Coagulation of Colloids – Hardy Schulze Rule, Flocculation Value, Protective Colloids and Gold Number

Have you noticed that whenever your father cuts himself while shaving, he applies alum?

After some time of alum application, the bleeding stops. What causes alum to cease bleeding?

Alum is a white crystal. Its chemical formula is $K_2\left(S{O}_4\right).A{l}_2(S{O}_4{)}_3.24H_{2}O$ Blood is a colloid. Alum is used as a blood coagulant and a disinfectant. Plasma proteins cluster together as a result of the positively charged ions in alum neutralising the negatively charged ions on plasma proteins. This process is called coagulation. Alum causes precipitation as a disinfectant by neutralising the ions on proteins in bacteria.

On this idea page, let's learn more about the coagulation of colloids.

TABLE OF CONTENTS

• Coagulation or Precipitation
• Coagulation of Lyophobic Sols
• Hardy-Schulze Rule
• Flocculating Value
• Coagulation of Lyophilic Sols
• Protective Colloids
• Gold Number
• Practice Problems
• Frequently Asked Questions – FAQ

Coagulation or Precipitation

Coagulation is the process of aggregating colloidal particles into an insoluble precipitate by adding an appropriate electrolyte.

The presence of tiny amounts of suitable electrolytes is required for colloidal stability. When a higher concentration of an electrolyte is applied, the particles of the sol take up the oppositely charged ions and neutralise them. The neutral particles subsequently begin to aggregate, resulting in bigger particles that are precipitated. The aggregation of particles at lower concentrations of electrolytes is called flocculation, which may be reversed by shaking, but coagulation occurs at greater concentrations of electrolytes and cannot be reversed by shaking. Colloidal particles contain charges and this contributes to the stability of lyophobic colloids. If the charge is eliminated in any way, the particles will cluster together to form aggregates and settle down under gravity's influence.

Coagulation of Lyophobic Sols

1. By electrophoresis:

The charged colloidal particles migrate towards the oppositely charged electrodes, discharge and precipitate.

2. Coagulation by mixing sols that are oppositely charged: When sols that are oppositely charged are mixed in appropriate amounts, they precipitate by neutralising the charges completely or partially. This type of coagulation is known as mutual coagulation.

Example: Mixing hydrated ferric oxide (positive sol) and arsenious sulphide (negative sol) precipitates them.

3. Coagulation by boiling: Boiling a sol causes the dispersion medium molecules to collide more, which disturbs the adsorbed layer. Increased collisions result in a reduction in particle charge, which causes particles to settle as a precipitate.

4. Coagulation by prolonged dialysis: If the electrolyte present in the sol is fully eliminated during continuous dialysis, the colloid becomes unstable and eventually coagulates.

5. Coagulation by the addition of electrolytes: When an excess of an electrolyte is added to the colloidal solution, the colloidal particles get precipitated. Colloid particles neutralise by interacting with oppositely charged ions, known as coagulating (flocculating) ions which leads to their coagulation. A positive ion causes the precipitation of negatively charged sol and vice versa.

Hardy-Schulze Rule

The following findings were drawn from experimental observations of the coagulation of various sols with various electrolytes.

1. Coagulation can be caused by ions whose charge is the opposite of that of the colloidal particles.

2. The greater the valency of the flocculating ion, the lesser is the amount of electrolyte required to coagulate a given amount of sol at a given time.

The above generalisations are known as the Hardy-Schulze rule.

As the valency of the added flocculating ion increases, its coagulation power increases. This power of flocculating ions to coagulate the colloidal sol is known as the coagulating (or flocculating) power.

For positively charged sols, the order of coagulating power of anions (coagulating ions) is

[Fe(CN)6]4-> PO43- > SO42- > Cl-

For negatively charged sols, the order of coagulating power of cations (coagulating ions) is

Al3+ > Ba2+ > Na+

Flocculating Value

Coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has a certain minimum concentration in the solution. The flocculating value is the lowest amount of an electrolyte, measured in millimoles, needed to cause coagulation in one litre of colloidal solution.

$Flocculating Value = \frac{Millimoles of electrolyte}{Volume of sol in litre}$

SI. unit is millimoles litre-1

Coagulating power ∝ 1Coagulating value

Thus, coagulating value decreases with increase in charge on the coagulating ion.

Coagulation of Lyophilic Sols

Coagulation of Lyophilic Sols can be done by following two methods:

1. By adding an electrolyte: When an excess electrolyte is added to the colloidal sol, the colloidal particles are precipitated.

