AnaChem: Gravimetric & Volumetric Analysis & Potentiometry

Gravimetric analysis

• is a quantitative method to determine an analyte concentration by weighing a pure, solid form of the analyte.

• measurement of mass

Solid

The ____ form is obtained by adding a precipitating reagent to a solution containing the analyte

Theodore W. Richards

Gravimetric analysis was developed by

Analyte

a substance whose chemical constituents are being identified and measured.

Precipitant

also known as the precipitating agent refers to the chemical that is used to cause precipitation

Precipitate

the insoluble substance formed in the gravimetric method

Size

Particle retention depends on the ____ of the filter’s pores.

Suspended solids

• suppose you are to determine the total suspended solids in water released from a sewage-treatment facility.

• _____ are just that solid matter that has yet to settle out of its solution matrix. The analysis is easy.

• You collect a sample and pass it through a pre-weighed filter that retains the suspended solids. After drying to remove any residual moisture, you weigh the filter. The difference between the filter’s original mass and final mass gives the mass of _____.

Cellulose fiber filters 

commonly referred to as filter paper, range in pore size from 30 µm to 2–3 µm.

Glass fiber filters

constructed from chemically inert borosilicate glass, range in pore size from 2.5 µm to 0.3 µm.

Membrane filters

which are made from a variety of materials, including cellulose nitrate and PTFE, are available with pore sizes from 5.0 µm to 0.1 µm.

Recrystallization 

• If the analyte is an aqueous ion, use

• In this case we cannot isolate the analyte by filtration because the ion (Pb2+ ion) is dissolved in the solution’s matrix. We can still measure the analyte’s mass by chemically converting it to a solid form.

• With use of Pt electrodes, The Pb2+ ion in solution oxidizes to PbO2 and deposits on the Pt electrode serving as the anode. If we weigh the Pt before and after, the difference in the two measurements gives the mass of PbO2 (calculated using reaction’s stoichiometry).

Precipitation

• easiest way using the precipitation process (the reaction that creates an insoluble solid product from the reaction of two soluble solid products)

• Analyte is converted to sparingly soluble precipitate.

Solution preparation, precipitate formation, filtering, washing, heating, weighing

Key steps in precipitation

Volatilization

• using thermal or chemical energy to determine their masses

• ____ gases (that can be easily evaporative), like carbon dioxide, chlorine, etc., can be separated with the help of _____ gravimetry.

• Example: In determining the moisture content of food, thermal energy vaporizes the H2O.

Larger particles, supersaturated

Particle size and filterability of precipitates

Selective, specific, readily filtered, low solubility, known composition

Properties of precipitant (precipitating agent)

Errors

There are different causes of ___ in gravimetric analysis:

• Inaccurate weighing

• Incomplete and imperfect precipitation

• Use of substandard reagent and apparatus.

• Haste and impatience

Volumetric Analysis

Measurement of the volume of a solution whose concentration is known and applied to determine the concentration of the analyte.

Titration or Titrimetric method

delivery of a solution of known concentration into a solution of unknown concentration

Acid–base titrations

in which an acidic or basic titrant reacts with an analyte that is a base or an acid

Complexometric titrations

involving a metal–ligand complexation reaction

Redox titrations

where the titrant is an oxidizing or reducing agent

Precipitation titrations

in which the analyte and titrant react to form a precipitate

Equivalence point.

• For a titration to be accurate we must add a stoichiometrically equivalent amount of titrant to a solution containing the analyte. We call this stoichiometric mixture the

• a theoretical point reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample

Titrant/standard solution

solution of known concentration used in titration (added from burette)

Titrate/Analyte

unknown solution which we determine its concentration

End point

physical change associated with the condition of chemical equivalence.

Indicators

• are often added to the analyte solution to produce an observable physical change (signaling the end point) at or near the equivalence point.

• Complex organic compound which shows clear visual change after the reaction between titrant and titrate is just complete.

• Phenolphthalein

• Methyl orange, blue

• Thymol blue

Titration error

• The difference between the end point volume and the equivalence point volume is a determinate method error, often called the ____.

