Analytical Chemistry I

SCIT 1543


Volumetric Methods of Analysis

Learning Objectives

Volumetric Analysis Video Lecture

Concentration Calculations Video Lecture

Back Titration Video Lecture


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Titrimetry is based on determining the quantity of a reagent of a known strength that is required to react completely with the analyte.  There are three types of titrimetry. The most widely used type is volumetric titrimetry and will be the focus of this unit.

Volumetric Titrimetry  -

 

based on the volume of titrant or standard reagent used.
Gravimetric Titrimetry -

 

based on the mass of titrant or standard reagent used.
Coulometric Titrimetry -

 

based on the charge in coulombs required to complete a reaction with the analyte.

Terms

The process in which a standard reagent is added to an analyte until the reaction is judged complete.

The point at which the amount of added titrant is chemically equivalent to the analyte.

The point at which a physical change is observed that is associated with chemical equivalence.

A substance that is added to the analyte solution to give an observable physical change at or near the end point.

The difference between the equivalence point and the end point. This is a systematic error present in all titrations. The magnitude of a titration error can be identified and the subsequent effects on the analysis minimized by running a blank determination.

The process of performing all steps in an analysis in the absence of the sample.

The process where an excess of a standard solution used to react with an analyte is determined by titration with a second standard solution.

The process of determining the concentration of a volumetric solution using a primary standard.


Establishing the Concentration of a Solution

The concentration of a solution can be established by using one of two methods. 

In the direct method a carefully weighed quantity of primary standard is dissolved in a solvent and diluted to an exactly known volume in a volumetric flask.

The second method is standardization. Here the titrant to be standardized is used to titrate either a weighed quantity of primary standard (or secondary standard) or a measured volume of another standard solution.


Concentration Terms

Solutions are made up of solute and solvent.  Concentration terms are basically a relationship between the amount of solute present and the amount of solution.  Terms like percent, parts per million (ppm), molarity and normality each describe the concentration of a solute in a solution.

Molarity (moles of solute per liter of solution) and Normality (equivalents of solute per liter of solution) are generally used for calculations involving chemical reactions (stoichiometry) since moles and equivalents are essentially counting units.  Remember that chemical reactions are the result of ions, atoms or particles "colliding" with each other.

Normality and equivalents are frequently used in volumetric analyses because they simplify the calculations.  They are regularly used in industrial laboratories. However, many chemistry journals do not permit the use of normality and equivalents in their articles and require the use of molarity and moles.

Percent, parts per thousand (ppt), parts per million (ppm) and parts per billion (ppb) are useful for describing concentrations on a mass or volume basis.  Concentrated acids and bases are given in percent concentration.

To convert these types of concentrations to molarity, you must know the density or specific gravity of the solution.  For extremely dilute solutions, the density is taken to be that of water (1.0 g/ml).


Percent Concentrations

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Molarity (M)

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Normality (N)

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 Note that equivalents can be defined in several ways.  Here are just a few.


p-Values

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The value X can be any chemical species or value.    The square brackets around a chemical species indicates molar concentration.    Some examples of p-values are pH,  pOH,  pCl, and  pKa.

The equation pH = -log[H+] means that the pH is equal to the negative logarithm of the molar concentration of the hydrogen ion.

Likewise, the equation pCl = -log[Cl-] means that the pCl is equal to the negative logarithm of the molar concentration of the chloride ion.  

For values such as Ka , the equation pKa = - log(Ka) means that the pKa is equal to the negative logarithm of the acid dissociation constant, Ka.

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