2. By adding suitable solvent.

Example: Alcohol and acetone are added to hydrophilic sol. Dehydration of the dispersion phase occurs when alcohol and acetone are added to the hydrophilic sol.

Protective colloids

Since lyophobic sols are less stable than lyophilic sols, lyophilic sols have a unique property to

protect lyophobic sols. Lyophilic sols used for this purpose are known as protective colloids. When lyophilic sol is added to lyophobic sol, lyophobic particles are surrounded by a layer of hydrophilic particles. Lyophilic particles protect lyophobic sol from electrolytes. In other words, on mixing both types of colloids, lyophilic colloid particles surround lyophobic colloid particles and thus lyophobic colloids are protected by the lyophilic sols.

Example: Gelatin (lyophilic) protects gold sol (lyophobic).

Gold Number

The minimum weight in milligrams of a protective colloid required to prevent the coagulation of 10 mL of a standard gold sol when 1 mL of a 10% NaCl solution is added to the sol is called the Gold number of the lyophilic sol.

The smaller the gold number, the greater will be protecting power of the protective colloid.

Protecting power of protective colloid ∝ 1Gold number

$Gold number = \frac{weight of lyophilic sol in mg \times 10}{volume of gold sol in mL}$

The gold numbers of some protective colloids are given as follows:

Practice problems

1. Protective colloids A, B, C and D have the gold numbers 0.40, 0.01, 0.10 and 0.0045 respectively. The correct increasing order of their protective powers is:

1. B < D < A < C
2. A < D < B < C
3. C < D < A < B
4. A < C < B < D

Answer: D

Solution: Higher is the value of gold number, lower is the protective power. Thus, the correct order of their protective powers is A (0.40)<C (0.10)<B (0.01)<D (0.0045).

So, option D is the correct answer.

2. Which of the following has the minimum gold number?

1. Gelatin
2. Haemoglobin
3. Potato starch
4. Egg albumin

Answer: A

Solution: The gold numbers of some protective colloids are given as follows:

Therefore, gelatin has the minimum gold number among the given options.

So, option A is the correct answer.

3. Arsenic trisulphide (As2S3) colloidal solution contains negatively charged particles. Which 0.0005 M solution would coagulate this colloidal solution the best?

1. $KCl, MgC{l}_2, AlC{l}_3 or N{a}_{3}P{O}_4$
2. $KCl, MgC{l}_2$
3. $AlC{l}_3, N{a}_{3}P{O}_4$
4. $AlC{l}_3$

Answer: D

Solution: Since As₂S₃ is a colloidal sol that is negatively charged, positively charged ions will lead to its coagulation. By the Hardy-Schulze rule, as the charge on the ion increases, its coagulating power of oppositely charged particles increases. Hence, out of $K^{+}, M{g}^{2+}, A{l}^{3+} and N{a}^{+}, A{l}^{3+}$ would be the most effective.

So, option D is the correct answer.

4. By which of the following processes coagulation of colloidal sol can be done?

1. Electrophoresis
2. By adding electrolytes
3. By boiling
4. All of the above

Answer: D

Solution: Coagulation of colloidal sol can be brought by various processes like electrophoresis, by adding electrolytes, by boiling, etc.

So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. Why are lyophilic sols more stable than lyophobic sols?
Answer: The existence of a charge and the solvation of colloidal particles both contribute to the stability of lyophilic sols. Lyophobic sols, on the other hand, are only stable because of the presence of a charge. As a result of the extensive solvation, the lyophilic sol is more stable than the lyophobic sol.

2. Is blood a colloid?
Answer: Blood is a colloid that has a dispersing phase that is liquid and a dispersed phase that is solid. Blood cells are scattered as solids in liquid plasma proteins, which causes colloids to form in the blood. Blood viscosity is affected by colloids. To put it another way, blood is a colloid known as a sol, which is a distributed solid in a liquid.

3. What are the uses of the protective action of colloids?
Answer: The following are a few uses of the protective action of colloids.

1. When making ice cream, gelatin is used to protect the ice particles.
2. Protargol and Argyrol are silver sols that are protected by organic material and utilised as eye drops.

4. Why is the pH of the solution adjusted before adding the coagulant?
Answer: Controlling the pH level would considerably improve the coagulation process because pH values have an impact on the charges on the surface and impurities to be eliminated. To maximise the elimination of pollutants contained in raw water, it is therefore important to optimise not only the dosage of the coagulant but also the pH value.

Related Topics