P.GOUTAMI,

M.PHARM(Pharmaceutical analysis and quality assurance)

JNTU, Hyd.

Flow of presentation :

• Definition• Principle• Criteria for performing amperometric titrations• Apparatus• Rotating platinum electrode• Types of amperometric titrations• Commercially available amperometers• Advantages• Applications

Definition:

Amperometric titration refers to a class of titrations in which the equivalence point is determined through measurement of the electric current produced by the titration reaction. It is a form ofquantitative analysis.

•Otherwise called as Polarographic or polarometric titrations.

Indicator microelectrode: DME or RPE

Reference electrode: SCE

Principle:

According to Ilkovic equation

• id is proportional to concentration keeping all other factors of the equation constant.

• So, if some of the electroactive material in the solution is removed by interaction with some other reagent the diffusion current will decrease proportionally.

 id = 607 n C D1/2m 2/3 t1/6

• The diffusion current at an appropriate applied voltage is measured as a function of the volume of the titrating solution.

• The end point is the intersection of two lines giving the change of current before and after the equivalence point.

• The migration current is minimised by adding sufficient qty of supporting electrolyte.

• Mechanism of the supporting electrolyte???

Difference between amperometry and polarography?????

Criteria for performing amperometric titrations:

• Either the titrate or titrant ot both should be electroreducible.

• The potential applied should correspond to the limiting current.

Apparatus Rotating Pt micro electrode

burette

SCE

Apparatus

• Pyrex glass, that is a three necked, flat−bottomed flask

• a micro burette,• dropping mercury electrode(ind’- polarisable)• gas outlet tube for N2• with an additional inlet N2 provision• connected to a reference electrode(SCE - non-

polarisable)

Rotating platinum electrode

Y Platinum???

• Mercury cannot be used as electrode at positive potentials because of its oxidation

• rotating platinum electrode is used

y rotating ???

• With platinum as electrode, the attainment of steady state diffusion current is slow

• One has to wait for a considerable time after each addition of the reagent

• Therefore platinum electrode is rotated at 600 rpm.

RPE

RPE

• Glass rod with a bent Pt wire at its tip – 600rpm.

• Wire contacts are made through a mercury reservoir so that potential can be applied and the current is measured.

• Nitrogen gas is bubbled through the titrate to remove oxygen.

Types of amperometric titrations

• Titration of electro reducible ion Vs non reducible ion

• Titration of non reducible ion Vs electro reducible ion.

• Titration of electro reducible ion Vs electro reducible ion.

• Biamperometry

Titration of electro reducible ion Vs non reducible ion

• Ex: lead( )Vs sulphate ions ( )

Addition of sulphate ions

Titration of non reducible ion Vs electro reducible ion.

• Ex: chloride( ) Vs silver ions( )

Addn of silver ions

Titration of electro reducible ion Vs electro reducible ion.

• Ex: lead ( ) Vs dichromate ions( )

Addn of dichromate ions

At the applied potential substance gives anodic current and titrantcathodic current.

Ex: (eg. I− ion with Hg(NO3)2 )

Biamperometry • Difference from amperometry ???• a pair of identical metallic or solid

microelectrodes/polarized electrodes are used.

• SCE is replaced with platinum electrode in the electrolysis cell.

• a magnetic stirrer is used.

• A fixed potential difference of 10-20 mV is applied between the two electrodes through the adjustment of resistance variable potentiometer, R (1000 ohms).

• One electrode as anode and the other cathode.

• Applicable when redox system is present before and after the end point.

• Ex: KARL FISCHER TITRATION (end pt)• A small potential is applied between the 2

similar Pt electrodes.• Initially when water is present both electrodes

are depolarised.

• Addition of KF reagent (sol of iodine and in pyridine and methanol) till the end pt- decreases at the end point.

• is almost zero at the end point.• After the end point gradually increases due

to the addition of the titrant. • Dead- Stop End Point Method

KF Titrator manufactured by Metrohm India ltd

Other examples of dead stop end point technique:

• Titration of sodium thiosulphate with iodine.• Determination of nitrate ion by titrating with

ammonium iron (II) sulphate solution in a strong sulphuric acid medium.

