Understanding Titration Technique in Chemistry: A Comprehensive Guide, Study notes of Analytical Chemistry

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7.00 Let the Titrations Begin

Dr. Fred Omega Garces

Analytical Chemistry 251

Miramar College, SDCCD

Titration

A technique of chemical analysis to determine the amount of a substance in a

sample. Common titration include

A sample can be tested by titration. In a titration experiment, a known

volume of a standard concentrated solution (the titrant) is used to analyze

a sample (the analyte). One is usually an acid, the other a base. An

indicator is added to the analyte to signal when the titration is complete.

This is called the endpoint. When the moles of acid(H 3 O

) and moles of

base (OH-) are equal in a titration experiment, the stoichiometric

equivalent point is reached. This is called the equivalent point.

Indicator changes color @ endpoint

moles titrant = moles analyte.

@ equivalent point.

Titration

A technique of chemical analysis to determine the amount substance in a sample.

Common titration include:

Acid-Base

Oxidation-Reduction

Complex Formation

Precipitation Reaction

The requirement for these classic techniques are that the reactions are fast, have

large Keq, and the analyte are consume and used up.

Common Terms:

Titrant Analyte Indicator

Endpoint Equivalent point Titration Error

Blank Titration Primary Standard Standardization

Direct Titration Back Titration

Titration Setup

Setting up a titration experiment.

Titration is a common laboratory method of quantitative/chemical analysis which can be used to determine the concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant, of known concentration (a standard solution) and volume is used to react with a measured quantity of reactant (Analyte). Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint is the point at which the titration is stopped. This is classically a point at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in di- or tri- protic acids). In the classic strong acid-strong base titration the endpoint of a titration is when the pH of the reactant is just about equal to 7, and often when the solution permanently changes color due to an indicator. There are however many different types of titrations (see below). Many methods can be used to indicate the endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes color). In simple acid-base titrations a pH indicator may be used, such as phenolphthalein, which turns (and stays) pink when a certain pH (about 8.2) is reached or exceeded. Methyl orange can also be used, which is red in acids and yellow in alkalis. Not every titration requires an indicator. In some cases, either the reactants or the products are strongly colored and can serve as the "indicator". For example, an oxidation-reduction titration using potassium permanganate (pink/purple) as the titrant does not require an indicator. When the titrant is reduced, it turns colorless. After the equivalence point, there is excess titrant present. The equivalence point is identified from the first faint pink color that persists in the solution being titrated. Due to the logarithmic nature of the pH curve, the transitions are generally extremely sharp, and thus a single drop of titrant just before the endpoint can change the pH significantly — leading to an immediate colour change in the indicator. That said, there is a slight difference between the change in indicator color and the actual equivalence point of the titration. This error is referred to as an indicator error, and it is indeterminate. (wikipedia.org)

Acid-Base Titration of Mixture

A mixture containing K 2 CO 3 (FW 138.21) and KHCO 3 (FW 100.12)

requires 15.00mL of 1.000M HCl. Find the mass of each component.

Stoichiometry Problem:

K 2 CO 3 + 2 HCl g 2 KCl + H 2 O + CO 2

KHCO 3 + HCl g KCl + H 2 O + CO 2

moles K 2

CO

3

• moles KHCO 3 = moles H+ =

1. 000 mole L

• 15. 00 mL = 15. 00 mmol moles K 2

CO

3

Mass (g) K 2 CO 3 FW K 2 CO 3

x (g) K 2 CO 3

138. 21 g mol

K 2 CO 3

moles KHCO 3 = Mass (g) KHCO 3 FW KHCO 3

2. 000 - x (g) KHCO 3
3. 12 g mol

KHCO 3

15. 00 mmol = x (g) K 2

CO

3

138. 21 g mol

K 2 CO 3

2. 000 - x (g) KHCO 3
3. 12 g mol

KHCO 3

x = 1. 811 g = Mass K 2

CO

3

1. 189 g = Mass KHCO 3

The Precipitation Titration

In a precipitation titration, the reaction is monitored by weighing out the

precipitate and knowing the stoichiometric ratio.

A precipitation titration curve is monitored like an acid-base titration

curve. MA D M+^ + A-^ : M+ is the titrant, Ksp is used for rxn.

pX = - log [X], where [X] is the concentration of X. X = M+

Before Equiv Pt: [M+] = Ksp / [A-],

A

is in excess before equivalent pt and determines the pX of the solution.

The excess A- concentration is corrected for by dilution factor.

At the Equiv Pt: Ksp = [M+] [A-] = s^2 , pM = - log s

Past Eqivalent Pt. All M+ added has precipitated. There is now excess M+

which determines the pM. Remember the M+ must be corrected for by

dilution factor.

Titration of a Mixture

In a mixture, two ions are titrated and less soluble precipitates first.

Consider the addition of AgNO 3 to KI and KCl, Ksp (AgI) << Ksp (AgCl)

Titration with a Spreadsheet

Titration of I-^ with Ag+^ via Excel

End-Point Detection

Volhard Titration : The technique is to measure

Cl-. Cl- is precipitated by addition of Ag+ to

form AgCl. AgCl is precipitated out and worked

up and the Ag+ is titrated with KSCN. With the

formation of FeSCN2+^ indicating the end point of

the titration.

Fajans Titration: Uses adsorption indicator.

When Ag+ is added to Cl-, AgCl forms and its

surface attracts (adsorbs) Cl- ions. After

equivalence point, the excess goes from Cl- to

Ag+. The abrupt change from negative to

positive occurs at the equivalence point. An

adsorption indicator such as will change color

when the charge is reverse.

Ag+^ (aq) + SCN-^ (aq) ® AgSCN (s).

Fe^3 +^ (aq) + SCN-^ (aq) ® FeSCN2+^ (aq).