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Back Titration

You being here, and reading this article simply shows that you feel nostalgic about going back to your analytical chemistry lessons, to once again read through all different chemical elements, atomic structures, lab experiments like revising the subject of what is titration, redox or back titration and stuff like that! Well, to begin with defining back titration, let me brief up for you in short the definition of titration first.

Titration is a process which calculates the property of one solution (molarity) to generalize another unknown property of solution. Now with this take, we’ll see the definition of back titration below.

What is Back Titration

Back titration is basically, an analytical technique in chemistry, which is performed backwards in method. That is, a user needs to find the concentration of a reactant of a given unknown concentration by reacting it with an excess volume of another reactant of a known concentration. Once these mixtures are reacted with each other, there will be a resulting mixture, which is titrated back in the process. The molarity of the excess which was added in the process is taken under consideration too. Back titration is very useful if the end point of a reverse titration is simpler to identify than the end point of a normal titration, and are trying to compute the amount of acid/base in a non-soluble solid.

With this back titration definition there’s another definition you can learn about the same, that is, back titration is a technique that is used in the analysis of solids which are insoluble or partially soluble in water. Its also used, when the sample consists of certain impurities which are bound to interfere with a regular forward titration. And even useful if the reaction between the titrant and the analyst is extremely slow.

After learning all these lessons on back titration, let’s consider some back titration examples for you to experiment with.

Best Back Titration Example

Example #1
Using the method of back titration to measure the amount of aspirin in a solution. This reaction will be a reaction between a strong base and a strong acid, reacting at high speed and producing an abrupt end-point result. (You cannot use simple titration method since aspirin is a weak acid, the identification of the end point will get difficult and the reaction will proceed slowly).

Solution
The stage one of this reaction is of alkaline hydrolysis involving reacting of aspirin solution with a measured amount of sodium hydroxide. Sodium hydroxide is the amount that exceeds the amount of aspirin present. Also, at room temperature the hydrolysis reaction proceeds at a very slow rate, thus it’ll be heated to increase the rate of reaction.

Equation of Reaction: Stage 1
CH3COOC6H4COOH + 2NaOH ———-> CH3COO.Na + HOC6H4COO.Na + H2O

Here: HOC6H4COO.Na + H2O is
Sodium Ethanoate + Sodium-2-hydroxybenzoate + Water

After this reaction, stage two involves a back titration consisting of the hydrolyzed sodium hydroxide solution with hydrochloric acid.

Equation of Reaction: Stage 2
NaOH + HCl ——-> NaCl + H2O

Here: NaOH + HCl ——-> NaCl + H2O is
Sodium Hydroxide + Hydrochloric acid ——> Sodium Chloride + Water

Conclusion
By performing back titration, the amount of hydrochloric acid required to neutralize the unreactive sodium hydroxide present in the solution can be easily determined. With all this knowledge, the amount of sodium hydroxide added to the amount of aspirin is also determined.

Well, this was one best example of back titration wherein, the use of aspirin to determine the end result was expended. Take a look at some more examples to determine back titration.

Other Back Titration Examples

1. A 25.00 ml aliquot of diluted sample is pipetted in a digestion flask. Concentrated H2SO4 and H2O are added and the whole solution is heated for 45 mins. The result would be, the organic molecules are broken down and organic nitrogen is converted into NH4+
2. A 10.00 ml sample of organic bound nitrogen is diluted into 100 ml with distilled water.
3. An easy example: If a boy has 5 oranges and 5 lemons and another friend wants to know the number of lemons in that boys hand. What they both friends could do is, first count the number of lemons and then subtract them from 5. Both will get an instant result.

Well, now that you have read the back titration definition and conceptual idea of what is back titration, here is one example which I think you can attempt to solve all by yourself. Let me know what the answer is! All the best!

Homework Exercise On Back Titration

Problem
A 50 ml volume of 0.10 M nitric acid is mixed well with 60 ml of 0.10 M calcium hydroxide solution. Calculate the volume of 0.050 M sulfuric acid which is required to naturalize the mixture?

Well, in conclusion I would just say that, the laboratory and experimental skills that are needed for conducting a back titration experiment in chemistry are almost similar to a common acid/base titration in the same breath.

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