Complexometric Titrations Detection of End Point, Application and Question Answer PART-2

DETECTION OF END POINT :-

1. Metal ion indicator :-

It is a compound whose colour changes when it binds to metal ions.

M + Indicator → Metal ion indicator

M¯ + EDTA → M¯- EDTA Complex

2.Eriochrome black T  :-

 Free indicator is blue in colour (pH 10). On complexation with metal ions, a pink colour is formed. Below pH 6.3 and above11.5 it is reddish in colour hence it is used at about pH 10. It is used in the estimation of metal ions like calcium, magnesium, zinc, cadmium, manganese etc.

3.XYLENOL ORANGE :-

It has yellow colour in acid solution and red colour Used Solution. The metal complexes are red In colour. It is estimation of Aluminium hydroxide gel, Aluminium sulphate Aluminium hydroxide. Titanium dioxide and Zinc undecylenate

4.Murexide:-

(Ammonium purpurate) It Is used in the estimation of Calcium in the presence of magnesium. This is because magnesium murexide complex is less stable than calcium – murexide complex.

5. pM indicator – 

commonly used.

Before titration :- Metal – indicator complex

During titration :- Metal – EDTA + free indicator +metal-indicator

After titration :- Metal – EDTA + free indicator

REq. for pM indicator

1. It must be sequestering agent.
2. Free indicator must have some colour which is diff. from the colour of metal- indicator complex.
3. K_M-EDTA/K_M-INDICATOR = 10⁴ MIMIMUN

1.Masking agents :-

are reagents which prevent interfering ion from reaction without physical separation.

These reagent form complexes with interfering ions which are more stable than complexes with indicator & EDTA.

Example of masking agent

1. KCN :- It is used as masking agent for Ag⁺, CU⁺²,CO⁺²,Ni⁺²

M⁺ + 2CN^–   ⇒ [M(CN)₂]^–

M⁺ +4CN^– ⇒[M(CN)₄]^–

2.Fluoride (NH₄F) :-It is used as masking agent for Fe³⁺ and Al³⁺ to give hexafluoro complex [FeF₆]^3- & [AlF₆]^3-

3.Iodide (KI) :-It is used as masking agent for Hg²⁺ to give tetraiodo complexe (HgI₄).

2.Demasking agent :-

Is the process in which masking substance reveres back to its ability to take part in the reaction. Are reagent which regian the ability of masked ion to enter the reaction with Ind. & EDTA. EXAMPLE :-  The masking by CN^– can be removed by mixture of formaldehyde, acetic acid on addition of demasking agent to [Zn (CN)₄]^2- Zn is liberated and titrated.

APPLICATION OF COMPLEXOMETRIC TITRATION :-

1. To determine the concentration of metal ions in solutions.
2. To determine calcium present in a food products.
3. To determine conc. of bad metals in the environment.
4. It is used to determined hardness of water.
5. It is used to assay of following drugs.

• Aluminium hydroxide gel
• aluminium hydroxide tablet
• calcium carbonate
• calcium phosphate dibasic
• zinc chloride
• Magnesium trisilicate

MCQ 

1. The end point of complexometric titrations is shown by means of 

a.Colorimetry

b.pH meter

c.pM Indicator

d. Acid base titration

2.Assay of Bismuth subcarbonate is 

a.0.05 M disod. edetate

b.0.1 M disod. edetate

c.0.01 M disod. edetate

d.1 M disod. edetate

3. Assay of  Magnesium trisilicate is

a.0.05 M disod. edetate

b.0.1 M disod. edetate

c.0.01 M disod. edetate

d.1 M disod. edetate

4.Which is not application of complexometric titration ?

a. Determination of hardness of water.

b.To determine conc. of bad metals in the environment.

c.To determine the concentration of metal ions in solutions.

d. To assay of barbiturates.

5. Which is Example of Demasking agents ?

a.KCN

b.KI

c.[Zn (CN)₄]^2-

d.NH₄F

6.Which indicator are used to detected calcium in presence of magenisum ?

a.EDTA

b.XYLENOL ORANGE

c.Eriochrome black T

d.Murexide

7. Erichrome black -T used indicator in which type of titration ?

a.NMR

b. non aqueous titration

c. acid base titration

d. complexometric titration

8.Which sentence is false about masking agent ?

a.NH₄F is masking agent.

b.These reagent form complexes with interfering ions which are more stable than complexes with indicator & EDTA.

c.It is reagents which prevent interfering ion from reaction without physical separation.

d.this is reagent which regian the ability of masked ion to enter the reaction with Ind. & EDTA.

ANSWER KEY

1.c

2.b

3.a

4.d

5.c

6.d

7.d

8.d

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REFERENCE :-

TEXT BOOK OF PHARMACEUTICAL ANALYSIS THIRD EDITION BY DR. S . RAVI SANKAR (PG.NO 22.1-22.13)