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Saturday, February 23, 2013

Analytical Chemistry Basics


Most of the scientists and chemists consider analytical method as tool for their research or work and are primarily interested in the results they can provide. all are concerned about the accuracy, precision and reproducibility and  reliability of the methods as well as the sources of any errors that can be introduced.One should always seek for ways to improve them so as to exact the very best results (results as close to true values as possible).

Accuracy & Precision
Accuracy of a determination is defined as the concordance between result (observed value) or the true or most probable value (IE, accepted correct value).

The precision of determination is defined as the concordance of a series of measurements of the same quantity.

"Accuracy expresses the correctness of a measurement while precision of a set of results expresses the reproducibility of  a measurement."

Accuracy is always followed by precision, but high degree of precision doesn't imply accuracy.

True Value & Error
Error is at least a small deviation from the true value. True value of a physical measurement can never be known with absolute certainity. only the most probable value can be arrived at. a better value may be arrived subsequently from better techniques.

Expression of errors
Errors may be expressed as absolute errors or relative errors.
The absolute error is the measure of difference between observed value and true value or most probable value of the quantity measured.

absolute error E, = X(obs) - X(t)

Absolute error is always expressed in the units of measurement.It is the measure of accuracy of an experiment.

Relative Error is the ratio of the error to the true value or the most probable value.

Relative error E(r) = (( X(obs) - X(t))/X(t)) * 100%

Classification of Errors
Errors can be classified into two classes : Determinate Errors and Indeterminate Errors.

  • Determinate  Errors (Systemic Errors)

Determinate errors can be avoided or  whose magnitude can be determined.These errors can be eliminated in an improved repetition of the measurements.

  • Indeterminate  Errors (Accidental Errors or Random Errors)

Indeterminate errors are those which cannot be discovered, avoided or eliminated in a repetition of measurements.

Types of Determinate Errors

  1. Operational and Personal Errors
  2. Instrument and Reagent Errors
  3. Method Errors
  4. Additive or Proportional Errors

Types of Indeterminate Errors

  1. Personal Errors
  2. Instrumental Errors
  3. Condition Errors