Complexometric titration

Complexometric titration

Complexometric titration

-complexometric titrations are also known as chelatometry ( volumetric analysis )

-endpoint of the titration is indicated by the coloured complex.

-an indicator capable of producing an unambiguous colour change is usually used to detect the end-point of the titration

-this titration is particularly useful for the determination of a mixture of different metal ions in solution.

PRINCIPLE

-the reaction reaches equilibrium rapidly after each portion of titrant is added.

-interfering situation does not arise, for instance, the stepwise formation of several different complexes of the metal ion with the titrant, resulting in the presence of more than one complex in solution during the titration process.

-a complexometric indicator capable of locating equivalence point with fair accuracy is available.

MASKING AGENTS

masking agents act by either precipitation or by the formation of complexes more stable than the interfering ion-EDTA complex.

a) by precipitation:-

-masking by precipitation is a mechanism of masking

-many heavy metals eg. Co, Cu & Pb can be separated either in the form of insoluble sulphides using sodium sulphide or as insoluble complexes using thioacetamide.

-these are filtered, decomposes & titrated with disodium EDTA

-other common precipitating agents are sulphate for Pb & Ba, oxalate Ca & Pb, fluoride for Ca, Mg & Ph, ferrocyanide for Zn & Cu & 8-hydroxy quinoline for many heavy metals.

-precipitation of Cu in the assay of lotion containing Cu & Zn thioglyceral.

b) masking by complex formation:-

-masking agents form more stable complexes with interfering metal ions.

-the most important aspect is that the masking agent must not form complexes with the metal ion under analysis.

the different masking agents used are enlisted below.

1.ammonium fluoride

2.ascorbic acid

3.dimercaprol

4.potassium cyanide

5.potassium iodide

6.triethanolamine

DEMASKING

-the use of masking & selective demasking agents permit the successive titration of many metals

-masked substance regain its ability to enter into a particular reaction.

-it is used to determine a series of metal ions in one solution.

-eg. using masking & demasking agents in Complexometry is the analysis of 3 metals

-Cu, Ca & Cd

-direct titration c EDTA;- it will give the amount of 3 metals

 

Cu + EDTA → Ca – EDTA

Ca + EDTA → Ca – EDTA

Cd + EDTA →Ca -EDTA

Cu & Cd masked by cyanide only Ca can be determined

Cu + cyanide ion → Cu- cyanide complex

Cd + cyanide ion → Cd – cyanide complex

Ca + cyanide ion → no reaction ( free Ca ) + EDTA → →Ca -EDTA

-formaldehyde/ chloral hydrate is added to demask the Cd & titrate the solution to determine the Ca & Cd

Cu-cyanide complex + HCHO → Cu (free)

Cd- cyanide complex + HCHO → no reaction

then titration reaction will be

Ca + EDTA → Ca – EDTA

Cd + EDTA → Ca -EDTA

-so we can determine the values of Cu, Ca & Cd separately

-Ca is directly determined after masking Cu & Cd

Cd= ( Ca +Cd ) – Ca

Cu= ( Cu + Ca + Cd ) – ( Ca + Cd )

TYPES OF COMPLEXOMETRIC TITRATION

  1. DIRECT TITRATION

-simple & convenient method

-in this meth0d, the standard chelon solution is added to the metal ion solution until the endpoint is detected eg. calcium gluconate injection, calcium lactate tablets & compound sodium lactate injection for the assay of calcium chloride

2. BACK TITRATION

-in this method, excess of a standard EDTA solution is added to the metal solution which is to be analyses & the excess is back titrated with the standard solution of a second metal ion eg. determination of Mn.

-this metal cannot be directly titrated with EDTA because of precipitation of Mn(OH)2. an excess of a known volume of EDTA is added to an acidic solution of Mn salt & then ammonia buffer is used to adjust the PH to 10 & the excess EDTA remaining after chelation is back titrated with a standard Zn solution kept in burette using eriochrome black T as an indicator.

-this method is analogous to back titration method in acidimetry eg. ZnO

3. REPLACEMENT TITRATION

-In this method the metal, which is to be analyzed, displaces quantitatively the metal from the complex.

-when direct/back titration do not give sharp endpoints, the metal may be determined by the displacement of an equivalent amount of Mg/Zn from a less stable EDTA complex

-Mn displace Mg from Mn EDTA solution. the freed Mg metal is then directly titrated with a standard EDTA solution. In this method, the excess quantity of Mg EDTA chelate is added to Mn solution.

