Analytical Terms

Homogeneous

Homogeneous

same throughout

Heterogeneous

differs from region to region

milli (m)

10^-3

micro (u)

10^-6

nano (n)

10^-9

kilo (k)

10³

Molarity (M)

number of moles of a substance per liter of solution

Molality (m)

concentration expressed as moles of a substance per kilogram of solvent

Solution

homogeneous mixture of two or more substances

Solute

a minor species in a solution

Solvent

the major species in a solution

Concentration

how much solute is contained in a given volume or mass of solution or solvent

Electrolyte

substance that dissociates into ions in solution

Tare

mass of an empty vessel that is used to receive the substance to be weighted

Sensitivity

the smallest increment of mass that can be measured

Parallax error

the shift in an object’s position as it is viewed from different angles

Weighing by difference

for hygroscopic reagents that rapidly absorb moisture from the air

Volumetric flasks

prepare solutions with known volume

Transfer pipet

deliver fixed volumes

Calibration

process of measuring the actual quantity of mass, volume, etc that corresponds to an indicated quantity on the scale of the instrument

Significant Figures

minimum number of digits needed to write a given value in scientific notation without loss of precision

Addition & Substraction (Sig. Fig)

limited by the least certain digit

Rounding (Sig. Fig)

round to nearest EVEN digit

Multiplication & Division (Sig. Fig)

limited by the smallest amount of sig. figs.

Logarithms & Antilogarithms (Sig. Fig)

determined by the mantissa

Precipitation titrations

quantity of titrant required for complete precipitation of analyte tells us how much analyte was present

Combustion analysis

sample burned in excess oxygen and products are measured

Gravimetric analysis

mass of product used to calculate quantity of original analyte

absorbed impurities within the crystal

inclusions or occlusions

inclusions

impurity ions that randomly occupy crystal lattice sites

occlusions

impurity pockets that are literally trapped inside the growing crystal

Systemic error

(determinate) affects accuracy (nearness to true value)

Random error

(indeterminate) affects precision (reproducibility) of result

Accuracy

degree of agreement between measured value and accepted true value

Precision

degree of agreement between replicate measurements of same quantity

Null Hypothesis (H0)

difference explained by random error

Alternate Hypothesis (HA)

difference cannot be explained by random error

linear response

corrected analytical signal (from sample - blank)= proportional to quantity of analyte

linear range (analytical method)

range over response = proportional to concentration

dynamic range

range over measurable response to analyte, even if response = not linear

False positive

concentration exceeds legal limit when concentration is actually below limit

False negative

concentration below limit when concentration is actually above limit

Raw data

individual values of measured quantity

Treated data

concentration or amounts found by apply calibration procedure to raw data

Blanks

interference by other species in sample

Purpose of Blanks

frequent measurements of blanks detect whether analyte from previous samples is carried into subsequent analyses by adhering to vessels or instruments

Method blank

• all components except analyte

• taken through all steps of analytical procedure

• substract response of method blank from response of sample before calculating quantity of analyte

Reagent blank

similar to method blank but has not been subjected to all sample preparation procedures

Field blank

indicates if analyte is inadvertently picked up by exposure to field conditions

Solubility Product (Ksp)

equilibrium constant for the reaction in which a solid salt dissolves to give its constituent ions in solution

Saturated solution

contains excess undissolved solid

Common ion effect

a salt will be less soluble if one of its constituent ions is already present in the solution

Coprecipitation

a substance whose solubility is not exceeded precipitates along with another substance whose solubility is exceeded

Acid

a substance that increases the concentration of hydronium ion when added to water

Base

a substance that decreases the concentration of hydronium ion

Matrix

everything in unknown, other than analyte

Matrix effect

change in analytical signal caused by anything in sample other than analyte

Specificity

ability of analytical method to distinguish analyte from everything else that be in sample

Linearity

measures how well calibration curve follow straight line, shows response proportional to quantity of analyte

Range

concentration interval over which linearity, accuracy & precision are all acceptable

Ionic Atmosphere

Region of net positive or negative charge around an anion or cation in a solution.

Ionic strength

μ is a measure of the total concentration of ions in solution.

