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INTRODUCTION TO VOLUMETRIC ANALYSIS
Objectives
At the end of this unit the student is expected to be able to :
1- Answer questions such as : what is volumetric analysis? , Titration? , equivalent point? , end point? , primary standard ?, titrant? and what is Standardization? 2- Calculate the volume of titrant at the equivalent point. 3- Understand the methods that are used for the detection of the equivalent point. 4- Know the requirements for a reaction to be applied in volumetric analysis. 5- Answer the questions : what is titration curve? Why it is important? How it can be derived? 6- Calculate the concentrations of all species participating in the titration reaction at any point during titration. 7- Do all sorts of volumetric calculations.
Unit 6 Subjects
Introduction
In volumetric analysis, a chemical called a titrant is added to a solution of unknown concentration called analyte (titrand) together with an indicator that will mark the time at which all of the analyte has been reacted. At this point, the analyst can determine how much analyte is originally present from the amount of titrant added. It is some times called titrimetric analysis because all experiments under this type of analysis are completed by titration
reactions.. It is highly precise , easy process , fast result and only
Simple apparatus is required. Volumetric analysis takes place in a wide variety of situations such as in industrial laboratories, hospital laboratories, forensic science laboratories .Titration is important in environmental chemistry , food , beverage and the pharmaceutical industries.
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What is Volumetric Analysis?
The reaction is generally carried out in a conical
flask containing the liquid or dissolved sample.
Titrant solution is volumetrically delivered
slowly with shaking to the reaction flask using a
burette. Delivery of the titrant is called a
titration. The titration is complete when
equivalent titrant has been added to react with
all the analyte according to the balanced
titration reaction equation. This is called
the equivalence point VIDEO
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What is Volumetric Analysis?
An indicator is often added to the reaction flask to signal when all of the analyte has reacted. The titrant volume where the signal is
generated is called the end point volume or
titer. The equivalence and end points are rarely
the same ( titration error ). Volumetric methods have the potential for a precision of up to ± 0.1% or less.
Calculating volume of titrant at equivalent point ( Veq.p ) If the number of moles or mmoles of the analyte in the conical flask is known ( as in
What is Volumetric Analysis?
2 NaOH + H 2 SO 4 Na 2 SO 4 + 2 H 2 O
mmoles H 2 SO 4 = mmoles NaOH X 1/
10 X 0.1 = 0.2 X Veq.p (mL) X 1/
Veq.p (mL ) = 10 ml of NaOH soln.
Example : In the case of titrating 10 mL of 0.1 M of H 2 SO 4 solution by 0.2 M of NaOH solution , calculate Veq.p of NaOH solution?
Solution :
Detection of the equivalent point
The equivalent point can be detected by observing or measuring a sudden and sharp change in one of the physical properties of the resulting conical flask solution due to sudden and sharp change of the concentration of one of the reactants or product of the titration reaction. There are different methods to detect (determine) the equivalence point include :
(a) Color change that can be detect by naked eye : (1) In some reactions, the solution changes color without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colors as we will see with permanganate MnO 4 -^. This sometimes called auto detection. MnO 4 -^ + 5 Fe2+^ + 8 H+^ Mn2+^ + 5 Fe3+^ + 4 H 2 O ( violet ) ( colorless )
Detection of the equivalent point
Regardless of the nature of the titration reaction reactants or product. This type of indicators will be discussed in detail in later lectures
(b) Measured properties : This method depends on the measurement by an instrument of a physical property (not noticed by the naked eye) of the titration solution during titration such as electrical conductivity of the solution or its voltage or the intensity of electric current passing through the solution or absorbance of electromagnetic radiation by the titration solution ….etc. This method is outside the scope of this course and will be discussed in detail in subsequent courses.
Detection of the equivalent point
Endpoint and equivalence point
Though equivalence point and endpoint are used interchangeably, they are different
terms. Equivalence point is the theoretical
completion of the reaction i.e the point at which equivalent moles of titrant is added to the analyte solution in the conical flask. But at this point no change in the indicator color can be noticed. To notice this color you must add one or two drops of titrant solution. this point is
called endpoint. Endpoint is what is actually
measured when a physical change in the titration solution as determined by an indicator is occured.
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Primary Standard
primary standard is a substance that is 100 % pure or its purity is well known. For example, if the weight required for the preparation of a substance solution is 9.5 g and if the substance powder is 100% pure we weigh 9.5 g and dissolve it in the desired volume to get a standard solution. But if it is let us say 95% pure in this case to get 9.5 g of this substance we have to weigh 10 g of the substance powder. But If it is not pure and its purity is accurately unknown in this case we weigh 9.5 g and dissolve it in the desired volume and then standardize this solution with another standard reagent solution to fined its real concentration .This is what is called standardization. Note that if the substance is pure or its purity is well known there will be no need for standardization.
Standard Solution
As mentioned earlier the solution, which is usually placed in the burette is called standard or reagent or titrant solution. Its concentration must be exactly known because any error in the preparation of this solution will be reflected on the result of the analysis and therefore needs to be a 100 % true. Not any substance can be used as a titrant only the substance that has the following requirements :
(1) It should preferably be primary i.e it is pure or its purity is well known in order to avoid the process of standardization.
Standard Solution
(5) It’s molecular weight preferably be high in order to reduce the weighing error. For example to prepare one liter of 1.0 M of NaOH ( mw = 40 g ) we will weigh 40 g of NaOH , while if we want to prepare one liter of 1. M of Ba(OH)2 [ mw = 171 g ] we will weigh 171 g. Since the balance error is constant , let us say ± 0.001 g , its effect on NaOH will be more compared to Ba(OH) 2.
Titration Reaction
Not any chemical reaction can be used in the volumetric analysis. There are some conditions to be met in order for a chemical reaction to be used as a basis for a titration :
1- The reaction must proceed according to a definite balanced chemical equation i.e no side reactions .In other word it must be stoichiometric.
2- For volumetric methods to be useful, the reaction must be at least 99% complete ( Keq 108 ).
Titration Reaction
Types of reactions used in volumetric analysis :
Although any type of chemical reaction that meet the above requirements may be used for
titrimetric analysis, the reactions most often used fall under the following four categories :
- Acid-Base Titrations :
HA + B ↔ HB+^ + A-
- Oxidation-Reduction Titrations :
AOx + BRed ↔ ARed + BOX
Titration Reaction
- Precipitation Titrations :
M(aq) + nL (aq) ↔ MLn (s)
- Complex Formation Titrations :
M (aq) + nL ↔ MLn (aq)
These reactions will be the subjects of the rest of this course.
