Accuracy & Precision & Unit Conversions Flashcards
What is accuracy?
Accuracy is the closeness of the results to the true value.
What is precision?
Precision is the closeness of the replicate results to each other.
What is a primary standard?
A primary standard is a compound which is easily purified, dried and stable, and is used to standardise solutions.
What is a secondary standard?
A secondary standard is a compound which is easily dissolved to form stable solutions, and is standardised against a primary standard.
What is a certified reference material?
A certified reference material is a substance used for calibration, whose values were certified by a technically valid procedure from several well-established laboratories, using different techniques, and is accompanied by a certification/ document.
When would a certified reference material be used in analytical science?
calibrate instruments,
validate analytical methods,
quality control,
transfer results between labs
What is absolute analysis?
Absolute analysis involves chemical reactions that go to stoichiometric completion.
What is comparative analysis?
Comparative analysis involves preparing standards and comparing a measurement of a(n) (unknown) sample against known standards (of known concentrations).
Give an example of absolute analytical measurement.
pH, conductance
Give an example of comparative analytical measurement.
pH, absorbance
What are the characteristics of a certified reference material?
A certified reference material has properties which have been certified by a technically valid procedure, and is accompanied by certification.
What are the requirements of a primary standard?
Primary standards should:
- be easy to purify and dry, and maintain in a pure state
- not change in the air during weighing (i.e. absorb moisture or be photochemically unstable)
- have a high molar mass to minimise weighing errors
- be at least 99.98% (w/w) pure.
- react instantaneously with the solute, according to a stoichiometric ratio
- be soluble in the conditions which it will be used
What are the requirements of a secondary standard?
Secondary standards can be dissolved to form stable solutions.
Give an example of a primary standard.
Potassium iodate, sodium chloride
Give an example of a secondary standard.
Sodium thiosulphate, sodium hydroxide
What are external standards?
External standards involve preparing a set of standards of pure analyte at different concentrations, and measuring a signal with an instrument, then drawing a calibration curve.
Define molarity.
Molarity is the number of moles per litre.
Define molality.
Molality is the number of moles per kg.
Define normality.
Normality is the number of equivalence per litre.
How is precision measured?
Precision is measured with standard deviation.
How is accuracy measured?
Accuracy is measured with percentage error.
How would a graph that would be obtained when using external standard calibrations look like?
The line would go through the origin
How would a graph that would be obtained when using internal standard calibrations look like?
How would a graph that would be obtained when using standard additions calibrations look like?
