• An An analyteanalyte is the substance that is being is the substance that is being analyzed. For example, the concentration of analyzed. For example, the concentration of glucose in blood is commonly analyzed by glucose in blood is commonly analyzed by diabetics. Glucose is the analyte.diabetics. Glucose is the analyte.

• A A titranttitrant is a solution of reagent that reacts is a solution of reagent that reacts with the analyte. The concentration of this with the analyte. The concentration of this reagent is accurately and precisely known.reagent is accurately and precisely known.

• In a In a titrationtitration, incremental volumes of titrant , incremental volumes of titrant are added to the analyte until the reaction is are added to the analyte until the reaction is complete.complete.

• A A buretburet is often used to measure the volume is often used to measure the volume of titrant added to the analyte.of titrant added to the analyte.

CH 103: ACID-BASE TITRATIONSCH 103: ACID-BASE TITRATIONS

1.1. The reaction must be The reaction must be stoichiometricstoichiometric. For example, the net ionic . For example, the net ionic equation for the reaction of potassium hydrogen phthalate (KHP, equation for the reaction of potassium hydrogen phthalate (KHP, HOOC-CHOOC-C66HH44-COOK) and sodium hydroxide (NaOH) is quantitative. -COOK) and sodium hydroxide (NaOH) is quantitative.

Exactly 1 mole of KHP reacts with exactly 1 mole of NaOH.Exactly 1 mole of KHP reacts with exactly 1 mole of NaOH.

HOOC-CHOOC-C66HH44-COO-COO-- + OH + OH-- → → --OOC-COOC-C66HH44-COO-COO-- + H + H22OO

2.2. The reaction should be The reaction should be rapidrapid..

3.3. The reaction should be The reaction should be specificspecific; that is, there should be no ; that is, there should be no competing reactions. Systematic error caused by competing reactions. Systematic error caused by interferencesinterferences must must be eliminated or reduced. For example, HOOC-Cbe eliminated or reduced. For example, HOOC-C66HH44-COO-COO-- should be should be

the only acid in the above reaction.the only acid in the above reaction.

4.4. There should be a There should be a marked change when the reaction is completemarked change when the reaction is complete. . For example, there is a marked increase in pH when the above For example, there is a marked increase in pH when the above reaction is complete. This increase in pH can be observed with an reaction is complete. This increase in pH can be observed with an indicatorindicator that changes color immediately after the reaction between that changes color immediately after the reaction between the titrant and analyte is completed.the titrant and analyte is completed.

REQUIREMENTS OF A TITRATIONREQUIREMENTS OF A TITRATION

• The The equivalence pointequivalence point occurs when the volume of titrant occurs when the volume of titrant added to the analyte is the exact stoichiometric amount added to the analyte is the exact stoichiometric amount that is needed to bring the reaction to completion.that is needed to bring the reaction to completion.

• The The end pointend point occurs when the indicator changes color. occurs when the indicator changes color.

• We want to measure the equivalence point. We actually We want to measure the equivalence point. We actually measure the end point. We need to select an measure the end point. We need to select an indicatorindicator that that has the same end point as the equivalence point.has the same end point as the equivalence point.

• The indicator is added to the analyte. It is NOT added to The indicator is added to the analyte. It is NOT added to the titrant.the titrant.

EQUIVALENCE POINT, END POINT, AND INDICATORSEQUIVALENCE POINT, END POINT, AND INDICATORS

Titration using Phenolphthalein as an IndicatorTitration using Phenolphthalein as an Indicator

• When do you stop adding titrant to the analyte?When do you stop adding titrant to the analyte?• At the end point.At the end point.

EQUIVALENCE POINT, END POINT, AND INDICATORSEQUIVALENCE POINT, END POINT, AND INDICATORS

• The titration of 0.100 M acetic acid (CHThe titration of 0.100 M acetic acid (CH33COOH) with 0.100 M COOH) with 0.100 M

NaOH has an equivalence point at pH 8.73. What indicator NaOH has an equivalence point at pH 8.73. What indicator should you use for this reaction?should you use for this reaction?

• Phenolphthalein.Phenolphthalein.

EQUIVALENCE POINT, END POINT, AND INDICATORSEQUIVALENCE POINT, END POINT, AND INDICATORS

• Today we will standardize NaOH against potassium hydrogen phthalate Today we will standardize NaOH against potassium hydrogen phthalate (KHP, HOOC-C(KHP, HOOC-C66HH44-COOK).-COOK).

• KHP is a KHP is a primary standardprimary standard..