• If the end point and equivalence point volumes coincide closely, then the ____ is insignificant and can be safely ignored. Clearly, selecting an appropriate end point is critical

Buret

Typical setup for carrying out a titration. The apparatus consists of a ___, a ____ stand and clamp with a white porcelain base to provide an appropriate background for viewing indicator changes, and a wide-mouth Erlenmeyer flask containing a precisely known volume of the solution to be titrated. The solution is normally delivered into the flask using a pipet.

Erlenmeyer flask

Typical setup for carrying out a titration. The apparatus consists of a buret, a buret stand and clamp with a white porcelain base to provide an appropriate background for viewing indicator changes, and a wide-mouth ____ containing a precisely known volume of the solution to be titrated. The solution is normally delivered into the flask using a pipet.

0.01 mL

Detail of the buret graduations. Normally, the puret is filled with titrant solution to within 1 or 2 ml. of the zero position at the top. The initial volume of the buret is read to the nearest ____.

Pink

Before the titration begins. The solution to be titrated an acid in this example, is placed in the flask, and the indicator is added as shown in the photo. The indicator in this case is phenolphthalein, which tums ____ in basic solution.

Over titrate 

If you overshoot the endpoint, it will cause an error in the calculation of the solution being titrated. If the volume of titrant is in error because it is higher than it should be, the moles of titrant will be incorrect and the resulting calculation for the solution titrated will not be accurate.

Potentiometry

• It measures the electric potential across the substance.

• It is used to measure the potential of an electrochemical cell to determine the concentration of a solution.

• It's used in many fields, including pharmaceuticals, food, beverages, clinical chemistry, environmental chemistry, and agriculture:

Potential

amount of work to move a unit of charge

Current

flow of electricity, amount of electricity flowing in a circuit.

Charge

property of sub atomic particles

Electrode

The electrochemical cell is divided into two half-cells, each containing an ___ immersed in a solution containing ions whose concentrations determine the electrode’s potential.

Redox reaction

This separation of electrodes is necessary to prevent the ____ from occurring spontaneously on the surface of one of the electrodes, short-circuiting the electrochemical cell and making the measurement of cell potential impossible.

Anode

• electrode where oxidation occurs

• Electrons flow from the

• A negative electrode

Cathode

• electrode where reduction occurs

• Electron flows to the

• Positive electron

Reference Electrode

electrode potential is known

Indicator Electrode

unknown electrode potential

Salt Bridge

to maintain electrical neutrality in an electrochemical cell by allowing ions to flow between the solutions in the cell's half-cells

Electric circuit

A salt bridge containing an inert electrolyte, such as KCl, connects the two half-cells. The ends of the salt bridge are fixed with porous frits, allowing ions to move freely between the half-cells and the salt bridge, while preventing the contents of the salt bridge from draining into the half-cells. This movement of ions in the salt bridge completes the

Electrochemistry 

• Relationship between Chemical Reactions and Electricity

• Certain chemical reactions can create electricity

• Electricity can make certain chemical reactions happen

Oxidation-Reduction Reactions

• Also known as Redox Reaction

• In an _____, electrons are transferred from one reactant to another. An example is the oxidation of iron(II) ions by cerium(IV) ions. The reaction is described by the equation

Oxidation

An increase in oxidation number

Reduction

A decrease in oxidation number

Two half-reactions 

We can split any oxidation/reduction equation into ____that show which species gains electrons and which loses them. 

Galvanic Cell or Voltaic Cell

is an electrochemical cell where chemical energy is transformed into electrical energy.

Loses

Zn ___ electrons = oxidation

Gains

Cu ___ electrons = reduction

Electric current 

• Here the redox reaction has been physically separated into two half-cells, each corresponding to one half-reaction.

• The flow of electrons from the anode to the cathode produces _____.

Oxidation number

is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

Zero

The oxidation number of any uncombined element is

Equals its charge

The oxidation number of a monatomic ion

-2

The oxidation number of oxygen in compounds is ___, except in peroxides, such as H2O2 where it is -1.

Hydrogen

The oxidation number of ___ in compounds is +1 if it is combined with nonmetals and -1 if it is combined with metals.

Sum

The ___ of the oxidation numbers of the atoms in the compound must equal 0.

Ionic charge

The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its

+1, +2

The Group 1 metals have an oxidation state of ___ and Group 2 an oxidation state ___

-1, -2, -3

Group 17 elements have an oxidation state of ___, Group 16 elements of , and Group 15 elements of _.