The Mohr method is a variant of the Argentometry, which in turn is one of the many areas of the volumes used in determining the content of chloride ions in water samples. The concentration of Cl - indicates the quality of the water, affecting its organoleptic properties such as its taste and smell.
This method, devised in 1856 by the German chemist Karl Friedrich Mohr (106-1879), continues in force due to its simplicity and practicality. One of its main drawbacks, however, is that it relies on the use of potassium chromate, K 2 CrO 4, a salt that is harmful to health when it pollutes water.
The brick-colored precipitate of silver chromate marks the end point of chloride titration by the Mohr method. Source: Anhella As it is a volumetric method, the concentration of Cl - ions is determined through titrations or titrations. In these, the end point, indicative that the equivalence point has been reached. It is not a color change as we see in an acid-base indicator; but the formation of a reddish precipitate of Ag 2 CrO 4 (upper image).
When this reddish or brick color appears, the titration is finished and, after a series of calculations, the concentration of the chlorides present in the water sample is determined.
Fundamentals
Silver chloride, AgCl, is a milky precipitate that is formed as soon as the Ag + and Cl - ions are in solution. With this in mind, it might be thought that by adding enough silver from a soluble salt, for example silver nitrate, AgNO 3, to a sample with chlorides, we can precipitate all of them as AgCl.
By then weighing this AgCl, the mass of the chlorides present in the aqueous sample is determined. This would correspond to a gravimetric and not a volumetric method. However, there is a problem: AgCl is a fairly unstable and impure solid, as it decomposes under sunlight, and it also precipitates quickly, absorbing all the impurities that surround it.
Therefore, AgCl is not a solid from which reliable results can be obtained. This is probably the reason why the ingenuity of developing a volumetric method to determine Cl - ions arose, without the need to weigh any product.
Thus, Mohr's method offers an alternative: to obtain a silver chromate precipitate, Ag 2 CrO 4, which serves as the end point of a titration or titration of chlorides. Such has been its success that it is still used in the analysis of chlorides in water samples.
Reactions
What reactions take place in Mohr's method? To begin with, we have Cl - ions dissolved in water, where adding Ag + ions initiates a very displaced solubility equilibrium to the formation of the AgCl precipitate:
Ag + (aq) + Cl - (aq) ⇋ AgCl (s)
On the other hand, in the medium there must also be chromate ions, CrO 4 2-, since without them the reddish precipitate of Ag 2 CrO 4 would not form:
2Ag + (aq) + CrO 4 2- (aq) ⇋ Ag 2 CrO 4 (s)
So, in theory, there should be a conflict between both precipitates, AgCl and Ag 2 CrO 4 (white vs. red, respectively). However, in water at 25ºC, AgCl is more insoluble than Ag 2 CrO 4, so the former will always precipitate before the latter.
In fact, Ag 2 CrO 4 will not precipitate until there are no chlorides with which to form salts; that is, the minimum excess of Ag + ions will no longer precipitate with the Cl - but with the CrO 4 2-. We will therefore see the appearance of the reddish precipitate, this being the final point of the assessment.
Process
Reagents and conditions
The titrant must go into the burette, which in this case is a 0.01 M AgNO 3 solution. Since AgNO 3 is sensitive to light, it is recommended to cover the burette with aluminum foil once it has been filled. And as an indicator, a 5% K 2 CrO 4 solution.
This concentration of K 2 CrO 4 guarantees that there is not a considerable excess of CrO 4 2- with respect to Cl -; If it occurs, Ag 2 CrO 4 will precipitate first instead of AgCl, even though the latter is more insoluble.
On the other hand, the pH of the water sample must have a value between 7 and 10. If the pH is greater than 10, the silver hydroxide will precipitate:
Ag + (aq) + OH - (aq) ⇋ AgOH (s)
While if the pH is less than 7, the Ag 2 CrO 4 will become more soluble, being necessary to add an excess of AgNO 3 to obtain the precipitate, which alters the result. This is due to the balance between the species CrO 4 2- and Cr 2 O 7 2-:
2H + (aq) + 2CrO 4 2- (aq) ⇋ 2HCrO 4 - (aq) ⇋ Cr 2 O 7 2- (aq) + H 2 O (l)
That is why the pH of the water sample must be measured before the Mohr method is performed.
Assessment
The AgNO 3 titrant must be standardized prior to titration, using a NaCl solution.
Once this is done, 15 mL of the water sample is transferred into an Erlenmeyer flask, diluted with 50 mL of water. This helps that when the 5 drops of K 2 CrO 4 indicator are added, the yellow color of the chromate is not so intense and does not prevent the end point from being detected.
The titration is started by opening the buret tap and dropping the AgNO 3 solution. It will be seen that the liquid in the flask will turn cloudy yellowish, a product of the precipitated AgCl. Once the reddish color is appreciated, stop the titration, shake the flask, and wait about 15 seconds.
If the Ag 2 CrO 4 precipitate redissolves, add other drops of AgNO 3. When it remains constant and unchanged, the titration is concluded and the volume dislodged from the burette is noted. From these volumes, dilution factors and stoichiometry, the concentration of the chlorides in the water sample is determined.
Applications
Mohr's method applies to any type of aqueous sample. It not only allows to determine chlorides, but also bromides, Br -, and cyanides, CN -. Therefore, it is one of the recurring methods to assess the quality of water, either for consumption or for industrial processes.
The problem with this method lies in the use of K 2 CrO 4, a salt that is highly toxic due to chromate, and therefore has a negative impact on waters and soils.
That is why we have sought how to modify the method to dispense with this indicator. One option is to replace it with NaHPO 4 and phenolphthalein, where the AgHPO 4 salt is formed by changing the pH enough that a reliable end point is obtained.
References
- Day, R., & Underwood, A. (1965). Quantitative Analytical Chemistry. (fifth ed.). PEARSON Prentice Hall, p 277.
- Angeles Mendez. (February 22, 2012). Mohr's method. Recovered from: quimica.laguia2000.com
- ChemBuddy. (2009). Mohr Method. Recovered from: titrations.info
- Daniele Naviglio. (sf). Mohr Method. Federica Web Learning. Recovered from: federica.unina.it
- Hong, TK, Kim, MH, & Czae, MZ (2010). Determination of Chlorinity of Water without the Use of Chromate Indicator. International journal of analytical chemistry, 2010, 602939. doi: 10.1155 / 2010/602939