What is Precipitation?

Sea salt is composed of various kinds of salts. On evaporation of sea water, the dissolved salts are left behind. More interestingly, if seawater is evaporated from a surface that has a slight curve (like a watch glass), the different salts appear as rings at different distances (Fig 1). As seawater evaporates, it starts to become saturated and salts become insoluble at different saturation levels due to their different solubility's. On becoming insoluble, the salt precipitates out from the solution in the form of a solid. Calcium carbonate being the least soluble (outermost layer, Fig 1) precipitates first. A solid that forms out of a saturated solution is called precipitate and this process is called precipitation.  

Precipitation Titration

The process of precipitation is used in a titration technique, called precipitation titration. Titration is the determination of concentration of an unknown solution using a solution of known concentration. Before moving forward, let’s understand some important terms associated with titrations.

Standard solution: The solution with known concentration

Titrant: The standard solution which is used to determine the concentration of unknown solution

Analyte: The solution with unknown concentration that is being determined  

Equivalence point: It is the point in titration at which the volume of titrant added is just enough to react with the given volume of analyte   

Indicator: A substance that undergoes a visible change on completion of the reaction between titrant and analyte

End point: The point in the titration at which the indicator undergoes the change marking the completion of reaction  

During titration, the titrant is added to the measured volume of analyte in small quantities until the reaction is complete (Fig 2). Completion of the reaction is indicated at the end point with the help of indicator. With stoichiometry of the reaction and volume of the standard solution required to react with the given volume of the analyte, concentration of the analyte can be determined.

A titrimetric method during which a sparingly soluble precipitate is formed is called precipitation titration. The combination of ions from titrant and titrate is such that when they find each other in a solution they react to form a sparingly soluble solid product (precipitate). After the analyte is consumed, the end point is indicated in the presence of an indicator which reacts with the excess of titrant to bring a visible change. Potentiometric titrations are usually utilized to estimate the concentration of halides and other anions like carbonate, chromate, phosphate, and arsenate in solutions.

Precipitation and Solubility Product

For discussing precipitation reactions, it is important to understand solubility and solubility product. Both calcium chloride (CaCl₂) and lithium fluoride (LiF) are ionic compounds, still their solubility in water varies a great deal. CaF₂ is so soluble in water that it is hygroscopic. LiF, on the other hand, has very little solubility in water. For a salt to be soluble in a solvent, its solvation energy (ion-solvent interactions) must overcome its lattice enthalpy (strong forces of attraction in ionic compound). For precipitation, we need to focus on sparingly soluble salts.         

When a sparingly soluble ionic compound like AgCl is added to water, it will dissolve to a small degree forming a dynamic equilibrium between the undissolved solid and dissolved ions. It can be represented as:

$AgCl \left(s\right) \rightleftharpoons A{g}^{+}\left(aq\right) + C{l}^{-}\left(aq\right)$

Like any other equilibrium, we describe the above equilibrium with an equilibrium constant.

$K=\left\{\right[A{g}^{+}\left]\right[C{l}^{-}\left]\right\}/AgCl$

Concentration for the pure solid substance remains constant, thus we have:

Or $K_{sp} = K\left[AgCl\right] = \left[A{g}^{+}\right] \left[C{l}^{-}\right]$

K_sp is called solubility product. It is the product of the concentrations of the two ions in the solution each raised to the power of their stoichiometric coefficients. Smaller the K_sp value, lesser would be the ions in the solution, hence lower would be the solubility of the compound.

Argentometric Titration

The precipitation titrations that use silver ions as the titrant are referred to as argentometric. Suppose concentration of chloride ions in a solution (analyte - unknown concentration, known volume) has to be determined. This solution can be titrated with a silver nitrate solution (titrant- known concentration) to determine its volume required to react with all of chloride ions in the solution. Silver ions (Ag⁺) and chloride ions (Cl^-) react to from sparingly soluble silver chloride (AgCl) that can be seen in the form of a white precipitate in the solution. The chemical reaction that occurs is:      

$A{g}^{+}\left(aq\right) + C{l}^{-}\left(aq\right) \to AgCl\left(s\right)$

According to the stoichiometry of the reaction, the amount of silver ions used till equivalence point is equal to the amount of chloride ions that were present in the solution originally.

There are many methods used for determination of end points of such precipitation titrations. Let’s look at them.

Mohr Method

Principle and Methodology

The method is used for estimation of chlorides and bromides. In this method, the indicator reacts with the titrant and forms a precipitate. Indicator used is potassium chromate (K₂CrO₄). Consider estimation of chloride ions with silver ions as titrant (Fig 3). After reaction of all chloride ions with silver ions to form silver chloride (AgCl), sparingly soluble red-brown colored silver chromate (Ag₂CrO₄) is formed at the end point by reaction of silver ions and chromate ions (indicator) (Fig 4). The reactions are:

Before end point:

$A{g}^{+}\left(aq\right) + C{l}^{-}\left(aq\right) \rightleftharpoons AgCl\left(s\right) \left(white\right)$

At the end point:

$2A{g}^{+}\left(aq\right) + Cr{O_4}^{2-}\left(aq\right) \rightleftharpoons A{g}_{2}Cr{O}_4\left(s\right) (brick-red)$

This method is an example of fractional precipitation. Fractional precipitation is separation of ions from the same solution based on the difference in their solubilities. Here, the two possible precipitates are silver chloride (K_sp = 1.2 x 10^-10) and silver chromate (K_sp = 1.7 x 10^-12). Though silver chloride has a lower K_sp value than silver chromate, it precipitates out first because of the initial high concentration of chloride ions.

