Dichromatic Titration - Titrations, Redox Titrations, Dichromate Titration, Practice Problems and FAQs

I'm sure everyone has heard about chlorine water treatment or chlorination in the pools.

Do you know what chlorination is, though?

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Chlorination is used to disinfect drinking water in order to get rid of germs and viruses. When a public water supply is chlorinated, several chlorine-containing species are produced and referred to as the total chlorine residual. The species is unhealthy for people.

Hence, Chlorine should be added to water in balance for best health effects.

Redox titrations are used to evaluate the chlorination of public water supplies.

In Redox titration, chlorine is oxidized by the application of various oxidizing agents. The optimal level of chlorine can be found by adding indigo as an indicator, which is colorless before the equivalence point. However, once the equivalence point is reached, unreacted indigo gives the solution a permanent color.

Table of Contents

Titration
Redox Titrations
Potassium dichromate titrations
Practice Problems
Frequently Asked Questions

Titration:

Titration is a technique used to compare a solution's concentration to that of another solution with a known concentration. Titration is a quantitative and volumetric method for deriving the concentration of an unknown solution from the concentration of a known solution when there is an indicator present.

Procedure used for titration:

A solution is added to a titration flask during this procedure.
The titrant is the solution used in the burette, whereas the other solution used in the titration flask is known as the analyte.
The titration is carried out until the stoichiometric ratios of the two reagents, titrant and analyte, have become equal.

For instance, when a strong acid like HCl and a strong base like NaOH are titrated, the following reaction occurs.

$H C l (a q) + N a O H (a q) \to N a C l (a q) + H_{2} O (a q)$

(i) The point at which complete neutralization of HCl and NaOH takes place, it is known as an endpoint or the equivalence point.

(ii) This is the simplest technique in which indicators are added to solutions that change color at the equivalence point into the solution.

Redox Titrations:

The basis of redox titration is a redox reaction that takes place between the analyte and the titrant. Between the analyte and the titrant, an oxidation-reduction process takes place simultaneously.

Numerous oxidizing agents like acidified KMnO₄(Potassium permanganate), acidified K₂Cr₂O₇(Potassium dichromate) and iodine solution, are used to oxidize the reducing agents such as ferrous sulfate , oxalic acid, sodium thiosulfate, potassium iodide, etc.

There are basically three type of redox titrations and these are:

(i) Potassium permanganate titration

(ii) Iodine titration

(iii) Potassium dichromate titration

Potassium dichromate titrations:

Acidified potassium dichromate K₂Cr₂O₇ is also used as an oxidizing agent in redox titrations. It is a somewhat weaker oxidizing agent than KMnO₄.

However, since these titrations do not produce a noticeable auto-color shift, it is not a self-indicator titrations like KMnO₄ titration. These titrations typically employ indicators such diphenylamine, N-phenylanthranilic acid, and the potassium ferricyanide, K₄[Fe(CN)₆] external indicator.

For instance, K₂Cr₂O₇ oxidizes the indicator diphenylamine just after the equivalence point or at the endpoint to create a strong blue color that signifies the endpoint.

Titration of Cr₂O₇^2- against Fe²⁺ions:

Reduction Half Reaction: ${C r}_{2} O_{7}^{2 -} (a q) + {14 H}^{+} (a q) + 6 e^{-} \to 2 {C r}^{3 +} (a q) + 7 H_{2} O (a q)$

Oxidation Half Reaction: ${F e}^{2 +} (a q) \to {F e}^{3 +} (a q) + e^{-} \dots \dots \dots \dots \dots \times 6$

Overall reaction: ${C r}_{2} O_{7}^{2 -} (a q) + {14 H}^{+} (a q) + 6 {F e}^{2 +} (a q) \to 2 {C r}^{3 +} (a q) + 6 {F e}^{3 +} (a q) + 7 H_{2} O (a q)$

Practice Problems:

Q1. Find n-factor of Fe in Fe2+ in the given reaction- ${C r}_{2} O_{7}^{2 -} (a q) + {14 H}^{+} (a q) + 6 {F e}^{2 +} (a q) \to 2 {C r}^{3 +} (a q) + 6 {F e}^{3 +} (a q) + 7 H_{2} O (a q)$

Answer: (D)

Solution:

Let O.S of Fe in Fe²⁺= x

$\Rightarrow x = + 2$

Let O.S of Fe in Fe³⁺ = y

$\Rightarrow y = + 3$

n-factor of Fe in Fe³⁺ = |3 - 2| = 1

Hence, option (D) is the correct answer.

