Back Titration

Back Titration

Up for a short chemistry tutorial on back titration? Go through this article to know its exact definition, its entailing examples and some more relative information under this topic.
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, revising what is titration, back or redox titration and stuff like that! Well, before I begin with the definition, let me brief you, in short, on the definition of titration.

Titration is a process which calculates the property of one solution (molarity) to generalize another unknown property of a solution.

What is Back Titration

It is basically, an analytical technique in chemistry, which is performed backwards in the 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 react 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. This method of titration is very useful as:
  • The endpoint of a reverse titration is simpler to identify (than the endpoint of a normal titration)
  • You can try to compute the amount of acid/base in a non-soluble solid
There's also another definition (of the same) that you can learn, i.e., it is a technique that is used in the analysis of solids, which are insoluble or partially soluble in water. It's also used, when:
  • The sample consists of certain impurities which are bound to interfere with a regular forward titration
  • If the reaction between the titrant and the analyst is extremely slow
After learning all these lessons, let's consider some examples for you to experiment with.

A Solved Example

Let's use this titration method 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 endpoint result. You cannot use simple titration method since aspirin is a weak acid, the identification of the endpoint will get difficult and the reaction will proceed slowly.

Solution
The 'stage one' of this reaction is of alkaline hydrolysis involving reaction 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 this method of 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 of the best examples of wherein, the use of aspirin to determine the end result was expended. Take a look at some more examples given below.

Other 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.
Well, now that you have read the definition and gone through an example (that was solved for you), here is one example which I think you can attempt to solve all by yourself. Let me know what the answer is! Wishing you luck!

Your Homework Exercise

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.