Charge Balance

an algebraic statement of electron neutrality: The sum of the positive charges in solution equals the sum of the negative charges in solution.

Mass Balance

called the material balance, is a statement of the conservation of matter.

The mass balance states that the quantity of all species in a solution containing a particular atom (or group of atoms) must equal the amount of that atom (or group) delivered to the solution.

End-point detection methods:

1. Metal ion indicators (most common)

2. Mercury electrode

3. Ion-selective electrode

4. Glass (pH) electrode

Direct titration

Analyte is titrated with standard EDTA. Analyte is buffered to a pH where for the metal-EDTA complex is large and the color of the free indicator is different than that of the metal-indicator complex.

Back titration

A known excess of EDTA is added to the analyte. Excess EDTA is then titrated with a standard solution of a second metal ion.

Displacement Titration

Use when the analyte, such as Hg²⁺, does not have a satisfactory indicator.

Indirect Titration

Anions that precipitate with certain metal ions can be analyzed with EDTA using an indirect titration.

Masking

A masking agent is a reagent that protects a component of the analyte from reaction with EDTA.

Ohm’s Law

Current is directly proportional to the potential difference across a circuit and inversely proportional to the resistance, R, across the circuit.

Galvanic cell

(also called a voltaic cell) uses a spontaneous reaction (ΔG < 0) to generate electrical energy.

potentiometer (voltmeter)

shows a positive voltage when electrons flow from the anode to the cathode; if electrons flow the other way, it shows a negative voltage.

Potentiometry

the use of electrodes to measure voltages that provide chemical information.

Electroactive species

a substance that can accept or donate electrons at an electrode.

Indicator electrode

one that develops a potential whose magnitude depends on the activity of one or more species in contact with the electrode.

Reference electrode

one that maintains constant potential against which the potential of another half-cell may be measured.

Formal potential

is the reduction potential that applies under a specified set of conditions (including pH, ionic strength, and concentration of complexing agents).

Metal electrodes

develop an electric potential in response to a redox reaction at the metal surface.

Ion-selective electrodes

are not based on redox processes but on selective binding of one type of ion to a membrane, which generates a potential.

Error in pH Measurement

1. Standards

2. Junction potential

3. Junction potential drift

4. Sodium error

5. Acid error

6. Equilibration time

7. Hydration of glass

8. Temperature

9. Cleaning

Standards

Limit us to ±0.01 pH unit.

Junction potential

Exists at the porous plug near the bottom of the electrode and gives an uncertainty of ~0.01 pH unit.

Junction potential drift

Caused by precipitation of AgCl(s) or reduction of Ag⁺ to Ag(s).  Recalibrate every 2h.

Sodium error

When [H⁺] is low and [Na⁺] is high, the electrode responds to [Na⁺] giving a measured pH lower than the actual pH.

Acid error

In strongly acidic solutions, the glass is saturated with H⁺ giving a measured pH higher than the actual pH.

Equilibration time

Requires ~30s with stirring to equilibrate with a solution.

Hydration of glass

A dry electrode must be soaked for several hours before it responds correctly to H⁺.

Temperature

A pH meter must be calibrated at the same temperature at which the measurements will be made.

Cleaning

An improperly cleaned electrode will drift while it re-equilibrates with aqueous solution.

Classes of ISE

• Glass membranes

• Solid-state electrodes

• Liquid-based electrodes

• Compound electrodes

Redox indicator

changes colors when going from its oxidized to reduced state.

Electrolysis

the process in which a chemical reaction is forced to occur at an electrode by an imposed voltage.

Overpotential

voltage required to overcome the activation energy of an electrode reaction. A greater overpotential is required to drive a reaction at a faster rate.

Ohmic potential (= IR)

is that voltage needed to overcome internal resistance of the cell. A Luggin capillary allows us to measure an electrode potential with minimal ohmic loss.

Tests for completion of the deposition

disappearance of color, no deposition on freshly exposed electrode.

Advantages of coulometry

• precision

• sensitivity

• generation of unstable reagents in situ

Voltammetry

is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed.

Polarography

is voltammetry with a dropping-mercury working electrode.

The faradaic current

is what is measured in voltammetry; It is due to reduction or oxidation of analyte at the working electrode.