Conductometric Titrations

Conductometric Titrations

Submitted to: • Madam Arjumand • Madam Aashi Subject: Applied chemistry lab - II

Date: 8 June 2009

Department of Chemical Engineering University of Engineering and Technology Lahore

Submitted by: 1.

Muhammad Usman

(08 - Chem-02-B)

2.

T A H IR

(08 - Chem-16-B)

3.

SAQIB

4.

Sameullah

5.

Allah Ditta Khizar

6.

Faraz Naeem

(08 -Chem-118-B)

7.

Syed M. Hanan

(0 8- Chem-84- B)

8.

LIAQUAT ALI

9.

S H O A Ib H A Meed (08- Chem-122-B)

10. Ronald

Mehmood

R A O U F (08 - Chem-26-B)

Ojiambo

(08 - Chem-30-B) (0 8- Chem-42- B)

(08- Chem - 22-B)

(08- Chem-146-B)

Contents Submitted by:............................................................................................................2 History.....................................................................................................................6 Types of titrations...................................................................................................7 •

Acid-base titrations...........................................................................................7

•

Redox titrations................................................................................................7

•

Complexometric titrations................................................................................7 1.

PH indicator...................................................................................................8

2.

Redox indicators............................................................................................8

4.

PH meter.......................................................................................................8

5.

Conductance:................................................................................................8

Explanation:.........................................................................................................9 6.

Colour change:..............................................................................................9

7.

Precipitation:.................................................................................................9

8.

Thermometric titrimetry:...............................................................................9

9.

Spectroscopy:................................................................................................9

10.

Amperometry...........................................................................................10

Experiment:.............................................................................................................10 Conductance:........................................................................................................10 CONDUCTOMETRIC

TITRATION :.............................................................................................11

Theory:.....................................................................................................................11 GRAPHICAL

REPRESENTATION :.............................................................................................14

ALTERNATIVE 1.

METHODS OF DETERMINING THE ACID STRENGTH.......................................................16

By volumetric analysis:...................................................................................16

Procedure:............................................................................................................16 Calculations:.........................................................................................................16 2.

By pH method:................................................................................................17

Works Cited..............................................................................................................18 Error: Reference source not found

Summary: Titration is the method in which we determine the strength of an unknown solution by reacting it with a solution of known strength. In the late 18th century French chemists were using this technique.

Francois Antoine Henri Descroizilles invented the 1st burette. The other important names in the history of titration are Karl Friedrich Mohr, Joseph Louis Gay-Lussac. Titrations are of many types… some of the important types are… Acid base titration, redox titration, Complexometric titration etc There are many methods of determining the end point of a titration. A method is Conductometric method. In this method we determine the point where reaction is completed with the help of a conductometer that measures the changes in conductance of solution produced by the ions in the solution. Conductometer works on the principle of ohm’s law. If we are to find the strength of a acid then we take that acid into a beaker and dip the electrode of conductometer into the acid solution. This measures the conductance of acid. Now, we titrate this acid solution against the base of known molarity, the conductance starts decreasing. This is due to the binding of H+ ions of acid with the OH- of Base until a point is reached where conductance is minimum. When we move forward the conductance starts increasing again. This is now due to the free ions of Base present in solution. This point at which this transition occurs is called Equivalence point. At Equivalence point we measure the volume of base used to neutralize the acid ions completely in the solution. Putting these values in formula we can get the strength of acid. Alternate method to calculate the strength of acid is the simple volumetric analysis. Acid base are reacted in the presence of a pH indicator that changes its color at the completion of reaction. Again we can get the strength of acid when we know the Molarity of Base and the Volume of Base used. Another alternate method of doing this thing is the pH method. We measure the pH of acid so can calculate the concentration of positive and negative ions. At a specific temperature we already know the dissociation constant of an acid. So we can calculate the strength of acid very easily.

They ask thee about the (final) Hour― when will be its appointed time? Say: "The knowledge thereof is with my Lord (alone): none but He can reveal as to when it will occur. Heavy were its burden through the heavens and the earth. Only, all of a sudden will it come to you. They ask thee as if thou wert eager in search thereof: Say: "The knowledge thereof is with Allah (alone), but most men know not." (Al-Araf [7:187])

History The word "titration" comes from the Latin word “titulus”, meaning inscription or title. The French word titre, also from this origin, means rank. Titration, by definition, is the determination of rank or concentration of a solution with respect to water with a pH of 7 (which is the pH of pure H2O under standard conditions). The origins of volumetric analysis are in late-18th-century French chemistry.

1st burette: Francois Antoine Henri Descroizilles developed the first burette (which looked more like a graduated cylinder) in 1791.

