Acid-Base Titrations 2 (1)

7/31/2019 Acid-Base Titrations 2 (1)

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ACID-BASETITRATION

maidliu 02.08.2012


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Types of Acid-Base Titration

AlkalimetryDetermination of basic

content in a sample bytitrating it with a standardacid solution

Acidimetry Determination of acidcontent in a sample by

titrating it with a standardbase solution


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Titration Curve

The course of an acid-base titration is bestfollowed by examining the pH as the titrationprogresses.

A titration curve provides visualpicture of how the

property of thetitration reactionchanges as thetitrant is added to theanalyte.


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Importance of Titration Curve

1. Helps choose the proper titrants andindicators to be used for the analysis

2. Permits to study the effects of manyparameters

3. Determine the conditions for an acceptabletitration

pH calculations is necessary to construct atheoretical titration curve.


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Determining the EquivalencePoint

Equivalence points in acid-base titrationsare determined most often using

chemical indicators that change color ator near equivalence point.Acid-base indicators are weak organic

acids or bases whose conjugate formshave different colors.

It must be weaker than the analyte acid or base so that it reacts last with the titrant.


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Equilibrium for Indicator

The behavior of an acid-type indicator:HIn + H 2O H 3O + + In

(acid color) (base color)

The behavior of an base-type indicator:

In + H 2O OH + InH + (base color) (acid color)


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Acid-Base Indicator

Human eye is not sensitive to color difference ina solution containing a mixture of HIn and In .

An average observer visualizes color imparted bya typical indicator within concentration ratio:

[Hin]/[In] = 10 0.1

This ratio vary considerably from indicator toindicator.

=


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Acid-Base Indicator

Diagram showingthe relationshipbetween pH and anindicators color.


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Determining the Equivalencepoint

The inflection point (thepoint in titration havingthe greatest slope)

very nearly coincideswith the equivalencepoint.

To minimize titration error,an indicators entiretransition pH rangemust fall within the

rapid change in pH atthe equivalence point.


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Reagents for Acid/BaseTitrations

Standard Solutions (Titrant)

Use of strong acids or strong bases react more completely with an analyte,yielding a sharper endpoint

Primary Standards Acid/Base Indicators


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Acid Titrants

1. Hydrochloric acid , HClWidely used for titration of basesIt is a strong acid; dilute solutions are stable with

little or reducing power; chloride ion does notform precipitates with most cationsConcentrated acid is not a primary standard

2. Perchloric acid, HClO 4 3. Sulfuric acid, H 2SO 4

HSO 4 is a weak acidForms insoluble sulfates with a number of common cations


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Primary Standard Bases

Primary standard bases are used for standardization of acids.

1. Sodium carbonate, Na 2CO 3- Most often used for the standardization

of acid solutions- High purity solid are commercially

available, but it is hygroscopic

Na2CO

3+ 2H

3O+ H

2CO

3+ 2Na + + 2H

2O


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Conversion of CO 32 to HCO 3, pH 8.3Formation of CO 2

- The second endpoint

is used for thestandardization becchange in pH isgreater than thefirst.

Two endpoints are observed in the titration of Na 2CO 3


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2. Sodium tetraborate, Na 2B4O 710H 2O Also known as borax; it has high molar

massCannot be heated to remove water Stored in dessicator over an aqueous

solution saturated with NaCl and sucrose(ensures no dehydration to occur)

Na 2B4O 7 + 2H 3O+ + 3H 2O 2Na + + 4H 3BO 3


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3. Tris(hydroxymethyl)aminomethane,(HOCH 2)2CNH 2

TRIS or THAM, available in primary-standardpuritySubstantially greater molar mass per mole of

protons consumed

(HOCH 2)2CNH 2 + H 3O + (HOCH 2)2CNH 3+ +H2O


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Sample Problem

A 0.4512g of primary standard grade Na 2CO 3 (105.99 g/mol) required 36.44mL of an HClsolution to reach the endpoint in the reaction.What is the molarity of the HCl?

Answer: 0.2336M HCl


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Sample Problem

Titration of a 0.7439-g sample of impureNa 2B4O 7 required 31.64mL 0.1081M HCl.Express the results in terms of percent:

Na 2B4O 7B2O 3

Answer: 46.25% Na2B

4O

7, 32.01% B

2O

3


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Sample Problem

A solution of HClO 4 was standardized bydissolving 0.4125g of primary standardgrade HgO (216.59 g/mol) in a solution of KBr:

HgO (s) + 4Br + H 2O HgBr 42 + 2OH The liberated OH consumed 46.51mL of the acid.

Calculate the molarity of the HClO 4.

Answer: 8.190 x 10 2 M HClO 4


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Base Titrant

1. Sodium hydroxide, NaOHUsed almost exclusively for titrating acids insolutions

Primary-standard-grade purity is not availableDilute NaOH solutions are stable can be storedin plastic containers.It slowly reacts with glass to form solublesilicates.Has a cementing property; often glass stoppersare freeze tightly to the mouth of thecontainer.It must be protected from carbon dioxide

CO + 2OH CO 2 + H O


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Primary Standard Acids

1. Potassium hydrogen phthalate, KHC 8H4O 4Often abbreviated KHP, an excellentprimary standard

Available in pure state, soluble in water,has high molar massNot hygroscopic and can be heated to

135oCwithout decomposition

KHC 8H4O 4 +OH K+ + C 8H4O 4 + H 2O


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Primary Standard Acids

2. Benzoic acid, C 6H5COOH Available in sufficient purityNot very soluble in water; most often mixed

with water-alcohol and non-aqueoussolvents

C 6H5COOH + OH C 6H5COO + H 2O3. Potassium hydrogen iodate, KH(IO 3)2

Strong acid that can be titrated using anyindicator with transition range between pH

4-10


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Sample Problem

A 300.0-mg sample containing H 3PO 4 andinert material was diluted with water andtitrated with 0.05000M NaOH. The endpointwas reached after 29.00mL of the titrant wasadded. Calculate the % H 3PO 4 in the sample.

H3PO 4 + NaOH Na 2HPO 4 + 2H 2O

Answer: 23.68% H 3PO 4


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Acid-Base Indicators

1. Phthalein indicatorsPhenolphthalein is the best known indicator in thisgroup.

Indicator transition range: pH 8.0-9.8 Acid form (colorless) Base form (red)Not soluble in water; usually prepared in ethanol


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Acid-Base Indicators

2. Azo indicatorsExhibit color change from red to yellowMethyl red , pH 4.2-6.2

Methyl orange, pH 3.1- 4.4

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Acid-Base Titrations 2 (1)

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