PRECIPITATION, COMPLEXOMETRIC, REDOX

precipitating agent

Analyte is titrated with a standard solution of a ______ in accordance with defined reaction stoichiometry

appearance of excess titrant; disappearance of the reactant

Detection of the end point (at completion of the precipitation) is usually by either the ____ or the ______

Halides (Cl-, Br-, I-)

Pseudohalides (S²-, HS-, CN, SCN-)

Silver ion is extremely useful in precipitation reactions including:

stoichiometric

The precipitate formation is ____

over titration

To allow the titrant to be added quickly, the equilibrium between the precipitate and its ions in solution much be attained rapidly. A slow attainment of equilibrium will cause ____

small equilibrium constant (Ksp)

The precipitate must be of low solubility in the solution. This is indicated by ______

instrumental technique

A method to detect the stoichiometric point of the titration must be available. Although a number of indicators are available, in general, the best method for detecting the end point in precipitation titration is by an ____

Plots of titration curves

____ are normally sigmoidal curve consisting of pAg (or pAnalyte) versus volume of AgNO3 solution added

Pre-equivalence region

Equivalence point

Post-equivalence region

The titration curve is normally broken down in three regions for the purposes of calculations and a function for pAg is determined for each region

argentometric titrations

Titrations with Ag+ are called _____

Mohr titration

Volhard titration

Fajans titration

For argentometric titrations, three classical methods based on color indicators can be used for end point detection:

Mohr titration

formation of colored precipitate at the end point

Volhard titration

formation of a soluble, colored complex at the end point

Fajans titration

adsorption of a colored indicator on the precipitate at the end point

chloride analysis

The mohr method was first published in 1885 as a method for ____

reddish-brown

In the precipitate of chloride by silver ion, chromate ion is used as an indicator in the formation of A2CrO4, a ____ precipitate formed when excess Ag+ is present

ph > 10

The titrations are performed only in neutral or slightly basic medium to prevent silver hydroxide formation at ___

ph < 7

the formation of chromic acid at ____

iron (III)

Determination of Cl. For titration of silver ion with thiocyanate (SCN-) and ___ as an indicator. First published in 1874

white precipitate

The silver chloride precipitate is filtered, and the excess silver ion is titrated with thiocyanate producing a _____ of AgSCN

red

Once the silver is consumed, the excess thiocyanate reacts with the iron(III) ion producing a ___ FeSCN2+ comlex. Thus, the appearance of the ___ color at the endpoint

6.4 × 10^6 M

The red FeSCN2+ complex color is detectable at ____ concentrations and above

shift to the right

since AgSCN is less soluble than AgCl, equilibrium will ____ causing a negative error for the chloride analysis

Adsoption indicators

_____ are organic compound that tend to be adsorbed onto the surface of the solid precipitate in a precipitation titration

adsorption indicators work best when:

They do not precipitate out silver ion when the indicators are at low concentration

adsorption indicators work best when:

They bind to the precipitate only when excess silver ion is present to produce color

yellow-green

A polycylic compound the ionizes in solution to yield ____ fluoresceinate ions

reddish-colored

Fluoresceinate adsorbs to silver ions on the surface of a precipitate when excess silver on is present, producing a _____ surface

redox (oxidation-reduction) reaction

______ is a chemical process in which electrons are transferred between chemical substances

oxidation; reduction

In a redox reduction, one substance loses electron (____) while another gains them (____)

oxidized substance; reduced substance

The _____ is called the reducing agent since it facilitates reduction by donating electrons, while the _____ is the oxidizing agent, as it accepts electrons

rapid

Reaction should be ____

completion

Reaction should go to the _____

stoichiometric

Reaction should be _____

detection of end point

Reaction should provide easy ____

Electrode potential

______ is the parameter which changes during the course of titration and magnitude of the change is maximum in the vicinity of the equivalence point

Redox Indicator

So indicator used for redux titration must change color due to sudden change in the electrode potential of the system at the equivalence point. Such indicator are known as ____

Theory of Redox Indicator

Certain organic compounds are capable to undergo oxidation-reduction reactions and the color of the oxidized form of indicator in solution is significantly different than that of reduced form

Nitroferron

Oxidized from Pale blue

Reduced from Red

Transition Potential 1.25 v

Ferroin

Oxidized from Pale blue

Reduced from Red

Transition Potential 1.06 v

Diphenyl amine-Sulphonic acid

Oxidized from Purple

Reduced from Colorless

Transition Potential 0.84 v

Diphenyl amine

Oxidized from Violet

Reduced from Colorless

Transition Potential 0.76 v

Methylene blue

Oxidized from Colorless

Reduced from Blue

Transition Potential 0.53 v

Indigo tetrasulfonate

Oxidized from Blue

Reduced from Colorless

Transition Potential 0.36 v

titration curve

For Redox titration, ____ is the plot of electrode potential of the system versus volume of the titrant (reducing or oxidizing agent)

electrochemical cell; nearst equation

Electrode potential of the system can be obtained by setting up ______ or by calculating theoretically using ____

Fe(II) vs Ce (IV)

Titration where H3O or OH do not participate directly in the redox reaction

Fe(II) vs KMnO4

Titration where H3O+ or OH- participate directly in the redox reaction

Ce(IV)

In the redox titration, acid solution of Fe (II) is titrated with standard solution of ___

Fe+2 ions

At the beginning of the titration, No Cr2O7^-2 is added to Fe+2 solution, so solution will contain only ____

electrode potential

At the beginning no Fe+3 will be present so at this stage the ___ is not significant

Reference electrode

give constant potential electrode towards the changes in a mixed solution (SHE, NHE, calomel)

Indicator electrode

shows change of potential quantitatively towards changes in mixed solution

potential (V) against titrant volume

The titration curve - plotting the ___