small amounts of compounds. In paper chromatography, water absorbed on the cellulose fibres of the paper acts as the stationary phase, while the solvent moving through the paper acts as the mobile phase.

Stationary phase remains fixed in position during separation.
Mobile phase moves through or over the stationary phase.
Paper chromatography uses water absorbed on paper as the stationary phase.
A locating agent helps to show colourless spots after development.
Ninhydrin is a locating agent for amino acids and produces coloured spots with many amino acids.
The retention factor is Rf = distance travelled by solute divided by distance travelled by solvent front.
Chromatography can be used to identify substances, check purity and separate components of a mixture.
A pure substance generally gives one spot under fixed chromatographic conditions, while an impure substance may give more than one spot.

Electromagnetic Radiation and Spectroscopy

Spectroscopy is the study of the interaction of electromagnetic radiation with matter. A spectrum may result from absorption, emission or scattering of radiation. The position of a line or band gives information about energy changes, while its intensity may give information about the amount of substance.

Atomic spectra are produced when electrons in isolated atoms move between definite energy levels. Molecular spectra are generally made of bands because molecules also have vibrational and rotational energy levels. Atomic spectroscopy deals with the measurement of wavelengths, frequencies and intensities of radiation emitted or absorbed by atoms.

Electromagnetic radiation travels as waves and has wavelength, frequency and energy.
The relation between wavelength and frequency is c = νλ.
Energy of radiation is directly proportional to frequency and inversely proportional to wavelength.
Absorption spectroscopy measures radiation absorbed by a substance.
Emission spectroscopy measures radiation emitted by an excited substance.
Atomic spectra are line spectra because atoms have definite electronic energy levels.
Molecular spectra are usually band spectra because molecular energy changes include electronic, vibrational and rotational changes.
Radiation can excite a molecule when its energy matches an allowed energy difference in that molecule.

Ultraviolet, Visible and Infrared Spectroscopy

Ultraviolet and visible spectroscopy is based on the absorption of radiation that causes electronic transitions. In many transition-metal compounds, absorption in the visible region is related to d-d transitions. This absorption gives many compounds their characteristic colours.

Infrared spectroscopy is mainly used to identify functional groups. Bonds absorb infrared radiation when the frequency of the radiation matches a vibration of the bond. The position of an absorption band is commonly expressed in cm-1 as wavenumber.

The infrared region extends approximately from 16 to 1000 μm in the FSc treatment.
A strong absorption near 1720 cm-1 indicates a carbonyl, C=O, bond.
Infrared absorption may be caused by stretching or bending vibrations of bonds.
The visible region may involve d-d electronic transitions in transition-metal compounds.
Ultraviolet and visible spectroscopy is based on absorption of light radiation.
Infrared spectroscopy is useful for detecting functional groups in organic compounds.
The exact position and strength of an absorption band depend on the type of bond and its chemical environment.

Nuclear Magnetic Resonance and Mass Spectrometry

Nuclear magnetic resonance spectroscopy studies the absorption of radiofrequency radiation by nuclei placed in a strong magnetic field. It gives information about the number of chemically different nuclei and their environments. Proton NMR is commonly written as H-NMR or 1H-NMR.

In low-resolution proton NMR, each chemically different group of equivalent hydrogen atoms generally produces one signal. Ethane thiol, CH3CH2SH, gives three proton environments: CH3, CH2 and SH. Some nuclei are difficult to observe because of their nuclear properties. In the FSc treatment, N14 is considered invisible or unsuitable for ordinary NMR observation because it is a quadrupolar nucleus and gives broad, weak signals.

The number of H-NMR signals depends on the number of chemically different proton environments.
Ethane thiol, CH3CH2SH, gives three H-NMR signals in the simple low-resolution treatment.
Equivalent hydrogen atoms give the same type of signal.
Carbon-12 is NMR inactive, while carbon-13 is NMR active but naturally less abundant.
N14 is treated as invisible in the stated NMR context because its quadrupole causes very broad signals.
Mass spectrometry separates ions according to their mass-to-charge ratio, m/e.
The relative abundance of an ion having a definite m/e value is measured from the strength of the electric current produced by that ion.
The mass spectrum can provide molecular mass and information about fragments of a molecule.

X-rays and Other Radiation-Based Methods

X-rays have sufficiently high energy to produce characteristic effects in atoms and solids. Henry Moseley used X-rays in 1914 to determine atomic numbers. He showed that the frequency of characteristic X-rays is related to atomic number rather than atomic mass.

