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- Accuracy :- It is the measurement of correctness of analysis.
- Accuracy of the determination may be defined as the concordance between it and the true or the most probable value.
- Methods for determinig analysis :- - - - -
- Absolute method :- Synthetic sample of known amount of constituents are prepared by weighing about pure elements or compounds of known stoichiometric composition.
- These substances are available commercially or prepared by analysis and subjected to vigorous purification by recrystallization.
3. The substances must be of known purity.
- Test of Accuracy :- - - - -
- amount of constituents are taken and proceed according to the specify instruction.
- The amount of constituents is varied because the determinate error in the procedure may be the function of amount used.
- Here, accuracy = Mean of results-Amount of constituents present.
- Comparitive method :- - - - -
- In analysis of minerals, it is impossible to prepare solid synthetic sample of desired composition.
- We use accuracy standards.
Precision : - - - - - - -
- It is the concordance of the series of measurement of same quantity.
- Primary Standards:- - - - - -
Primary standards are the referenced solution that are used frequently in defined conentration.
Primary compounds are made of compound of sufficient impurity.
- Requirement of primary standards: -- - - - - -
- It must be easy to obtain purity,dry or preserved in pure state.
- It should get unaltered in air during weighing.
- It should have relatively higher molecular masses such that weighing errors must be negligible.
- It should be readily soluble under conditions under which it is employed.
- The reaction with a standard should be stoichiometric or practically instantaneous.
- Substances commonly employed as primary standards: -- - - - - -
- Acid- base reaction: - - - - -
Sodium carbonate, Sodium tetraborate, Potassium hydrogn pthalate, Potassium hydrogen iodate.
- Compex formation reaction: - - - - - - -
In this reaction we use pure metals as primary standard.
Examples: - - - - -
Zinc, Magnesium, COPPER, Manganese.
- Precipitation reaction: - - - - - - -
Examples: - - - - -
Silver, Silver nitrate, KCl, NaCl.
- Oxidation and reduction reaction: - -- - - - -
Potassium dichromate, Potassium bromate , Potassium iodate, sodium oxalate.
- Secondary standard: -- - - - - - -
In this the content of active substances have been found by comparison aganist a primary standard.
In primary standard, the conc. of dissolved solute is determined by direct weight while in secondary standard the conc. of dissolved solute is determined by the reaction of the volume of the solution aganist measured volume of the primary standard.
- Classification of errors: - - - - - - -
- Systematic or determinate error: -- - - -
These are those errors which can be avoided & whose magnitude can be determined.
- Operational or personal error :- - - - -
This is connected with individual but not with method.
Example: - - - --
Inability of a person to judge burette reading or colour change.
The chemical loss of material during sample dissolution from bumping.
- Instrumental & reagent error : - - - -
This is due to the faulty construction of instruments.
Example: - -- - - - -
Use of uncaliberated weight.
Use of reagent containing impurity and attack on glass & porcelain etc.
- Error of method: - - - - - -
This is the most serious error & difficult to detect.
example: - - - --
Solubility of precipitate.
- random or indeterminate error: - - - - - These errors are due to causes overwhich analyst has no control.
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