CHEM211 Lecture Notes - Efflorescence, Equivalence Point, Analyte
10 May 2018
School
Department
Course
Professor
Gaussian (Normal) Distribution
❏Population mean (true) = Average
❏Standard Deviation
❏+/- 1 SD = 68%
❏+/- 2 SD = 95%
❏+/- 3 SD = 99.7%
Confidence Intervals
❏Range of values within which the specified probability that the true value lies.
❏Tcalc > Ttable → values are different based on specific confidence limit → systematic bias is
present.
❏Tcalc<Ttable → values are the same within that confidence limit.
Power of Replicates
❏As replicates increase, the CI interval decreases.
❏Reduce the chance that random errors will result in incorrect results.
❏More accurate due to partial compensation when averaging random errors.
Propagation of Random Error
❏Uncertainty for adding and subtracting → S total = ( (S1)^2 + (S2)^2 )^1/2 .
❏Uncertainty for multiplying and dividing → %RSD = ( (%RSD1)^2 + (%RSD2)^2)^½
Titration Reaction Requirements
❏Reaction must be quantitatively complete
❏Greater than or equal to 99.9% complete.
❏Less than 0.1% (or 1 ppt) remains unreacted.
❏Can be predicted from equilibrium constants
❏Reaction must be rapid → performed within reasonable time period.
❏Stoichiometry must be well defined and known
❏Method available to accurately estimate equivalence point.
Volumetric Titrations (Direct Titration)
❏Standardization → Pipet known volume concentration standard into flask.
❏Standardization → Titrate standard to calculate exact concentration of titrant.
❏Analysis → Pipet known volume of unknown concentration of sample and titrate.
Standards
❏All measurements referenced to standards → accuracy only as good as quality and accuracy of
standards.
❏Purity and composition are well known and defined.
❏Primary Standards;
❏High purity → 99.9%
❏Atmospheric stability → Can be stored indefinitely
❏Low hydrogscopocity and efflorescence.
❏High solubility (for titration).
❏High molecular weight so that it can be weighted with high precision.
❏Modest cost.
❏Secondary Standards;
❏Solution standardized against primary standard solution.
❏Make titrant up to an approximate known concentration.
❏Titrate known amount of primary standard
❏Use stoichiometry to calculate titrant moles. → Use volume of titrant needed to reach
endpoint to calculate [titrant].
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Range of values within which the specified probability that the true value lies. Tcalc > ttable values are different based on specific confidence limit systematic bias is. Tcalc
