Positive and Negative Sols
The Origin of Charge, Preferential Adsorption, and the Excess Reagent Trap.
One of the most fundamental properties of a colloidal solution is that all dispersed particles in a given sol carry the exact same electrical charge (either all positive or all negative). This uniform charge causes the particles to continuously repel each other, preventing them from colliding, aggregating, and settling under gravity. In short, charge is the reason colloids are stable.
1. Origin of Charge on Colloidal Particles
Where does this electrical charge come from? While there are minor reasons (like electron capture by particles during Bredig's Arc method), the most widely accepted and universally tested mechanism is the Preferential Adsorption of Ions.
The Rule of Preferential Adsorption:
The Golden Rule: The particle will preferentially adsorb the ion that is COMMON to its own crystal lattice and is present in EXCESS in the solution.
2. The Classic Trap: Silver Iodide ($AgI$) Sols
Depending entirely on the method of preparation (which reagent is in excess), the exact same precipitate of Silver Iodide ($AgI$) can form either a positively charged sol or a negatively charged sol. This specific example is the undisputed favorite of JEE and NEET examiners.
Figure 1: Preferential adsorption of the common ion present in excess determines the sol charge.
Case 1: Forming a Negative Sol
When a highly dilute solution of Silver Nitrate ($AgNO_3$) is added to an excess of Potassium Iodide ($KI$) solution.
- Precipitate formed: $AgI$.
- Because $KI$ is in excess, there is an abundance of $K^+$ and $I^-$ ions in the medium.
- The $AgI$ particle looks for an ion common to its own lattice. It finds $I^-$.
- The particle selectively adsorbs $I^-$ ions onto its surface, acquiring a negative charge: $AgI / I^-$.
Case 2: Forming a Positive Sol
When a highly dilute solution of Potassium Iodide ($KI$) is added to an excess of Silver Nitrate ($AgNO_3$) solution.
- Precipitate formed: $AgI$.
- Because $AgNO_3$ is in excess, there is an abundance of $Ag^+$ and $NO_3^-$ ions in the medium.
- The $AgI$ particle looks for an ion common to its own lattice. It finds $Ag^+$.
- The particle selectively adsorbs $Ag^+$ ions onto its surface, acquiring a positive charge: $AgI / Ag^+$.
(Another common example is adding $FeCl_3$ to hot water vs. adding $FeCl_3$ to $NaOH$. Both form $Fe(OH)_3$ or hydrated ferric oxide. If $Fe^{3+}$ is in excess, it forms a positive sol ($Fe(OH)_3/Fe^{3+}$). If $OH^-$ is in excess, it forms a negative sol ($Fe(OH)_3/OH^-$)).
3. The Master Classification Cheat Sheet
Examiners will frequently give you a list of colloidal sols and ask you to identify which are positive and which are negative. You must memorize this standard list.
| Positively Charged Sols (+) | Negatively Charged Sols (-) |
|---|---|
| 1. Hydrated Metallic Oxides & Hydroxides: | 1. Pure Metals: |
|
• Ferric hydroxide: $Fe(OH)_3$ • Aluminum hydroxide: $Al(OH)_3$ • Chromium hydroxide: $Cr(OH)_3$ |
• Copper ($Cu$) • Silver ($Ag$) • Gold ($Au$) / Platinum ($Pt$) |
| 2. Basic Dyes (and their stuffs): | 2. Metallic Sulfides: |
|
• Methylene blue sol • Bismarck brown |
• Arsenious sulfide: $As_2S_3$ • Antimony sulfide: $Sb_2S_3$ • Cadmium sulfide: $CdS$ |
| 3. Specific Oxides: | 3. Acidic Dyes: |
| • Titanium dioxide: $TiO_2$ |
• Eosin • Congo red |
| 4. Specific Biological Fluids: | 4. Naturally Occurring Organics: |
|
• Hemoglobin (Blood) (Note: While Hemoglobin itself is positive, whole blood plasma acts as a negative sol when interacting with Alum). |
• Starch, Gum, Gelatin • Clay, Charcoal, Dirt |
4. A Note on the Isoelectric Point
For certain complex macromolecular colloids, specifically Proteins and Amino Acids, the charge on the particle is entirely dependent on the pH of the dispersion medium.
- In highly acidic medium (low pH, excess $H^+$): The amino groups ($-NH_2$) accept protons to become $-NH_3^+$. The sol becomes Positively Charged.
- In highly basic medium (high pH, excess $OH^-$): The carboxyl groups ($-COOH$) lose protons to become $-COO^-$. The sol becomes Negatively Charged.
Mastery Check: Charge on Sols
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