Unit 1 - 3 terms

• Analytical Chemistry

  • branch of chemistry involved with the analysis of chemical substances
  • measurement science consists of a set of powerful ideas and methods that are useful in all fields of science and medicine. (Skoog, 2011)
• Types of Chemical Analysis

  1. Qualitative Analysis – what is present?
  2. Color, Odor, Temperature
  3. Quantitative Analysis – how much is present?
  4. Volume, Mass, Density
• Classification of Analytical Methods

  1. Classical Method - chemical properties of analytes
  2. Instrumental Method - physical properties of analyte
• Classical Method “Wet chemical method”

  • Earliest method of analysis
  • Relied mainly on analytes’ chemical properties
  • Separation:
  • Precipitation
  • Extraction
  • Distillation
  • Qualitative Analysis - yielded products that could be recognized by their
  • color
  • boiling points
  • solubilities
  • optical activities
  • refractive indexes
  • Quantitative
  • gravimetric
  • titrimetric
• Instrumental Methods

  • measures physical properties of analyte
  • conductivity
  • electrode potential
  • light absorption or emission
  • mass-to-charge ratio
  • fluorescence
  • Separation employs:
  • chromatography
  • electrophoresis or field flow fractionation
  • Qualitative
  • color
  • boiling or melting points
  • solubilities
  • optical activities
  • refractive indexes
  • Quantitative
  • gravimetric
  • titrimetric
• Classifying Quantitative Analysis

  • The results of a typical quantitative analysis are computed from two measurements:
  • Mass or volume of the sample being analyzed
  • Measurement of some quantity (mass, volume, intensity of light, or electrical charge) that is proportional to the amount of analyte in the sample
• Classification of Quantitative Analysis

  1. Gravimetric methods:
  2. determine the mass of the analyte or some compound chemically related to it.
  3. Volumetric (Titrimetric) methods:
  4. determine the volume of a standard reagent that reacts completely with the analyte
  5. Electroanalytical methods:
  6. measurement of electrical properties (i.e., potential, current, resistance or quantity of electrical charge)
  7. Spectroscopic methods:
  8. measurement of interaction between emr and analyte atoms or molecules / production of radiation by analytes
• Steps in a Quantitative Analysis

  1. Selection of a method - Needs to consider the following:

  • Level of accuracy required
  • Economic factors
  • Complexity and number of samples

  1. Acquiring the sample 

  Sampling - process of collecting a small mass of material whose composition accurately represents the bulk of the material being sampled.

  • Obtain a representative sample
• 1. Complete analysis: determine the amount of each component in the sample
  2. Ultimate analysis: amount of each element present without regard to actual composition
  3. Partial analysis: most common; determining one or limited number of species in a sample.
• Interference: species other than the analyte that affect the final measurement; it may enhance or attenuate the final measurement.

  Masking: elimination of an interference by converting it to a non-interfering form.
• Solvent: the fraction of a solution in which the other components are dissolved.

  Solute: a substance that is dissolved in a solvent to produce a solution.
• 1. Saturated solution: the solvent contains the maximum amount of a solute that can be dissolved at equilibrium at a given temperature.
  2. Unsaturated solution: contains less than the maximum amount of a solute that can be dissolved
  3. Supersaturated solution: contains more than the equilibrium amount of a solute that can be dissolved. When a supersaturated solution is disturbed in any way, the excess solute separates and the equilibrium solubility is restored.
• Colligative Properties

  • Any property of a solution that depends on the number of solute particles
  • Not affected by the nature of the solute particles
  • Colligative properties are physical properties of a solution.
  • This is only applicable to dilute solutions (≤0.20M)

  1. Vapor pressure lowering
  2. Boiling point elevation
  3. Freezing point depression
  4. Osmotic pressure
• Vapor Pressure

  • It refers to pressure exerted by vapor on the surface of the liquid (result of evaporation of liquid).
• Raoult’s Law

  • If solute is nonvolatile, the vapor pressure of the solution is always less than of the pure solvent
• Changes in Boiling and Freezing Points

  • The presence of solute particles affects the boiling and freezing points of a solvent.
  • To boil the solution, vapor pressure above the solution must be equal to the external atmospheric pressure.
  • Then, the temp of the solution must be raised!
• Osmosis

  • is a selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one
• Osmotic Pressure 

  Osmotic pressure (π) is the pressure required to stop osmosis
• Electrolyte Solution

  • The effects of the electrolytes on colligative properties are larger than nonelectrolytes.
  • This is because the number of particles released in solution is greater for electrolytes.

