The titrimetric analysis is an analytical method involving measuring the volume of a reagent reacting stoichiometrically with the analyte.
The volumetric analysis measures the volume of the solution of known concentration required to react completely with the analyte. The volumetric analysis is also known as the titrimetric analysis.
Terms involved in titrimetric analysis
An analyte is a component (element or compound) in a sample whose amount is determined by a chosen method.
The solution of known concentration used in titration is known as a standard solution. There are two types of standard solutions.
A. Primary standard solution: It is the standard solution prepared by dissolving a calculated mass of a substance in a certain volume of solution and then is directly used to determine the concentration of the unknown solution. The following pre-requisites should be fulfilled for the preparation of the primary standard solution of the substance
1. It should be available in a pure state and non-toxic.
2. It should have a higher equivalent weight to minimize error during weight.
3. The substance should not be hygroscopic and deliquescent.
B. Secondary standard solution: It is a standard solution whose concentration is first determined by using the primary standard solution and then only used to determine the concentration of the unknown solution.
The solution containing 1 g eq. wt. of substance dissolved in 1 liter of volume is called a normal solution.
The solution containing 1/10 of one g eq. wt. of substance dissolved in 1 liter of volume is called a decinormal solution.
The solution containing 1/2 of one g eq. wt. of substance dissolved in 1 liter of volume is called a semi-normal solution.
A titrant is a solution with a known concentration that is added to another solution to determine the concentration of the unknown solution. Titrant’ is the compound in the titration burette.
A titrand is a solution whose concentration is determined by titration, using the solution of known concentration.
It is the point at which the titrant becomes chemically equivalent to the analyte in the sample.
It is the point at which the indicator changes color.
The general principle of titrimetric analysis
A titrimetric method of analysis is based on the following chemical reaction:
aA+ bB → product
a molecule of analyte reacts with b molecules of reagent to give the product. The reagent B is called the titrant. It is the solution with a known concentration. The titrant is gradually added from the burette until the amount of B is chemically equivalent to the amount of A.
Requirement for reaction used in the titrimetric analysis
1. The reaction must follow the definite chemical equation. There should be no side reaction. The substance to be determined should react completely with the reagent in Stoichiometric proportions.
2. The reaction should be rapid.
3. The reaction should proceed to virtual completion at the equivalence point.
Classification of reaction in titrimetric analysis
There are four different types of titrimetric analysis:
Acid-Base titration (Acidimetry and Alkalimetry)
This includes the titration of a base with standard acid (acidimetry) and titration of acids with standard base (alkalimetry). This reaction involves the combination of hydrogen and hydroxide ions to form water.
HA + OH– → A– + H2O
Types of acid-base titration
- Strong acid- strong base
- Weak acid-strong base
- Strong acid-weak base
- Weak acid-weak base
Acid-base indicators are substances that change color or become turbid at a specific pH. They determine the equivalence point and also measure the pH. They are stable and exhibit strong color.
There are three different types of acid-base indicators. They are as follow:
- The phthaleins (e.g., Phenolphthalin)
- Azo indicators ( e.g., Methyl orange)
- Triphenylmethane indicators (e.g., Malachite green)
Oxidation-Reduction (Redox) reaction
The titration involving the change in oxidation number or transfer of electrons among the reacting substances is called redox titration. In this titration, one reactant is oxidized and the other is reduced. So it involves the reaction of oxidant with a reductant in presence of a redox indicator. The standard solution is either oxidizing agent or a reducing agent.
Ce4+ + Fe 2+ ⇌ Ce3+ + Fe 3+
For detailed information watch this video
There are different types of redox indicators they are as follows:
Self-indicator: A colored substance may act as the self-indicator. For example, KMnO4 during the reaction between KMnO4 and Fe (II) solution. In this reaction, Fe (II) ions are oxidized to Fe (III) by MNO4- and KMnO4 itself gets reduced. The solution remains colorless before the endpoint when Fe (II) ions are completely oxidized by KMNO4. The added KMNO4 in slight excess imparts pink color to the solution.
