TITRIMETRY

 Titrimetry, also known as Titration is a procedure in which definite volumes of solutions of known and unknown concentration react with each other.

Some important terms in titrimetry

a) Titre/Titrate/Analyte – The substance of unknown concentration.

b) Titrant – The substance dissolved in solution of known concentration.

c) Standard solution – A solution with known concentration.

d) Primary standard

e) Secondary standard

Different types of titrations in titrimetry are

a) Direct Titration

A normal titration in which concentration of analyte is determined by adding titrant till end point

b) Back Titration –

When a reaction is very slow ,the end point is not obtained quickly, an excess titrant is taken. After the reaction with analyte is complete, the amount of excess titrant is determined by titration with another standard solution .

c) Differential Titration

Differential titration is a type of titration where two or more analytes are present in a solution, and the titrant reacts with them differentially. In other words, each analyte reacts with the titrant at a different rate or in a different manner. Differential titration techniques often involve using indicators or monitoring changes in properties such as pH, conductivity, or absorbance to detect the endpoints of the titration reactions for each analyte. These methods require careful selection of titrants, indicators, and experimental conditions to ensure accurate and reliable results. Differential titration can be particularly valuable in complex analytical chemistry scenarios where precise identification and quantification of multiple components in a mixture are necessary.

Standard materials contain a precisely known concentration of a substance for use in quantitative analysis.[1]

A standard is a reference that is used in determining unknown concentrations of substances.

Types of standards in Titrimetry

  1. Primary standard
  2. Secondary standard

Primary standard

A primary standard is a reagent that is extremely pure and stable; it is not a hydrate/it has no water of hydration, and it has a high molecular weight. Further,

Further, a primary standard is a chemical or reagent which has properties such as-

(a) Highly stable (b) It is less hygroscopic (c) Should be ready to use and available (d) Should be preferably non toxic (e) Should not be expensive

In practice, an ideal primary standard is difficult to obtain. Primary standard and its solution is prepared by accurate weighing required quantity of the substance by dissolving in water to produce the required volume of the solution accurately [2]

Examples of primary standards for titration of acids is sodium carbonate, for titration of bases are potassium hydrogen phthalate (KHP), KHC8H4O4, potassium hydrogen iodate: KH(IO3)2 and for redox titrations are potassium dichromate: K2Cr2O7, sodium oxalate: Na2C2O4 

Secondary Standard

A secondary standard is a substance which is used for standardizations, whose content of the active substance has been found by comparison against a primary standard. A secondary standard is a solution which contains exactly the known amount of the substance in unit volume of the solution. By titrating against a primary standard concentration of secondary standard is known.

A secondary standard is a substance which cannot be used as a primary standard. e.g., sodium hydroxide cannot be used as a primary standard for the reason that it absorbs water and carbon dioxide from the atmosphere.

Volumetric solutions also known as Standard Solution are solutions of reagents of known concentration, primarily for use in quantitative determinations. 

Standardization of sodium hydroxide using KHP[3]

Sodium hydroxide is a deliquescent[4] substance. It is not possible to weigh it accurately. Therefore, it is not possible to prepare a standard solution of sodium hydroxide of known concentration by simply weighing NaOH.

This problem can be overcome by titrating it against a sample of potassium acid phthalate (KHP), HOOC-C6H4-COOK , a primary standard acidic substance. A primary standard substance can be weighed accurately as it is stable and is not hygroscopic[5]. The reaction between NaOH and KHP (molar mass 204.23 g/mole) is as follows

NaOH + HOOC-C6H4-COOK → NaOOC-C6H4-COOK + H2

Procedure

1.Weigh approximately 0.3 -0.4 g of KPH in to three conical flasks and label them 1,2 and 3

2. To prepare sodium hydroxide solution of approximate concentration  measure 8 to 8.5 ml of 6M NaOH solution in a conical flask and add distilled water to it.

3.For standardisation of NaOH solution fill the burette with the prepared NaOH solution.To the flasks labelled 1 (Step1) add 50 ml distilled water and 1-2 drops of Phenolphthalein indicator.Swirl the flask until all KHP dissolves.

4. Now add NaOH solution from burette into the conical flask containing KPH solution with continuous swirling until the end point . At this point a faint pink colour appears.This Titration.

5.Close the nozzle of the burette and note the volume of NaOH consumed.

6. Refill the burette with NaOH solution and repeat the same titration with KPH containing flasks labelled 2 and 3 as well.

Calculations

By looking at the balanced equation we can understand that number of moles of OH(from base) = Number of moles of H+(from acid)

NaOH + HOOC-C6H4-COOK → NaOOC-C6H4-COOK + H2O

Molarity of NaOH = (mol KHP) / (Volume of NaOH used for titration)

Standardisation of a hydrochloric acid solution using a standard solution of sodium carbonate[6] 

Laboratory grade hydrochloric acid is not sufficiently pure to be a primary standard.

A standard solution of sodium carbonate(0.1M) is used in determining the exact concentration of a hydrochloric acid solution. 

The neutralisation reaction that occurs is as follows: Na2CO3 + 2HCl →  2NaCl + H2O + CO2 

Procedure

1. Fill the burette with hydrochloric acid solution.

2.Using a clean pipette out 25 ml of standard sodium carbonate solution into a conical flask.

3.Add 2-3 drops of methyl orange indicator.

4.Add hydrochloric acid solution dropwise to the conical flask until the colour changes. This is ‘Titration‘.

5.Throw away the contents of the conical flask and repeat the titration for two times.

6. Note the values of burette reading for each titration. This gives the volume of hydrochloric acid consumed to neutralise sodium carbonate solution for each titration.

Calculations

If MA,VA, nA are the molarity, volume and number of moles of acid and MB,VB and nB are the molarity, volume and number of moles of base then MA can be calculated from the formula

VA x MA x nB = VB x MB x n

From the stoichiometric equation nA and nB are 2 and1 respectively.

Written by Uma Maheswari P

                                         

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