Commercially available amperometers

vintage amperometer (about 1950)

(mechanical guage)amperometer

Product details:

• Mnf company : Ruian Kangertai Automobile Parts Co., Ltd.

• Brand Name: Kangertai• Place of Origin: China• Model No: CS033

4-20 mA Amperometer(HI 931002)by Hanna instruments services,italy

Product details:

• Mnf company: Xiamen Dexing Imp. & Exp. Co., Ltd.

• Place of Origin: Fujian, China (Mainland)• Brand Name: Dexing• Model Number: D26-A,D61-A,D76-A,D9-A,T51-

A,T77-A,T63-A• Display Type: Analog and Digital• Measuring Range: 0-75-150-300-600V• Accuracy Class: 0.5

Hach company AutoCAT 9000 chlorine amperometric titrator

Advantages of amperometry:

• Rapid , simple apparatus.• Titrations can be carried out in cases in which

the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory. Ex:?

• can be carried out at dilutions (10−4 M) at which many visual or potentiometric titrations no longer yield accurate results.

• supporting electrolyte(without interference) - to eliminate the migration current which is not possible in case of conductometric titration.

• The temperature need not be known provided it is kept constant during the titration.

• The reactions can be reversible or irreversible.• Non reducible ions which cannot be

determined by polarography can be determined easily.

Applications of amperometric titrations

• wider range of applications than polarography because even electro-inactive substances can be determined using electro-active titrant.

• Quantitative (end point)• Determination of water using KF reagent

(biamperometry)• Quantification of ions or mixture of ions.

Determination of various species:Titrant  Electrode Species determined

1.Complexometric rxnsEDTA

DME Many metallic ions

2.Pptn titrations•Dimethyl glyoxime•Lead nitrate•Mercury(II) nitrate•Silver nitrate•Sodium tetraphenyl borate

•DME•DME•DME•RPE•Graphite

•Ni2+•Sulphate, F-•I-•Cl-, Br-, I-, CN-•K+

3.Oxidation rxnsKBrO3

RPE As(III), Na2S2O3

4.Addition rxns RPE Alkenes

Substitution rxns RPE Some phenols, aromatic amines

Amperometric detection in HPLC

• Amperometric Detection in HPLC for Determining Organic Acids in Fermented Foods. http://www.electrochem.org/dl/ma/203/pdfs/2933.pdf

• HPLC with amperometric Detection of Medroxyprogesterone Acetate in Human Plasma. http://www.scienceasia.org/2007.33.n4/v33_405_410.pdf

Amperometric detector

• Optimum for High-sensitivity Analysis of Carbohydrates and Polyols in the pharmaceuticals !!!

• Derivatization and other complex pretreatments are not necessary. The sensitivity is about 10 to 100 times greater than that of a differential refractometer.

• http://www.dionex.com/en-us/webdocs/5046-An117.pdf

AutoCAT 9000 Chlorine Amperometric TitratorPre-Programmed for Easier Use

AutoCAT 9000 Chlorine Amperometric Titrator

• Automatic calculation of analyte concentration. • Automatic hands-free operation. • Easy to use.• Real-time graphics and graphic print output. • Automatic archiving of results.• Titrant calibration. • Electrode cleaning. • Greater accuracy and precision than manual

titrations.

• Amperometry uses a carbon electrode to record changes in the chemical composition of the oxidized components of a biological solution.

• used for studying exocytosis in the neural and endocrine systems bcoz many monoamine tansmitters are oxidisable.

http://en.wikipedia.org/wiki/Electrophysiology

• Amperometric determination of acetylsalicylic acid in drug formulations using copper electrode in alkaline solutions.

http://144.206.159.178/FT/1000/73279/1255808.pdf

• Amperometer measures glucose levels from tears - A report appears in ACS journal Analytical Chemistry.

(ongoing effort)http://www.knowabouthealth.com/amperometer-measures-glucose-levels-from-tears/9097/

Amperometry Videos on :

• http://www.youtube.com/watch?NR=1&v=OUN8oY6kcXw&feature=endscreen

References

• Wikipedia• http://vedyadhara.ignou.ac.in/• Vogel's TB of Quantitative Chemical Analysis

5ed.

Thank u