It is a type of redox indicator which reacts with one of the reagents in a specific manner to give color. For example, starch reacts with iodine to give deep blue color.
It is not added to the solution being titrated but used outside the titrating system. For example, the ferrocyanide ion was used to detect Fe (II) ion by the formation of Fe (II) ferricyanide on a spot plate outside the titration vessel.
The redox potential can be measured during redox titration. The equivalent point is detected from the large change in potential in the titration curve. This is called potentiometric titration.
True-redox indicator (internal indicator)
These are the indicators that themselves undergoes oxidation-reduction and exhibit different color in oxidized and reduced forms. E.g., Diphenylamine, 1,10-phenanthroline
Characteristics of redox indicators:
1. The ideal redox indicator should mark the sudden change in oxidation potential at the equivalence point. l
2. These indicators should have the oxidation potential intermediate between that of titrand and titrant.
3. The oxidation and reduction reaction should be reversible and quick.
4. They should change color over a particular range.
This reaction involves the formation of a precipitate. Precipitation titration involves the precipitation reaction. Silver nitrate can precipitate quantitatively halide ions and thiocyanate ions (SCN-) from their solutions and these precipitation reactions form the basis of titrimetric estimation of the ions.
AgNO3 + X– → AgX↓ + NO3–
Where X= Cl– , Br– , I– or SCN–
For more information watch this video:
Indicators for the precipitation titration
The indicators used to determine the end point of precipitation reaction are as follows:
1. Color precipitate formation (Mohr’s method)
A dilute solution of a suitable substance that forms an intense color precipitate with the titrant is one type of indicator. For example, Mohr’s method of argentometric titration of Cl-. When NaCl solution containing potassium chromate as an indicator is titrated with AgNO3 solution, Cl- ions precipitate as AgCl. After the complete precipitation of Cl- as AgCl. Excess of Ag+ ions combined with CrO4–present as an indicator to give a brick red precipitate of silver chromate.
AgNO3 + NaCl → AgCl↓ (White ppt) + NaNO3
2AgNO3 + K2CrO4 → Ag2CrO4↓ (Brick red ppt.)+2KNO3
2. Color solution formation (Volhard’s method)
A solution of a suitable substance that forms a soluble colored compound with the titrant. E.g., in Volhard’s method of argentometric titration of Cl-, the excess of unreacted Ag+ ion present is detected by titrating mixture against the standard thiocyanate solution using Fe (III) ion as the indicator.
Ag+ + SCN– → AgSCN
Fe+ + SCN– →FeSCN 2+ (red)
3. Adsorption indicator (Fajan’s method)
An adsorption indicator is an organic compound that is absorbed on the surface of a precipitate, resulting in a color change. The mechanism of the action of the adsorption indicator is based on the properties of colloids. They are useful for the detection of end-point during precipitation titration. At the equivalence point, the indicator is adsorbed on the surface of the precipitate Changes in the indicator occur during adsorption, resulting in the formation of a substance of a different color. E.g., Dichlorofluorescein, Fluorescein, etc.
Conditions for selecting appropriate adsorption indicators
1. The precipitate should separate ideally in the colloidal solution and coagulation should be avoided as far as possible.
2. The solution should not be too dilute as the amount of precipitation formed will be small and color change far from sharp with certain indicators.
3. The titration solution must be at suitable pH for the indicator.
4. The indicator ion must be an opposite charge to the ion of precipitating agent.
5. The indicator ion should not be adsorbed before the particular compounds have been completely precipitated.
The titration between metal ion solution and the complexing agent (chelating agent) in presence of a suitable indicator is called complexometric titration. In this titrimetric analysis formation of the colored complex is used to indicate the endpoint. This involves the estimation of metal ions in solution titrimetrically through the complexation with a strong chelating ligand. Chelating ligand provides extra stability to metal complex due to the chelate effect.
Ethylenediamine tetraacetic acid (EDTA)is a very good complexing agent for metal ions.
For example, EDTA is written as H4Y. So, the disodium salt is represented by Na2H2Y. This salt gives the complex forming ion H2Y2– in an aqueous solution, which can form complexes with metal ions in the mole ratio 1:1 with a simultaneous release of H+ ions.
