Water quality — turbidity standard reference: The historical “1 turbidity unit” (Jackson scale) was defined as the turbidity produced in 1 litre of distilled water by 1 mg of finely divided which reference substance?

Introduction / Context:
Turbidity is an optical property related to light scattering by suspended particles. Historically, the Jackson Turbidity Unit (JTU) used a silica standard. Modern instruments report Nephelometric Turbidity Units (NTU), but many exam questions still reference the classical definition for clarity on standards and calibration.

Given Data / Assumptions:

• Reference is to the original Jackson candle turbidimeter standard.
• Standard suspension prepared by adding a known mass of a fine material to distilled water.
• Volume basis: 1 litre.

Concept / Approach:
The original JTU standard was set as the turbidity caused by 1 mg of finely divided silica (SiO2) per litre of distilled water. Although JTUs have largely been replaced by NTUs, silica remains the classical reference material in the historical definition. Understanding this helps when converting or interpreting older data and literature.

Step-by-Step Solution:

Verification / Alternative check:
Modern nephelometers use formazin standards (FTU/NTU), but the specific question explicitly references the Jackson-era definition tied to silica.

Why Other Options Are Wrong:

• Mud/clay/organic matter: poorly defined composition and particle size; unsuitable as universal standards.
• Calcium carbonate: used in some hardness/titration contexts, not for turbidity standardization.

Common Pitfalls:
Confusing NTU (formazin) with JTU (silica); assuming “clay” stands for a standard when it is not uniform.

Final Answer:
Silica (SiO2)