• For example, 25.71 mL of a NaOH solution are used to titrate 1.095 g of For example, 25.71 mL of a NaOH solution are used to titrate 1.095 g of KHP to a phenolphthalein end point. The molar mass of KHP is 204.2 KHP to a phenolphthalein end point. The molar mass of KHP is 204.2 g/mol. One mole of KHP reacts with 1 mole of NaOH. What is the g/mol. One mole of KHP reacts with 1 mole of NaOH. What is the molarity (M) of this NaOH solution?molarity (M) of this NaOH solution?

• Next week you will use your NaOH solution to determine the molecular Next week you will use your NaOH solution to determine the molecular weight of an unknown acid.weight of an unknown acid.

• This NaOH solution is a This NaOH solution is a secondary standardsecondary standard..

STANDARDIZATIONSTANDARDIZATION

1.1. A primary standard should be A primary standard should be 100.00% pure100.00% pure; although a ; although a 0.01% to 0.02% impurity is tolerable if it is accurately 0.01% to 0.02% impurity is tolerable if it is accurately known.known.

2.2. A primary standard should be A primary standard should be stable at drying stable at drying temperaturestemperatures, and it should be stable indefinitely at room , and it should be stable indefinitely at room temperature. (A primary standard is always dried before temperature. (A primary standard is always dried before weighing, unless it is a hydrate.)weighing, unless it is a hydrate.)

3.3. It should be It should be readily availablereadily available..

4.4. It should have a relatively It should have a relatively large formula weightlarge formula weight. . Therefore, a relatively large mass of it will be weighed for Therefore, a relatively large mass of it will be weighed for titration. This will reduce error.titration. This will reduce error.

• Explain this last point.Explain this last point.

REQUIREMENTS OF A PRIMARY STANDARDREQUIREMENTS OF A PRIMARY STANDARD

• Where is the equivalence point?Where is the equivalence point?• At pH 7.At pH 7.• Where is the end point?Where is the end point?• Between pH 8.0 and 9.5.Between pH 8.0 and 9.5.• Is there a significant difference between the volume of titrant used to Is there a significant difference between the volume of titrant used to

reach the equivalence and end points?reach the equivalence and end points?• No. Therefore, phenolphthalein is an appropriate indicator for this No. Therefore, phenolphthalein is an appropriate indicator for this

titration.titration.

TITRATION OF A STRONG ACID WITH A STRONG BASETITRATION OF A STRONG ACID WITH A STRONG BASE

• Where is the equivalence point?Where is the equivalence point?• At pH 7.At pH 7.

• Could phenolphthalein be used as Could phenolphthalein be used as an indicator for this titration?an indicator for this titration?

• Yes.Yes.

TITRATION OF A STRONG BASE WITH A STRONG ACIDTITRATION OF A STRONG BASE WITH A STRONG ACID

• Where is the equivalence point?Where is the equivalence point?

TITRATION OF A WEAK ACID WITH A STRONG BASETITRATION OF A WEAK ACID WITH A STRONG BASE

• Where is the equivalence point?Where is the equivalence point?

TITRATION OF A WEAK BASE WITH A STRONG ACIDTITRATION OF A WEAK BASE WITH A STRONG ACID

• Standard deviation is a measure of precision. It is used to Standard deviation is a measure of precision. It is used to quantify the agreement between repeated measurements of quantify the agreement between repeated measurements of the same sample.the same sample.

STANDARD DEVIATIONSTANDARD DEVIATION

• Give at least 1 safety concern for the following procedure.Give at least 1 safety concern for the following procedure.

• Using HOOC-CUsing HOOC-C66HH44-COOK, NaOH, phenolphthalein,-COOK, NaOH, phenolphthalein, and an and an

unknown acid.unknown acid.

• These are irritants. Wear your goggles at all times. These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in Immediately clean all spills. If you do get either of these in your eye, immediately flush with water.your eye, immediately flush with water.

• Your laboratory manual has an extensive list of safety Your laboratory manual has an extensive list of safety procedures. Read and understand this section.procedures. Read and understand this section.

• Ask your instructor if you ever have any questions about Ask your instructor if you ever have any questions about safety.safety.

SAFETYSAFETY

• Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc.York, NY: John Wiley & Sons, Inc.

• Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company.New York, NY: W.H. Freeman Company.

• Traverso M. 2006. Titration using Phenolphthalein as an Traverso M. 2006. Titration using Phenolphthalein as an Indicator. Available: Indicator. Available: www.chemistry.wustl.edu/.../AcidBase/phph.htm [accessed 14 September 2006]. [accessed 14 September 2006].

SOURCESSOURCES