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Titration is the process by which the unknown concentration of a substance can be determined by titrating it by adding measured amounts of a standard solution. The reaction’s stoichiometry can obtain the unknown concentration and the number of moles of the standard solution to get the desired endpoint.
Mohr’s method
Mohr’s method is a method that obtains the amount of chloride ion concentration in a solution by titrating it with silver nitrate solution.
Background
Chloride ions are one of the major components of wastewater when it comes to inorganic constituents of water. Its concentration in raw water is lesser than that of sewage because chloride ions, for example, in sodium chloride, are barely missed by the human body’s digestive system.
In the seaside places, seawater may pass into the sewage system, causing an increase in the concentration of salt percentage in the water systems. Also, industrial processes can be a reason behind the increase of salt content in the water systems. In drinking water, the salty taste we get is due to the presence of chloride ions, which varies in concentration and is highly dependent on the composition of water.
So, Mohr’s method can be used for the analysis of sodium chloride!
Standard solution in chemistry
Any chemical solution that has a known concentration is called a standard solution. In Mohr’s method, we use silver nitrate as the standard solution.
Endpoint in chemistry
The point determined experimentally where some signal marks the completion of a reaction is called the endpoint. We will be discussing the endpoint of Mohr’s reaction in the upcoming paragraphs.
Titration reactions use reactions that give ionic compounds of limited solubility. The most important of them is the silver nitrate solution (AgNO3). The reaction that involves silver nitrate solution is also sometimes called an argentometric reaction. So the method for the analysis of sodium chloride will be Mohr’s method.
Mohr’s method is highly sensitive to pH, so why is pH range so important in the determination of chloride concentration in Mohr’s method?
The pH range is very important in the determination of chloride concentration in Mohr’s method because at higher pH, silver ions may be eradicated, and the precipitation of hydroxide ions takes place, whereas, at a lower pH, chromate ions would be eradicated by the acid-base reaction to form hydrogen chromate or dichromate ions. This will affect the accuracy of the endpoint.
Potassium Chromate is used as an indicator of the endpoint achieved by the titration of sodium chloride to determine chloride ions. The chloride ions react with the silver nitration solution’s silver ions to form a brick red-coloured silver chromate precipitation as the endpoint.
The Mohr method uses chromate ions as a marker or, in the words of chemistry, an indicator for the titration of chloride ions with the silver nitrate solution. The first titrant forms silver chromate when the entire chloride concentration is precipitated as white silver chloride. This marks the endpoint (1).
𝐴𝑔(𝑎𝑞) + 𝐶𝑙(𝑎𝑞) → 𝐴𝑔𝐶𝑙(𝑠)
2𝐴𝑔+(𝑎𝑞) + 𝐶𝑟𝑂42-(𝑎𝑞) → 𝐴𝑔2𝐶𝑟𝑂4(𝑠)
Through stoichiometry and the number of moles consumed by the reaction of titration mentioned above, one can determine the value of the concentration of the chloride ions in the given sodium chloride solution. Thus, Mohr’s method is useful for analysing sodium chloride in this particular instance.
Materials and Method Used for the Analysis of Sodium Chloride
Equipment and Reagents required
The reagents used in this experiment are CaCO, NaCl, NaHCO3, KCrO, and AgNO3. The equipment used is listed below: A buret transfer pipette 25 mL, pipette pump, Erlenmeyer flasks 250 mL, Desiccator Volumetric flask 500 mL, Amber bottle Graduated cylinder 100 mL, and Wash bottle.
Preparation of standard AgNO3 solution –
9.0gm of AgNO3 solution is put into a 500mL flask. Fill up the remainder of the flask with distilled water. So the moles in this solution are 1.0 M. This solution is a standard against the sodium chloride solution (NaCl).
Experimental Work –
Steps:
Take 50mL of sodium chloride sample in a conical flask
Add 1.0mL of potassium chromate to the flask. This potassium chromate serves as an indicator
Indicator
An indicator, particularly a chemical indicator, is a substance that gives a signal, a colour change signal, to show the absence or presence of a threshold concentration of a chemical species.
The initial colour of the AgNO3 mixture is yellow, which is noted down for experimentation purposes and to determine the result
Titrate the standard solution slowly and steadily and stop immediately after reaching the brick-red colour endpoint. Note that the brick-red colour should be intensified, and titration should be stopped as soon as you get your brick red colour. Note the point on the titration tube where the brick-red colour is obtained: the volume of the titrant used
Calculate the chloride ion concentration by using the following equation
Chloride Ion Concentration (mg/L) =( mL of AgNO3 used-0.2) X500 / mL of sample used
Thus, we can conclude by saying that Mohr’s method is an effective method for analysing sodium chloride.
Precautions
Uniform sample usage is suggested, preferably 50mL, so the ionic concentration to achieve the endpoint remains constant.
The pH of the solution must be between 7–8 since chromate ions are converted to something else at lower pH concentrations. Therefore, the desired results are not achieved. So it can be said that pH range is important in determining chloride ions in Mohr’s method.
A good amount of indicator solution must be used. Otherwise, the endpoint might be achieved too soon, disrupting the chances of getting accurate results.
Caution should be taken to notice the colour change of the indicator solution from yellow to brick red, and the tube must be shaken to get accurate results.
Take a minimum of three results to obtain accurate readings for the experiment.
Additional pointers
Leftover silver ions can be re-used for the later recovery of silver metal. You can always ask for your teacher’s or lab assistant’s support in this matter
It is advised to do a ‘rough’ titration first to get hold of the process and get familiar with it. Excellency in titration takes a good amount of time
Mohr’s titration method is used to determine both the chloride ions and the bromide ions (if present). However, if one is using seawater, the concentration of the bromide ions will be negligible
Thus, today, you learned how to determine the concentration of chloride ions using Mohr’s method.
Conclusion
This article elucidates the topic of determining sodium chloride using Mohr’s Model. Mohr’s method is a method that obtains the amount of chloride ion concentration in a solution by titrating it with silver nitrate solution. Further, the article discusses how sodium chloride is obtained through various measures and methods.
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