COMPLEXOMETRIC TITRATIONS : 1 COMPLEXOMETRIC TITRATIONS BY : G.v.RAMANA REDDY
kcp INTRODUCTION : 2 INTRODUCTION Complexometric titrations have found important application in analysis of pharmaceuticals especially for metal ion such as Al , Ca , Mg , Zn etc PRINCIPLE : 3 PRINCIPLE Complexometric titrations are those reactions in which simple metal ion is transformed into complex ion by addition of a complexing agent
Comlexing agent is any electron donating group or ion , which by its ability to form one or more covalent bond with the metal ion produces a complex
The complexes have different property from those of free metal ion BONDING IN COMPLEXES : 4 BONDING IN COMPLEXES The bonds are either ordinary covalent bond in which both the metal and ligand contribute one electron
In cordinate bonds both electrons are contributed by the ligand CHELATING AGENT : 5 CHELATING AGENT Complexes involving simple ligands that is those forming only one bond are described as co-ordination compounds.
Ligands having more than one electron donating group are called chelating agents STABILITY OF COMPLEXES : 6 STABILITY OF COMPLEXES The general eqn: for the formation of a 1:1 chelate complex , MX is M + X MX Where M is the metal ion and X is the chelating ion
Therefore stability constant K = [MX] / [M] [X] TITRATION CURVE : 7 TITRATION CURVE In EDTA titration , titration curve can be plotted by pM on Y axis and volume of EDTA soln on X axis
A point of inflection occurs at the end point and this can be visually detected by using pM indicators Slide 8: 8 Titration curve END POINT DETECTION : 9 END POINT DETECTION BY USING pM INDICATORS The end point of complexometric titration is shown by means of pM indicators . The concept of pM arises as follows If K is a stability constant K = [MX] / [M] [X] [M] = [MX] / [X] K log [M] = log [MX] /[X] – log K pM = log [X] / [MX] - pK : 10 The pM indicator is a dye which is capable of acting as a chelating agent to give dye-metal complex
The complex is different in colour from the dye itself and also has a lower stability constant than the chelate metal complex. The colour of the soln remains that of dye complex untill the end point , when an equivalent amount of sodium edetate is added : 11 As soon as there is a slight excess of edetate , the metal dye complex decomposes to produce free dye , this is accomplished by a colour change
The colour of dye and of the metal complex vary with the pH therefore it is also essential to buffer the solution to maintain the required pH during titration EXAMPLES FOR pM INDICATORS : 12 EXAMPLES FOR pM INDICATORS Alizarin fluorine blue : For titration of lead , zinc , cobalt , mercury , copper .
Calcon : For titration of cal. carbonate , cal. chloride , cal. Gluconate .
Mordant black II : For titration of magnesium , calcium , zinc , manganese
Xylenol orange : for titration of bismuth lead mercury , lanthanum . : 13 BY USING PHYSICAL METHOD The end point of complexometric titrations can also be determined by some physical methods like spectrophotometic detection , amperometric titration , potentiometric titration , high frequency titrator TYPES OF EDTA TITRATIONS : 14 TYPES OF EDTA TITRATIONS DIRECT TITRATION Metal ion + buffer + indicator Titrate with EDTA soln
A blank titration may be performed by omitting the sample as a check on the presence of impurities in reagents The end point can be determined by change in Colour of the metal indicator EXAMPLE FOR DIRECT TITRATION : 15 EXAMPLE FOR DIRECT TITRATION Determination of percentage of Bi from bismuth sub carbonate PROCEDURE 0.2gm sample + 2M nitric acid + 50 ml water (adjust the pH 1-2 by the addition of either nitric acid (2 M ) or ammonia (5 M ) + mix of xenolol orange and pot. Nitrate (1:99, 30 mg ) + Titrate with 0.05M disodium edetate until the colour become yellow . BACK TITRATION : 16 BACK TITRATION In back titration an excess of std EDTA is added the resulting soln is buffered to the desired pH and the excess of EDTA is back titrated with a std metal ion soln
The end point is detected with the aid of the metal indicator which responds to Zn or Mg ions introduced in the back titration EXAMPLE FOR BACK TITRATION : 17 EXAMPLE FOR BACK TITRATION Determination of the percentage of calcium phosphate
PROCEDURE Calcium phosphate + excess of HCl (dissolved with the aid of heat ) + 0.05M disodium edetate ( add in excess quantity) + ammonium buffer (to bring the pH 10 ) + back titrated with 0.05M zinc chloride using mordant black II as indicator . REPLACEMENT TITRATION : 18 REPLACEMENT TITRATION In this titration metal may be determined by the displacement of an equivalent amount of magnesium or zinc from a less stable edetate complex
Calcium , lead and mercury can be determined satisfactorily using mordant black II by this method EXAMPLE FOR REPLACEMENT TITRATION : 19 EXAMPLE FOR REPLACEMENT TITRATION Determination of the percentage of sodium calcium edetate
PROCEDURE 0.5gm of sample + 90 ml water + 7gm hexamine +5ml dil. HCl + xenolol orange (indicator) + titrate with 0.05 M lead nitrate ALKALIMETRIC TITRATION : 20 ALKALIMETRIC TITRATION In this method protons from disodium edetate are displaced by a heavy metal and titrated with standard alkali accord MASKING AND DEMASKING : 21 MASKING AND DEMASKING With the introduction of a masking agent it is possible to estimate two or more metal ion in the same soln
It is a substance that will form complex more strongly with metal ion than the titrant
Eg : pot. Cyanide for the masking of heavy metals DEMASKING AGENT : 22 DEMASKING AGENT It is the substance which release masked metal ion
Eg formaldehyde can release the masked zinc ion potassium cyanide PREPARATION AND STANDARDISATION OF 0.05M EDTA : 23 PREPARATION AND STANDARDISATION OF 0.05M EDTA Preparation of disodium EDTA 18.6g of disodium EDTA in sufficient water to produce 100ml : 24 Standardisation 0.8gm of granulated zinc + 12ml dil. HCl + 5 drops bromine water (boil ) + water (sufficient to produce 200ml ) 20ml of resulting soln + 2N NAOH (for neutralizing ) +150 ml water + ammonia buffer ( upto pH 10 ) + 50mg mordant black II + titrate with disodium EDTA soln until the soln turns green REFERENCE : 25 REFERENCE Vogels text book of quantitative chemical analysis by J . Mendham ,R.C. Denny , M.J.K . Thomas
Practical pharmaceutical chemistry by A.H. becket and J.B. Stenlake Thank you : 26 Thank you