-Mn quantitatively displace Mg from Mg EDTA chelate.

-this displacement takes place because Mn forms a more stable complex with EDTA by this method Ca, Pb, Hg may be determined using eriachrome black T indicator.

4. indirect titration:-

-it is also known as alkalimetric titration.

-used for determination of ions such as anions, which do not react with EDTA chelate.

-protons from disodium EDTA are displaced by a heavy metal & titrated with sodium alkali.

eg-    barbiturates do not react with EDTA but are quantitatively precipitated from alkaline solution by mercuric ions as 1:1 complex.

barbiturates + EDTA →  no reaction

barbiturates -Hg + EDTA ( in excess ) → gives reaction & remaining EDTA are titrated.

INDICATORS

-essential to use indicators in complexometric titration

-common indicators are organic dyes eg. eriochcrome black T

-colour changes show that the indicators have been displaced ( usually by EDTA) from the metal cations in solution when the endpoint has been reached thus the free indicator ( rather than metal complex ) serves as the endpoint indicator.

Estimation of calcium gluconate

-preparation of 0.05 m disodium EDTA solution (disodium edetate 0.05m)

-material- disodium ethylene diamine tetracete (18.6)gm

Procedure-Weigh accurately  18.6gm of disodium ethylenediaminetetraacetate, dissolve to sufficient Distilled water, in a litre volumetric flask and make up the volume up to the mark.

Calculation;

C10 H14 N2 NA2 O8 2 H20= 1000ml M

372.24g C10 H14 N2 NA2 O8. 2H2O =1000ml M

18.612 g C10 H14 N2 NA2 O8 .2H2O =1ml o.05M

Disodium Edetate

Standardization of 0.05M disodium edetate

-Weigh accurately about 0.8gm of granulated zinc dissolve by gently warming in 12 ml of dilute HCL and 5 drops of bromine water.

-boil to remove excess bromine cool and add sufficiently distilled water to produce 200ml in a volumetric flask.

-Pipette 20 ml of the resulting solution into a Flask and neutralize carefully with z N NaOH

-Dilute to about 150 ml with Distilled water, add sufficient ammonia buffer (pH 10) to
dissolve the precipitate & add a further 5ml  quantity, in excess.

-Finally, add  50 mg of Mordont Black 2 the mixture of 0.2 port Mordant Black II With100 parts of NaCl)
Titrate with the disodium edetate solution Until the solution turns green.

-Each 0.003259 g of granulated zinc is equivalent to1 ml of  0.05 M disodium ethylene.

Diamine tetra acetate

Calculations:

Zncl2= Zn2+ = 1000ml M
65.38 g zn =1000ml M
3.269g Zn =1000ml 0.05M
0.003269 g Zn = 1 ml of 0.05 M Disodium ethylenediamine tetra state

Determination of calcium gluconate

-an accurately measured volume of the injection equivalent to 0.8 gm of calcium gluconate is taken in a titration flask and dissolved in water (150ml) and contains Dil HCL (5ml)

-To the acidified solution: NaOH (15ml) is added with murexide indicator (4mg) solution of naphthalene green (3ml) -The reaction mixture is titrated with m/20 Disodium EDTA until the solution is deep blue in colour.

1 ml of m/20 Disodium EDTA =0.02242gm of calcium gluconate.

Principle of complexometric titration :

-complexometric titration is a form of volumetric analysis in which the formation of a coloured complex is used to indicate the endpoint of the titration

-Complexometric titration are particularly used for the determination of a mixture of different metal ions in solution.
Magnesium can be easily determined by EDTA titration in the PH 10 against eriochrome black T

-If the solution initially contains also different metal ions, they should be removed or masked as EDTA react easily with most cations.
Mg+ eriochrome black T / Mordant black 2 (indicator)
Hooc – ch2

Procedure :
1. standardization of EDTA

-Pipette out 20 ml of calcium carbonate solution, add 10 ml of ammonia, ammonium chloride solution/ammonia buffer solution, titrant against EDTA using Mordant black 2 mixture till the colour changes from red to blue.

2. Assay of magnesium sulphate :
-Weigh 0.3 gm magnesium sulphate, add 10 ml of distilled water, add10 ml of ammonia, ammonium chloride solution or ammonium buffer solution. Titrant against EDTA using Mordant black 2 mixture till the colour changes from red to blue.

Calculations
Each ml of 0.05 M EDTA = 0.01232GM of magnesium sulphate.

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Wikipedia

Precipitation Titration

Precipitation Titration

 

 

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