Joseph Louis Gay-Lussac Developed an improved version of the burette that included a side arm, and coined the terms "pipette" and "burette" in an 1824 paper on the standardization of indigo solutions

Karl Friedrich Mohr, A major breakthrough in the methodology and popularization of volumetric analysis was due to Karl Friedrich Mohr, who redesigned the burette by placing a clamp and a tip at the bottom, and wrote the first textbook on the topic1

Types of titrations Titrations can be classified by the type of reaction. Different types of titration reaction include: •

Acid-base titrations are based on the neutralization reaction between

•

Redox titrations are based on an oxidation-reduction reaction between

•

Complexometric titrations are based on the formation of a complex

the analyte and an acidic or basic titrant. These most commonly use a pH indicator, a pH meter, or a conductance meter to determine the endpoint. the analyte and titrant. These most commonly use a potentiometer or a redox indicator to determine the endpoint. between the analyte and the titrant. The chelating agent EDTA is very commonly used to titrate metal ions in solution.

1 Lehrbuch der chemisch-analytischen Titrirmethode (Textbook of analytical-chemical titration

methods), published in 1855

Measuring the Endpoint of A Titration Different methods to determine the endpoint include: 1. PH indicator: This is a substance that changes color in response to a chemical change. Example:

An acid-base indicator (e.g., phenolphthalein) changes color depending on the pH. 2.

Redox indicators A drop of indicator solution is added to the titration at the start; when the color changes the endpoint has been reached.

3. . Potentiometer:

This is an instrument that measures the electrode potential of the solution. Example:

These are used for titrations based on a redox reaction; the potential of the working electrode will suddenly change as the endpoint is reached. 4. PH meter: This is a potentiometer that uses an electrode whose potential depends on

the amount of H+ ion present in the solution.

Explanation:

(This is an example of an ion-selective electrode.) This allows the pH of the solution to be measured throughout the titration. At the endpoint, there will be a sudden change in the measured pH. It can be more accurate than the indicator method, and is very easily automated. 5. Conductance:

The conductivity of a solution depends on the ions that are present in it. During many titrations, the conductivity changes significantly. Explanation:

This changes the conductivity of the solution.) The total conductance of the solution depends also on the other ions present in the solution (such as counter ions). Not all ions contribute equally to the conductivity; this also depends on the mobility of each ion and on the total concentration of ions (ionic strength). Thus, predicting the change in conductivity is harder than measuring it.

Example:

For instance, during an acid-base titration, the H+ and OH- ions react to form neutral H2O. 6. Colour change:

In some reactions, the solution changes color without any added indicator. Example:

This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours. 7. Precipitation:

If the reaction forms a solid, then a precipitate will form during the titration. Example:

A classic example is the reaction between Ag+ and Cl- to form the very insoluble salt AgCl. An isothermal titration calorimeter uses the heat produced or consumed by the reaction to determine the endpoint. This is important in biochemical titrations, such as the determination of how substrates bind to enzymes. 8. Thermometric titrimetry:

Instead, the endpoint is determined by the rate of temperature change. 9. Spectroscopy:

It can be used to measure the absorption of light by the solution during the titration, if the spectrum of the reactant, titrant or product is known. The relative amounts of the product and reactant can be used to determine the endpoint. 10.Amperometry

Current due to the oxidation or reduction of either the reactants or products at a working electrode will depend on the concentration of that species in solution. The endpoint can then be detected as a change in the current. Example:

This method is most useful when the excess titrant can be reduced, as in the titration of halides with Ag+. (This is handy also in that it ignores precipitates.)

Experiment:

To find the Strength of an acid using conductivity method

Principle: Basic principle is the changes in conductivity of a solution when no. of ions that are the responsible for the conduction of electricity is changing.

Explanation: Conductance: This is the method in which the capability of analyte to conduct an electrical current is monitored from ohm’s law.

Ohm’s law says that… V = IR V = VOLTAGE I = ELECTRIC CURRENT R = ELECTRICAL RESISTANCE

CONDUCTOMETRIC

TITRATION:

The conductance method can be employed to follow the course of a titration, provided that there is a significant difference in conductance between the original solution and the reagent of the product of reaction. It is not necessary to know the cell constants, since relative values are sufficient to permit locating the equivalence point. The conductance produced by an ion is proportional to its concentration (at constant temperature), but the conductance of a particular solution will in general not vary linearly with added reagent, because of the dilution effect of water being added along with reagent added. Hydrolysis of reactants or products, or partially solubility of a precipitated product, will also cause departures from linearity.

The shape of the titration curve can be predicted easily. The concentration of each ion at any point in the titration is calculated by the usual methods based on stoichometry, equilibria, and dilution.

Theory: The conductance of the solution mainly depends on two factors 1. Size of the ions The conductivity of the solution is inversely proportional to the size of the ions .if the size of the ions is increasing then the conductivity of the solution will decrease because the mobility of the ions will decrease by increasing the size of the ions. So the mobility of the hydrogen ions will be greater then the sodium ions so the conductance of the solution containing sodium ions will be less than the solution containing hydrogen ions. same principal is used here in this experiment ,initially solution contain the hydrogen ions when this solution is titrated against the base solution ,the base reacts with the acid and the number of the hydrogen ions go on decreasing. Finally a stage reaches when there is no hydrogen ion in the solution so the conductivity decreased and remains constant. If further base is added then the number of negative ions increases and the conductivity go on increasing. The point at which the conductivity becomes constant is the equivalence point. The volume of base used at equalance point is used to calculate the molarity of acid and then the strength of acid. Consider a solution of a strong acid, hydrochloric acid, HCl for instance, to which a solution of a strong base, sodium hydroxide NaOH, is added. The reaction occurs. For each amount of NaOH added equivalent amount of hydrogen ions is removed. Effectively, the faster moving H+ cation is replaced by the slower moving Na+ ion, and the conductivity of the titrated solution as well as the measured conductance of the cell fall. This continues until the equivalence point is reached, at which we have a solution of sodium chloride, NaCl. If more base is added an increase in conductivity or conductance is observed, since more ions are being added and the neutralization reaction no longer removes an appreciable number any of them. Consequently, in the titration of a strong acid with a strong base, the conductance has a minimum at the equivalence point. This minimum can be used instead of an indicator dye to determine the endpoint of the titration. Conductometric titration curve that is a plot of the measured conductance or conductivity values against the number of milliliters of NaOH solution