  EXAMPLE: Compare the number of particles released by one mole of sucrose and NaCl in the solution.
• The van’t Hoff factor (i), is used to introduce this effect into the calculations.

  It is a measure of the extent of ionization or dissociation of the electrolyte in the solution

  
• Sensitivity smallest weight producing a certain measurable response

  Readability - smallest discernible scale division which may or may not be the sensitivity of the equipment
• Meniscus the shallow curve on the surface of the liquid

  • Parallax Error happens when the experimenter reads the volume of the liquid at a height lower or higher the eye level.
• To Deliver - TD glasswares

  • burette, pipette 
  • refers to laboratory apparatus designed to deliver the specified volume at the calibration temperature
  • calibrated in milliliters (mL)
  • may be used to measure approximately or accurately the volume of liquids
• Accuracy - the agreement of a particular value with the true value.

  Precision - the degree of agreement among several measurements made in the same manner
• 1. Instrument errors - are caused by imperfections in measuring devices and instabilities in their components.
  2. Method errors - arise from non-ideal chemical or physical behavior of analytical systems.

  Personal errors - result from the carelessness, inattention, or personal limitations of the experimenter.
• Determinate or Systematic

  • Error due to procedural or instrumental factors that cause a measurement to be consistently too large or too small
  • error can, in principle, be discovered and corrected
  • consistent error.
  • Key Feature: reproducible
  • can be detected and corrected.
  • too high (+) or too low values (-)
  • may lead to bias in measurement techniques.
  • affects accuracy
• Indeterminate Errors/Random

  • type of error, which can be either positive or negative 
  • cannot be eliminated, based on the ultimate limitations on a physical measurement.
  • causes data to be scattered more or less symmetrically around a mean value.
  • errors occur whenever a measurement is made.
  • caused by many small but uncontrollable variables.
  • the errors are accumulative.
• Gross Errors

  • usually occur only occasionally, are often large, and may cause a result to be either high or low.
  • lead to outliers.
  • results that appear to differ markedly from all other data in a set of replicate measurements.
  • various statistical tests can be done to determine if a data point is an outlier.
• Absolute Error

  • The absolute error E in the measurement of a quantity xi is given by the equation
  • where xt is the true, or accepted, value of the quantity.
  • note that we retain the sign in stating the error
• Relative Error

  • a more useful quantity than the absolute error. The percent relative error is given by the expression:
• Median - middle value in a set of data that has been arranged in order of size.
• A normal error curve has several general properties: 

  (1)The mean occurs at the central point of maximum frequency, (2)there is a symmetrical distribution of positive and negative deviations about the maximum, (3)there is an exponential decrease in frequency as the magnitude of the deviations increases. – Small random uncertainties are more common
• Standard Deviation

  • measures how closely the data are clustered about the mean.
  • The smaller the standard deviation, the more closely the data are clustered about the mean
• The quantity N — 1 is called the number of degrees of freedom
• Spread or Range (w)

  • the difference between the largest value in the set and the smallest.
  • Another term to describe the precision of a set of replicate results. 
• CONFIDENCE INTERVAL – the range of values within which the population mean µ is expected to lie within a certain probability 

  CONFIDENCE LIMIT – boundaries of the confidence interval 

  CONFIDENCE LEVEL – probability that the true mean lies within the certain interval 

  SIGNIFICANCE LEVEL – probability that the result is outside the confidence interval
• Confidence Interval 

  • interval surrounding experimental mean within which the population mean μ lies with a certain degree of probability
  • the boundaries (range) of probability are defined by the confidence limits
  • the probability that the true mean lies within a certain interval is defined by confidence level expressed in %
  •  Generally accepted is 90 - 95%
• CI

  A) ts

  B) N

  C) z

  
• Outliers

  • suspect values.
  • consequence of determinate errors.
  • rejection may not need statistical test

  Q test

  • also known as Dixon’s Q-test 
  • use to test presence of outliers 
  • suspect values 
  • consequence of determinate errors 
  • rejection may not need statistical test

  How:

  1. Arrange numbers chronological 
  2. Solve for Q
  3. If Q calculated ≥ Q tabulated , data can be rejected 
• Q-test