The Mohr's method It is a variant of argentometry, which in turn is one of the many areas of volumetry used to determine 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, KtwoCrO4, salt that is harmful to health when it pollutes water.
When this reddish or brick color appears, the titration is concluded and, after a series of calculations, the concentration of the chlorides present in the water sample is determined..
Article index
Silver chloride, AgCl, is a milky precipitate that only forms Ag ions+ and Cl- they are in solution. With this in mind, it might be thought that adding enough silver from a soluble salt, for example silver nitrate, AgNO3, to a sample with chlorides, we can precipitate them all 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 has probably been the reason for the ingenuity of developing a volumetric method to determine Cl ions.-, without the need to weigh any product.
Thus, Mohr's method offers an alternative: to obtain a precipitate of silver chromate, AgtwoCrO4, which serves as the end point of an assessment or titration of chlorides. Such has been its success that it is still used in the analysis of chlorides in water samples.
What reactions take place in Mohr's method? To start with, we have Cl ions- dissolved in water, where when Ag ions are added+ a very displaced solubility equilibrium begins with the formation of the AgCl precipitate:
Ag+(ac) + Cl-(ac) ⇋ AgCl (s)
On the other hand, in the middle there must also be chromate ions, CrO4two-, since without them the reddish precipitate of AgtwoCrO4:
2Ag+(ac) + CrO4two-(ac) ⇋ AgtwoCrO4(s)
So, in theory, there should be a conflict between both precipitates, AgCl and AgtwoCrO4 (white vs. red, respectively). However, in water at 25 ºC, AgCl is more insoluble than AgtwoCrO4, so the first will always rush before the second.
In fact, the AgtwoCrO4 it will not precipitate until there are no chlorides with which the silver will form salts; that is, the minimum excess Ag ions+ no longer precipitate with Cl- but with the CrO4two-. We will therefore see the appearance of the reddish precipitate, this being the final point of the valuation.
The titrant must go in the burette, which in this case is an AgNO solution3 0.01 M. Because AgNO3 It is sensitive to light, it is recommended to cover the burette with aluminum foil once it has been filled. And as an indicator, a solution of KtwoCrO4 at 5%.
This concentration of KtwoCrO4 ensures that there is not a considerable excess of CrO4two- with respect to Cl-; because if it happens, the Ag will precipitate firsttwoCrO4 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+(ac) + OH-(ac) ⇋ AgOH (s)
While if the pH is less than 7, the AgtwoCrO4 it will become more soluble, being necessary to add an excess of AgNO3 to obtain the precipitate, which alters the result. This is due to the balance between the CrO species4two- and CRtwoOR7two-:
2H+(ac) + 2CrO4two-(ac) ⇋ 2HCrO4-(ac) ⇋ CrtwoOR7two-(ac) + HtwoO (l)
That is why the pH of the water sample must be measured before the Mohr method is performed..
The AgNO titrant3 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 indicator are addedtwoCrO4, 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 solution.3. 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 precipitate of AgtwoCrO4 redissolves, other drops of AgNO will have to be added3. 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 chlorides in the water sample is determined..
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 evaluate the quality of water, either for consumption, or for industrial processes..
The problem with this method lies in the use of the KtwoCrO4, salt that is highly toxic due to chromate, and therefore negatively impacts 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 NaHPO4 and phenolphthalein, where the AgHPO salt is formed4 changing the pH enough so that a reliable end point is obtained.
Yet No Comments