Chemical Titration

8/8/2019 Chemical Titration

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-Involves reacting a measured

volume of a standard solution

with a measured volume of thesolution of unknown

concentration.

-This process is carry out through

titration



There are three practical activities associated with

titrations:1.Preparation of solutions to be used in the titration

2.Preparation of glassware to be used in the

titration

3.Performing the titration, including the recording

of burette readings

At the conclusion of a titration, calculation must

be performed using data obtained from the

titration.





Many chemicals are impure or without

accurately supplied concentration e.g.KOH and NaOH is hygroscopic / deliquescenti.e. rapidly absorbs moisture from the

atmosphere. Moreover, these strong bases canreact with acidic oxide present in the air i.e.CO2

Strong acids are supplied in concentrated

form by manufacturers and have a viableconcentration from batch to batch

==== Therefore the amount of substance can·t

be calculated or obtained ACCURATELY!!!



Primary Standard

-Is a stable, colourless, water-soluble

solid, readily available in high purity,

used to make a solution of knownconcentration for volumetric analysis.

* The solution is called a

µ Standard solution¶



Criteria for a primary standard

Primary Standard must obey the following criteria

1. Obtainable in pure form (no. of moles can be

obtained)

2. Has a known formula (no. of moles can be obtained)

3. Do not absorb moisture or other chemicals in the air

i.e. stable in air. (no. of moles can be obtained)

4. Has a reasonable high molar mass (to minimizeweighing error)

5.Completely soluble in the solvent (water) at room

temperature.Exp: Anhydrous sodium carbonate & Hydrated oxalic acid







Determine the

end point of a

titration



-Usually there isn¶t a need to use an indicator

as there is a significant colour change

experienced by either the reductant or theoxidant ± self indicating

-Self indicating ± the ability of one of its

reactants to undergo visible colour changeduring the course of the reaction

-Example???



1.Using wrong indicator 2.Error in rinsing of glassware

3.P

arallax error while taking the readingof burette



If we have a analyte that has acid-base or redox properties

and is soluble but also fits into one or more of the following

categories?

Insoluble ± can¶t prepare solution

Volatile

Gaseous and in a mixture of gases

Fairly unreactive ± that is too weak to have clearly definedend-point. Exp: weak acids / bases

Then direct titration can¶t be used to determine the amount of

analyte. Back titration is the suitable method.



Back TitrationStep 1: Excess amount and accurately known amount of

S is added to the system to react with the R (analyte). Atthe end of the reaction, R is fully reacted i.e. no more R in

the system.R + S T + V

Step 2: Titration is carried out to determine the amount of unreacted S from step 1

S + W X + Y



In order to determine the concentration of ammonia, as % w/v, in a particular

brand of household cleaner, the following procedure was carried out:

Step 1: 10.0 mL of the household cleaner was pipetted into a volumetric

flask and the volume made up to 200 mL with water.

A 20.0 mL aliquot of the solution from the volumetric flask was transferred to

a conical flask. 20.0mL of 0.105 mol L-1 hydrochloric acid solution was then

added. The following reaction took place:

NH3 + HCl NH4Cl * for this reaction the HCl was in

excess

Step 2: To determine the surplus amount of hydrochloric acid from Step 1,

the solution remaining from Step 1 was titrated with 0.0500 mol L-1 sodium

hydroxide solution using phenolphthalein indicator to detect the end point.The average titre value obtained was 6.45 mL. The titration equation is as

follows:

NaOH + HCl NaCl + H2O

Calculating the % w/v concentration of ammonia in the cleaner

Chemical Titration

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