2. Temperature:

By increasing the temperature, the mobility of the ions in the solution will increase. So temperature has a direct effect on conductance of solution. E.g. by increasing the temperature the conductance will increase and vice versa.

Procedure: 1. 2. 3. 4. 5. 6.

100 Ml of acid was taken in beaker. Solution was kept at room temperature for at least 10 minutes. Then Dip the electrode of conductometer in solution. Measure the initial conductance of solution. First reading of conductometer was noted. Then this solution was titrated against the Base solution of known strength using burette. 7. Observe the change in conductance of acid solution after every 5 ml addition of base. 8. Plot these values to find the equivalence point. 9. Volume of base used till the equivalence point is the volume which is required to fully neutralize the acid.

Calculations:

Acid

Base

M1 V1 = M2 V2 M=molarity v=volume used M1=M2V2V1

Volume of Base used (mL)

Conductance

0

28.2

5

28

10

27.7

15

27.2

20

26.6

25

21.8

30

20.6

35

19.4

40

18.6

45

17.6

55

16.6

65

13.6

75

12

85

10.6

95

9

105

7.8

110

7.2

115

6.6

120

6

130

5

135

5.1

140

5.3

150

5.9

160

6.3

170

6.8

185

7.4

M1=M2V2V1 M1=0.1×130100 M M1=0.13 M

Strength of acid = molarity x molecular wt. =0.13 X 36.5= 4.745 g/mL

GRAPHICAL

REPRESENTATION :

APPLICATIONS OF CONDUCTOMETRIC TITRATIONS Acid-base titrations redox titrations are known to us in which commonly indicators are used to locate the end point e.g., methyl orange, phenolphlthalene for acid base titrations and starch solutions for iodemetry type redox process. How ever electrical conductance measurement can be used as a tool to locate the end point. 1. This method can be used with much diluted solutions 2. This method can be used with colored or turbid solutions in which end point can not Be seen by eye 3. This method can be used in which there is no suitable indicator 4. has many applications, i.e. it can be used for acid base, redox, precipitation, or complex titrations

a. Determination of sulphur dioxide in air pollution studies b. Determination of soap in oil c. Determination of accelerators in rubber d. Determination of total soap in latex e. Specific conductance of water

ALTERNATIVE

METHODS O F DETERMINING THE ACID STRENGTH

1. By volumetric analysis: Experiment: To determine the strength of an acid

Principle: It is an acid base titration.

Apparatus: 1. Burette 2. Conical Flask

3. Pipette 4. Burette Stand

Indicator: 1-2 drops of Phenolphthalein

Chemical reaction: HCL+NaOH→NaCL+H2O

End point: Light pink color

Procedure: 1. 2. 3. 4. 5.

Take 10ml of acid in the conical flask with the help of a pipette. Add 1-2 drops of indicator in it. Solution’s color was changed to pink. Take the base of known molarity in burette. Titrate the acid solution against the base of known molarity. Calculate the volume of base used to neutralize the acid as we see that end point is reached. 6. Perform following calculations to calculate the strength of acid…

Calculations: Acid M1 V1 = M2 V2 M=molarity v=volume used

Base

M1=M2V2V1

1. By pH method: Experiment: To determine the strength of an acid

Principle: Strength of acid is calculated from the pH of solution. Chemicals and apparatus: 1. PH meter 2. Pipette solutions (acid & base)

3. Thermometer 4. Chemical

Procedure: As we know that Ka=H+A-HA Ka has a fixed value at a given temperature that is given in literature. H+ Can be measured by using ph apparatus Also A- can be found as For monobasic acid H+=ASo initial concentration of acid can be calculated as HA=H+A-Ka

Works Cited Board, p. T. (2009). Chemistry I. punjab Text Book Board. encyclopedia Britinica. (n.d.). Retrieved 10 2009, from http://www.britannica.com/EBchecked/topic/131698/conductometry ewing, g. w. Instrumental Methods of Chemical Analysis. Mcgrawhil publisher. Ewing, G. W. Instrumental Methods of Chemical Analysis. McGrawhill Publisher. J.Welehor, F. Standard Methods of Chemical Analysis. D.VAN Nostrand company, INC. www.wikipedia.org. (n.d.). Retrieved 10 8, 2009, from wikipedia the free encyclopedia: http://en.wikipedia.org/wiki/Titration

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