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Titration Indicator

Titration

It is also known as titrimetry and volumetric analysis [1]. Titration is quantitative chemical analysis used in laboratories to find out the concentration of an identified analyte (a substance to be analyzed). A reagent which is termed as titrant or titrator is prepared on the basis known concentration and volume as a standard solution. The titrant react with analyte, the volume used is termed as titration volume. During the titration of strong acid with strong base the pH changes from 3 to 11, phenolphthalein indicator range from pH 8 to 10 that’s why mostly used for this type of titration.

Indicator

A substance that changes color of the solution in response to a chemical change. Phenolphthalein indicator used in acid-base titration.  A drop of indicator is added in the start of the titration, the endpoint has been appeared when color of the solution is changes. Redox indicators are also used which undergo change in color at specific electrode potential [2]. Different indicators are used but depend on the strength of an acid and alkali. Universal indicators are not used for titration because they give different color at different pH. Methyl orange or phenolphthalein are mainly used they cause change in color at neutralization which is easier to see the end point of titration. The most important property of an indicator is pH range which is depend upon the strength of an indicator.

Titration Procedure

This process contain a beaker which contain small amount of the analyte and very small amount of indicator like phenolphthalein placed underneath a calibrated burette which contain the titrant. Small amount of titrant is added in the solution until the indicators changes the color, representing the endpoint of the titration. Single drop or less than the drop of used make difference in between a permanent and temporary change in the indicator. The volume of titrant used is measured and calculate the concentration of analyte through

                       Ca = CtVtM

                                  Va

Ca = concentration of the analyte typically in molarity

Ct = concentration of the titrant

Vt= volume of the titrant used measured in liters

M= mole ratio of analyte and reactant from the balanced chemical equation.

Va= volume of analyte used typically in liters.

Types of Titration

There are many types of titration but the most common qualitative are acid-base titration and redox titration.

  • Acid- base Titration

This titration is used to find out the unknown concentration of an acid or base through neutralizing with an acid or base of known concentration. An appropriate pH indicator is used in the titration chamber which show the pH of the solution. Phenolphthalein indicator is used because it reduce the error. When more accurate results are needed pH meter or a conductance meter is used.

  • Redox Titration

This titration based on a reduction-oxidation reaction carried out in between an oxidizing agent and reducing agent. An indicator or potentiometer is used to find out the end point of titration, main constituent in the oxidizing agent is potassium dichromate. The color changes is not definite that’s why sodium diphenylamine is used. But some titration do not need an indicator because of intense color of constituent.

  • Gas phase Titration

This titration is carried out in gas phase. In this method reactive substances are reacted with an excess of some other type of gas which is called as titrant. It has many advantages over spectrophotometry like its measurement does not depend upon path length, not depend upon linear change in absorbance and also sample which interfere at wavelength used specially for analyte.

  • Complexometric Titration

In this titration complex formation between analyte and titrant. That’s why specialized indicators are used which make weak complexes. For example starch indicator is used iodometric titration and chelating agent EDTA is used metal ions titration in solution.

  • Zeta potential Titration

It is used to find out the iso-electric point when surface charge is zero, it may be achieved by changing the pH or addition of surfactant.

Indicators

As weak acid

  • Litmus

It is a weak acid, a complicated molecule consist of HLit. The “H” called as proton which is given away while “Lit” is a weak molecule [3]. The equilibrium will be established when the acid dissolve in water.

                    HLit ⇋ H+ (aq) + Lit(aq)

Litmus in unionized form give red color while in ionized form it gives blue color.

When hydroxide ions are adding:

Titration Indicator 1

When hydrogen ions are added:

Titration Indicator 2

When the concentration of HLit and Lit becomes equal so at that point the color appear is a mixture of two colors and called as neutral. Because at that point the pH of the solution is 7. That’s why litmus paper is used to test for acids and alkalis.

  • Methyl orange

It is the pH indicator commonly used in titration. In alkaline solution methyl orange is in yellow color. When acid is added in the solution it gives red color. Methyl orange changes color at the pH of mid strength acid.

  • Phenolphthalein

It is also very common indicator, a weak acid used in titration. The weak acid has no color while its ion gives bright pink color.

Titration Indicator 3

Due to addition of extra hydrogen ions shifts the position of equilibrium to left side which turns the indicator colorless. But addition of hydroxide ions and removal of hydrogen reverse the equation and turns the indicator into pink color. This is mostly happens at pH 9.3 but mixture of pink and colorless gives paler pink color that’s why it is very difficult to detect this indicator very accurately.

The pH range of indicator

Importance of pKind

Generally the indicator is represent as HInd where “Ind’’ is an indicator while “H” ion is given away.

Hind (aq) ⇋ H+(aq) + Ind(aq)

Ka is also called as KInd its expression is written as

              KInd = [H+] [Ind]/ [Hind]

              KInd = [H+] [Ind]/ [Hind]

              KInd = [H+]

Rearrange the equation so the hydrogen ion concentration is on left side,

           [H+] = KInd

            pH = pKInd

The value of indicators are as follow:

Indicator pKInd
Litmus 6.5
Methyl orange 3.7
Phenolphthalein 9.3

 The pH range of indicators

The range of indicator is not fix at one point of pH, they change very quickly over a narrow range of pH. The addition of anything will change the color of solution. For example: the dominant color of methyl orange in alkaline solution is yellow but addition of an acid will shift the pH of solution and the color appear to be an orange tint with the addition of more acid the red color will appear and there is no yellow color.

The exact values of the indicators are as follow:

Indicator pKInd pH range
Litmus 6.5 5-8
Methyl orange 3.7 3.1-4.4
Phenolphthalein 9.3 8.3-10.0

In laboratory, litmus is used it changes color around pH 7 but methyl orange or phenolphthalein would be used in small amount.

Titration Indicator 4

Choosing indicator for titrations

To choose an indicator it varies from titration to titration.

Strong Acid v Strong Base

The diagram shows the pH curve between strong acid to a strong base, for methyl orange and phenolphthalein.

Titration Indicator 5

The graph shows that there is no difference in color at equivalence point. But by using phenolphthalein it would titrate at pH 8.3 and it is very close to the equivalence point, while methyl orange would titrate the solution but further away from the equivalence point.

Strong Acid v weak base

Titration Indicator 6

In this graph the indicator methyl orange is used and change color of end product from yellow to orange which is very close to equivalence point but phenolphthalein is completely useless.

Weak Acid v strong base

Titration Indicator 7

In this condition only phenolphthalein indicator works and give accurate color. Which is very close to equivalence point.

Weak acid v weak base

In this case both acid and base are weak in nature that’s why neither methyl orange nor phenolphthalein indicators works to give color.

Titration Indicator 8

References

  1. Whitney, W.D.; Smith, B.E. (1911). “Titrimetry”. The Century Dictionary and Cyclopedia. The Century Co. p. 6504.
  2. https://en.wikipedia.org/wiki/Titration
  3. https://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html
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