Na2H2Y ⇌ H2Y+ + 2Na+
The reaction involved in the titration of divalent metal ions may be represented as
M2+ + H2Y2- → MY2- + 2H+
The complex formation depends upon the pH of the medium, each metal ion forms a stable complex with EDTA at a definite pH.
For more details watch this video:
Types of EDTA titration
The metal ions can be estimated by titration with EDTA by the following methods:
1. Direct titration
It is the most suitable method of complexometric titration of many metal ions. In this titration method, the EDTA solution is added to the sample containing metal ions till the end point is achieved. Some auxiliary complexing agents like tartrate or citrate can be used to prevent the precipitation of metal as hydroxide or basic salt. If the metallochromic indicator is used endpoint may be detected by a color change of the indicator. E.g., Determination of Ca2+ or Mg2+ in hard water.
2. Back titration
Many metals cannot be titrated directly with EDTA as they may precipitate from the solution in the pH range required for titration or the reaction between the EDTA and metal ion is a very slow, or due absence of a suitable indicator. So, in this excess amount of the standard solution of EDTA is added to the metal solution being examined and the excess EDTA is back titrated with metal ion solution using a suitable indicator.
For example, During the determination of Mg2+ and Mn2+ using EDTA from the mixture solution by direct titration, Mg2+, and Mn2+ precipitate as Mg (OH)2 and Mn (OH)2. Hence from the back titration, the amount of Mg2+ and Mn2+ can be estimated from their mixture.
Mixture of Mg2+ and Mn2+ + EDTA → Mg-EDTA + Mn-EDTA
If F- is added as a demasking agent, it demasks Mg2+ ion from Mg- EDTA complex. Then the standard solution of Mn2+ ion is added which will react with EDTA set free from Mg- EDTA complex.
MgY2- + 2F– + Mn 2+ → MgF3 + MnY2-
3. Replacement or substitution titration
It is a type of complexometric titration used to determine metal ions when direct and back titration fail to provide a sharp endpoint. This type of titration is appropriate for metal ions that do not react with the metal ion indicator or form a stable complex with EDTA than other metal ions. This titration method involves displacing magnesium or zinc ions from an EDTA complex with an equivalent amount of metal ions.
Mn++ MgY2- ⇌ (MY)(n-4)+ + Mg2+
The amount of magnesium ion set free is then titrated with a standard solution of EDTA in presence of a suitable metal ion indicator.
4. Indirect titration
Some metal cations form a precipitate with some anions. They cannot react with EDTA so these can be analyzed by indirect titration with EDTA. Gold and silver cannot be titrated directly with EDTA. So they are first reacted with [Ni (CN)4 ]2- ion from which Nickel is set free which is determined by EDTA.
[Ni (CN)4 ]2- + 2Ag+ 2 [Ag (CN)2 ]– + Ni2+
Metal ion indicator
Colored organic compounds that form chelates with metal ions are known as metal ion indicators. The color of the chelate must differ from that of the free indicators. They are chelating agents with several ligand atoms suitably arranged for coordination with metal ions. E.g., Erichrome -Black T (solochrome black), murexide, calmagite, xylenol orange etc.
Criteria for suitable metal ion indicators are as follows
1. The color reaction must be distinct.
2. The metal-indicator complex must be sufficiently stable, otherwise, dissociation results in no sharp color change.
3. There must be a strong color contrast between the free indicator and the metal-indicator complex.
4. The indicator must be extremely sensitive to metal ions in order for the color change to occur as close to the equivalent point as possible.
Advantages of titrimetric analysis
The advantages of titrimetric analysis are:
- It is a common technique in laboratories of schools, colleges, research laboratories, and industries.
- It is a rapid method.
- This method is less expensive than other analytical methods.
- Analysis can be performed by using a simple apparatus.
- Several types of analytes like acid-base, oxidants, reductants, and complexable species can be analyzed by using titrimetric methods.
- It tends itself to a variety of techniques as visual titrimetry, conductometry, voltametry, and spectrophotometric titrimetry.