Limitations of Mohr Method

• The solution has to be around neutral or slightly basic (pH 6.5 - 9). At low pH, the following reaction occurs:

         $2Cr{O_4}^{2-} + 2H^{-} \rightleftharpoons 2HCr{O_4}^{-} \rightleftharpoons C{r}_2{O_7}^{2-} + H_{2}O$

Concentration of chromate ions in the solution is reduced and silver chromate might not get precipitated. At high pH, silver might precipitate as silver hydroxide.  

• This method cannot be performed for iodide ions as silver iodide adsorbs chromate ions very strongly. Correct end point is not obtained in this case.  
• This titration is limited to neutral or slightly basic solutions only.

Volhard Method

Principle and Methodology

In Volhard method, silver ions are estimated by titration with a standard thiocyanate solution, like potassium thiocyanate (KSCN) in presence of nitric acid (acidic medium) using Fe (III) as an indicator. The precipitation is carried out in an acidic medium to prevent the precipitation of Fe (III) ions in Fe (OH)₃. Silver ions react with thiocyanate ions (SCN^-) to form silver thiocyanate precipitate (AgSCN).

Before end point:

$A{g}^{+}\left(aq\right) + SC{N}^{-}\left(aq\right) \rightleftharpoons AgSCN\left(s\right)$

At the end point, when all silver ions are consumed, the extra thiocyanate combines with Fe(III) ions of the indicator to form a dark red complex (Fig 5). Hence, end point is indicated.

At end point:

$F{e_3}^{+}\left(aq\right) + SC{N}^{- }\left(aq\right) \rightleftharpoons {\left[Fe\right(SCN\left)\right]}^{2+}\left(s\right) (dark red)$

Volhard method finds its application in estimation of chlorides, bromides and iodides. Say, for estimating chloride ions with silver ions as the titrate, a measured excess of silver nitrate is added to the analyte containing chloride ions (Fig 6). Silver chloride which is formed as the precipitate, is filtered. The extra silver nitrate is determined by back titration using Volhard method. The amount of silver that precipitates chloride in the analyte solution is calculated by subtracting the excess silver (determined from back titration) from initial silver concentration (known) (Fig 6).

Filtration of silver chloride is important otherwise silver chloride may react with thiocyanate as silver chloride is the more soluble salt. This leads to error in overall titration.

 $AgCl + SC{N}^{-} \rightleftharpoons AgSCN + C{l}^{-}$

In case of other halides, filtration is not required as their silver salts are less soluble than silver thiocyanate.

Limitations of Volhard Method

• The method cannot be used where the solution has to be neutral or basic.
• It is a time-consuming process.

Fajans Method

Principle and Methodology

This method is used to determine chlorides, bromides and iodides. Indicator used is dichlorofluorescein, which is an adsorption indicator. Adsorption indicators change color on getting adsorbed to a precipitate surface. Dichlorofluorescein molecules are present as anions in the solution. Consider the titration for estimating chloride ions with silver ions as the titrant. During titration, AgCl precipitates and the chloride ions adsorb on it to form a layer of negative charge (Fig 7). It is likely for a precipitate to adsorb its ions. Negatively charged dichlorofluorescein is thus repelled by this precipitate, and remains in the solution having greenish-yellow color. As the end point is passed, excess silver ions form a layer of positive charge on the precipitate surface by absorbing on it. The negatively charged dichlorofluorescein is attracted to the positively charged precipitate layer, gets adsorbed and changes color to pink (Fig 7). This change in color of the indicator indicates the end point. Color change occurs at the surface of the precipitate.

Limitations

It is limited to a few precipitation titrations only in which rapid formation of colloidal precipitates occur.

Practice Problems

Q1. In precipitation titrations ions from titrant and titrate react to form a ____________soluble ______________product.

1. completely liquid
2. completely, solid
3. sparingly, solid
4. sparingly, liquid

Answer: sparingly, solid

Q2. Which of the following statements is correct?

1. Smaller the K_spvalue,  lower would be the solubility of the compound.
2. Higher the K_spvalue,  lower would be the solubility of the compound.
3. Smaller the K_spvalue,  higher would be the solubility of the compound.
4. Ksp value has no relationship with solubility.

Answer: Smaller the K_spvalue,  lower would be the solubility of the compound.

Q3. Which is used as an indicator for the estimation of chloride ions using the Mohr method?

1. Silver chromate
2. Potassium chromate
3. Silver nitrate
4. Potassium thiocyanate

Answer: Potassium chromate

Q4. In Volhard Method, the dark red compound form at the end point is:

1. KSCN
2. AgSCN
3. [Fe(SCN)]²⁺
4. [Fe(SCN)₃]²⁺

Answer: [Fe(SCN)]²⁺

Q5. Fajan’s method is used to detect?

1. Chlorides
2. Bromides
3. Iodides
4. All of these

Answer: All of these

Context and Applications

Precipitation analysis is used to analyze drugs, and to measure the salt content in food and beverages. It is an important concept from the examination point of view many practical questions are asked on the topic. This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

Bachelors of Science in chemistry

Master of Science in chemistry