Titration

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In this presentation you will: Explore the titration method to find the

concentration of a solution

Titration

Titration is a method of analysis that allows us to determine the precise equivalence point of a reaction and therefore the precise quantity of reactant added to the titration flask. From this information we can calculate the concentration of the reactant.

To learn about the titration method, it is important to understand first what the pH level of a base or an acid is.

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Titration

The concentration of hydrogen ions in an acid aqueous solution can be measured with the pH scale, where the pH value is the negative logarithm of the oxonium ion concentration:

pH Scale

The concentrations of hydrogen ions in most acid aqueous solutions are between 10-14 mol dm-3 and 1 mol dm-3. Therefore, the range of values of these solutions are between 0 and 14 since:

–log10(1) = 0 and

–log10(10-14) = 14.

pH = -log[H3O+]

H+ concentration moles per liter

Sodium hydroxidePotassium hydroxide

Borax

Sodium bicarbonate

Distilled water

Boric acid

VinegarLemon juice

Nitric acid

pH values of some substances

Magnesia milk

Hydrochloric acid

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Titration

The pH of any solution when the concentration of hydrogen ions is written as 1 x 10-n will be the numeric value of n.

pH Scale

Pure water at 25 ºC has a concentration of hydrogen ions of 1 x 10-7 mol dm-3, hence a pH of 7. Every solution of pH 7 is neutral.

For example: 1 x 10-9 mol dm-3 = pH 9.

The higher the pH of a solution, the more basic or alkaline it is. The lower the pH, the more acidic it is.

Sodium hydroxidePotassium hydroxide

Borax

Sodium bicarbonate

Distilled water

Boric acid

VinegarLemon juice

Nitric acid

pH values of some substances

Magnesia milk

Hydrochloric acid

H+ concentration moles per liter

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Titration

1

What is the pH value of a neutral solution?

Question

A) 0

B) 1

C) 7

D) 14

Titration

Some chemical substances react to different pH levels by changing their color.

Indicators

These substances, called indicators, are used in titration experiments to establish the acidic-alkaline values of substances and mixtures.

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Titration

Indicators

Other indicators change color at different acid-alkaline levels.

By selecting the right indicator we can establish the pH level of nearly any aqueous colorless solution.

For example, the litmus indicator will show red in acid solutions and blue in basic solutions.

Different colorant indicators have different color combinations which change at different pH levels. One color will represent an acidic solution and the other color a more basic solution.

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Titration

2

What is an indicator used for in a titration experiment?

Question

A) To establish the acidity-alkalinity of a solution.

B) To increase the acidity of a solution.

C) To increase the alkalinity of a solution.

D) To alter the pH level of a solution.

Titration

Titration is a laboratory technique used to determine the concentration Cr of an acid or base solution.

Titration

The aim of the experiment is to find out the volume Vs of a titrating substance (with a known concentration Ck) necessary to neutralize the acidity or alkalinity of a substance.

With the volume of the titrating substance and its concentration, the number of moles necessary to neutralize the substance can be calculated.

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Titration

Titration

In a titration, an acid solution of accurately known strength called standard acid is gradually added to a known volume of base until a pH meter reads exactly 7 or an indicator pigment dissolved in the mixture changes its color.

The change of color of the indicator signifies the reaching of the equivalence point, where the substances neutralize each other.

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Titration

Titration

The unknown concentration of the base can then be calculated based on the number of moles necessary to neutralize each other, together with the base volume, with the equation:

NrC =r Vr

where Cr is the concentration of the base solution, Nr is the neutralizing number of moles, and Vr the volume of the titrated solution.

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Titration

3

What is the final aim of a titration experiment?

Question

A) To find out the volume of the titrating substance added to the titrated solution.

B) To find out the concentration of the titrating substance added to the titrated solution.

C) To find out the volume of the titrated solution.

D) To find out the concentration of the titrated solution.

Titration

Calculating Concentration

25 cm3 of a base changed color (was neutralized) when 15.8 cm3 of a 0.025 mol dm-3 acid was added.

number of moles Nr = volume in dm3 Vr x acid concentration in mol dm-3 Cr

To calculate the unknown concentration Cr of the titrated solution, the number of moles in the acid which neutralized the solution needs to be calculated first.

This is given by:

So the acid’s moles in this experiment = × 3 -315.8 dm 0.025 mol dm1000

= 3.95 x 10-4 molNext >

Titration

4

How is the number of moles of a titrating substance calculated?

Question

A) Divide the volume by its molarity concentration.

B) Multiply the volume by its molarity concentration.

Titration

Calculating Concentration

Knowing the number of moles that were necessary to neutralize the base solution, its concentration can be calculated.

Base moles Nr = volume in dm3 Vr x acid concentration in mol dm-3 Cr

3.95 x 10-4 mol = (25 dm3 / 1000 ) x Cr

= -4

-2 -3r3r

r

N 3.95 10C = = 1.58 10 mol dmV 25 dm / 1000

Therefore;

Applying the moles formula to the base solution, we obtain:

3.95 x 10-4 mol

The concentration of the base solution is therefore 0.0158 mol dm-3

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Titration

5

How is the concentration of a titrated solution calculated?

Question

A) Multiplying the concentration of the titrating substance by its volume.

B) Multiplying the number of moles in the titrating substance by its volume.

C) Dividing the concentration of the titrating substance by its volume.

D) Dividing the number of moles in the titrated substance by its volume.

Titration

Titration curves

The titration of a strong acid with a strong base produces the titration curve shown in the diagram.

The curve is drawn by plotting the data obtained during the titration, titrant volume on the x-axis and pH on the y-axis.

A titration curve is a graph representing the pH change in a unknown solution during a titration experiment.

The titration of a strong base with a strong acid will produce a descending curve in the pH axis.

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Titration

Titration curves

4. The pH range at equivalence.

1. The initial pH of the solution.

The important features of titration curves are:

2. The equivalence point when equimolar amounts of both acid and base are present.

5. The final pH.Initial pH

Final pH

3. The volume of titrating liquid required for equivalence.

equivalence pH range

Equivalence point

Equivalence titrating volume

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Titration

6

What does a titration curve represents?

Question

A) The change of the equivalence point of an unknown solution during a titration experiment.

B) The change of the number of moles in an unknown solution during a titration experiment.

C) The pH change in an unknown solution during a titration experiment.

D) The molarity change in an unknown solution during a titration experiment.

Titration

Show knowledge and understanding of what a titration experiment is.

Show knowledge and understanding of the pH scale.

Having completed this presentation you should be able to:

Summary

Show knowledge of what indicators are.

Show understanding of how to calculate the concentration of a solution.

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