Q2. In the Potassium dichromate redox titrations, Potassium dichromate behaves as:

Reducing agent
Oxidizing agent
Neither reduced nor oxidized
Self Indicator

Answer: (B)

Solution: It cannot act as a self-indicator because these titrations do not result in a noticeable auto-color shift and an external indicator needs to be added for detection of the endpoint.

In redox titrations, acidified potassium dichromate K₂Cr₂O₇ is also employed as an oxidizing agent.

If we consider the titration of Cr2O72- against Fe2+ions-

${C r}_{2} O_{7}^{2 -} (a q) + {14 H}^{+} (a q) + 6 {F e}^{2 +} (a q) \to 2 {C r}^{3 +} (a q) + 6 {F e}^{3 +} (a q) + 7 H_{2} O (a q)$

As oxidation state of Cr in Cr₂O₇^2- is +6 which decreases to +3 in Cr3+. If the oxidation state decreases, reduction occurs and hence Cr₂O₇^2- acts as an oxidizing agent.

Hence, option (B) is the correct answer.

Q3. Which of the given titration methods do not fall under the category of redox titrations?

Potassium permanganate titration
Iodine titration
Acid-Base titrations
Potassium dichromate titration

Answer: (C)

Solution: Redox titration is an oxidation-reduction process that takes place simultaneously.

Numerous oxidizing agents like acidified KMnO₄ (Potassium permanganate), acidified K₂Cr₂O₇(Potassium dichromate) and iodine solution are used to oxidize the reducing agents.

The three major redox titrations are

(i) Potassium permanganate titration

(ii) Iodine titration

(iii) Potassium dichromate titration

As acid-base titration do not fall under the category of redox titrations. It is a different class of titrations. Hence, the correct answer is option (C).

Q4. Pick out the correct statement about Potassium dichromate Titrations.

In Potassium dichromate titration, Potassium dichromate itself acts as an indicator.
Titrant used in Potassium dichromate titration is the Potassium dichromate whose concentration is unknown.
Analyte used in Potassium dichromate titration is the Potassium dichromate whose concentration is known.
All the given statements are incorrect.

Answer: (D)

Solution: A standard solution with a known concentration is known as the titrant and whose concentration is unknown is known as the analyte. Potassium dichromate (K₂Cr₂O₇) is taken in a burette and added dropwise in the titration flask. Hence, the concentration of Potassium dichromate (K₂Cr₂O₇) is unknown and acts as an analyte in this titration.

Potassium dichromate (K₂Cr₂O₇) does not function as a self-indicator because these titrations do not result in a noticeable auto-color shift. External indicators are frequently used in these titrations.

Hence, option (D) is the correct answer.

Frequently Asked Questions-FAQs:

1. Do excessive amounts of indicator impact titrations?
Answer: A too-great indicator addition will modify the concentration of the solution to which the titrant is added, which will have an impact on the titration process.

Assume for the moment that the indicator is acidic in nature. If you mix it with acid, it will increase the acidity of the acid beyond what you anticipated. Your entire experiment will be ruined since it will be inaccurate because more bases will be needed to neutralize the acid.

2. What kinds of titration errors are possible?
Answer: Just a few of the factors that could cause errors in titration findings include misreading volumes, choosing the incorrect concentrations, and applying substandard techniques. Care must be used when transferring a known concentration of solution into a specific volume of the unknown using laboratory glassware like a burette or pipette.

3. How can the peroxide concentration of teeth-whitening products be regulated using redox titration?
Answer: As it is well known, redox titration uses the oxidation-reduction reaction theory. Here, oxidizing chemicals such as acidified KMnO₄ (Potassium permanganate), acidified K₂Cr₂O₇(Potassium dichromate), and iodine solutions diminish the peroxide in teeth-whitening products. As a result, we can calculate the peroxide content.

4. What distinguishes an endpoint from an equivalence point?
Answer: The primary distinction between an equivalence point and an endpoint is that an endpoint is a point at which the color of a system changes, whereas an equivalence point is a point at which the equivalents of the acid and the equivalent of the base are equal.