Different analytical methods use different interactions between matter and radiation. Absorption of radiation is used in ultraviolet, visible and infrared spectroscopy. Rotation of plane-polarised light is measured by polarimetry.

Henry Moseley used X-rays for the determination of atomic number.
Characteristic X-ray frequency increases systematically with atomic number.
Polarimetry is based on the rotation of plane-polarised light.
Optically active substances rotate the plane of polarised light.
The direction and amount of rotation can help identify an optically active substance and estimate its concentration.
Spectroscopic methods commonly involve absorption or emission of light radiation.
The nature of the method depends on the physical or chemical property being measured.

Electrical and Photographic Analytical Methods

Conductometry measures the electrical conductance of a solution. The conductance depends on the concentration and mobility of ions. It can be used to follow changes during reactions and titrations.

Polarography is based on the current-voltage characteristics of a solution. The current changes as the applied voltage is varied, especially when reducible or oxidisable species undergo electrode reactions. Photographic analysis also involves chemical processing. Sodium thiosulphate dissolves unreacted silver halide and fixes the photographic image.

Conductometry is based on electrical conductance.
Conductance of an electrolyte solution depends on the number and movement of ions.
Polarography is based on current-voltage characteristics.
Polarography uses an electrode system to study oxidation or reduction processes.
The fixing solution used in developing a photographic negative contains sodium thiosulphate.
Sodium thiosulphate removes unreacted silver halide from photographic material.
Chemical and instrumental methods may be combined in a complete analytical procedure.

Key terms

Analytical chemistry: The branch of chemistry concerned with identifying substances and measuring their amounts in samples.
Qualitative analysis: Analysis used to identify the substances or ions present in a sample.
Quantitative analysis: Analysis used to determine the amount or concentration of a substance.
Trace constituent: A component present in an amount less than 0.01% of the sample.
Average deviation: The arithmetic mean of the deviations obtained from repeated measurements.
Chromatography: A separation technique based on the distribution of substances between stationary and mobile phases.
Stationary phase: The phase that remains fixed during chromatographic separation.
Mobile phase: The phase that moves through or over the stationary phase.
Locating agent: A reagent used to make otherwise colourless chromatographic spots visible.
Spectroscopy: The study of the interaction of electromagnetic radiation with matter.
Wavenumber: The number of waves per unit length, commonly expressed in cm-1 in infrared spectroscopy.
Infrared spectroscopy: A technique that identifies bonds and functional groups through absorption of infrared radiation.
Nuclear magnetic resonance: A technique based on absorption of radiofrequency radiation by suitable nuclei in a magnetic field.
Mass spectrometry: A technique that separates ions according to their mass-to-charge ratio.
Polarimetry: The measurement of the rotation of plane-polarised light by a substance.
Conductometry: An analytical method based on measuring electrical conductance.
Polarography: An electroanalytical method based on current-voltage characteristics.
Prussian blue: The blue ferric ferrocyanide compound, Fe4[Fe(CN)6]3, formed in the nitrogen test.

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Chemistry shortcuts

Finding the limiting reactant and percentage composition

Convert every given mass or volume into moles first. The reactant that produces the least amount of the required product is the limiting reactant.

Write the balanced equation and calculate moles using n = mass/Mr.
Use the mole ratio to calculate the product. For percentage composition, use percentage = mass of element in one mole of compound divided by molar mass, multiplied by 100.
Example: Percentage of nitrogen in KNO3 = 14/101 × 100 = 13.86%.
Answer: 13.86% nitrogen.

Use gas volume at molar volume only when the gas conditions are stated or are standard conditions.

Using gas volume, pressure and temperature relations

At the same temperature and pressure, gas volume is directly proportional to the number of molecules. For changing conditions, use P1V1/T1 = P2V2/T2.

At constant temperature and pressure, divide or multiply the volume in the same ratio as the number of molecules.
Example: 10 mL H2 contains 2 × 10^3 molecules. Oxygen in 200 mL contains 20 × 2 × 10^3 = 4 × 10^4 molecules.
Answer: 4 × 10^4 molecules.
For a rigid container, increasing temperature increases molecular speed and mean free path if the gas remains in the same phase.

The direct volume to molecule ratio does not apply when temperature or pressure changes.

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