The sulphate of _________________ is used with zinc paint so as to eliminate the risk of discoloration of a lead drier.
🔬 Understanding the Role of a Drier
A drier is a chemical compound added to oil-based paints to act as a catalyst. Its job is to speed up the curing process, where the liquid oil vehicle reacts with oxygen to form a hard, solid film. Without a drier, this process could take days or even weeks.
The Discoloration Problem: Lead-based driers are very effective but have a major flaw. If they are exposed to sulfur compounds (present in industrial air pollution or as impurities in pigments), they react to form lead sulfide (PbS), which is black. This causes unsightly dark stains on the paint surface.
⚖️ Detailed Analysis of the Options
The question asks for a substitute drier that avoids this discoloration issue, especially when used with zinc-based paints.
(a) Manganese
Function: The sulphate of manganese is an excellent drier. Like lead, it effectively catalyzes the oxidation of drying oils.
Key Advantage: Crucially, manganese compounds do not react with sulfur to form black precipitates. This makes manganese sulphate a stable and reliable drier, especially for white or light-colored paints (like those with a zinc oxide base) where any discoloration would be highly visible. It provides the necessary drying power without the risk of blackening.
(b) Magnesium, (c) Potassium, and (d) Calcium
These elements, while common in chemical compounds, are not used as primary driers in paint for several reasons:
- Lack of Catalytic Activity: Their sulphates (Magnesium Sulphate, Potassium Sulphate, Calcium Sulphate/Gypsum) do not have the specific catalytic properties needed to accelerate the oxidation of drying oils.
- Different Functions: Some of these compounds, particularly calcium sulphate (gypsum), are used in paints, but as extenders or fillers to add bulk and reduce cost, not as driers.
- No Solution to the Problem: They do not solve the problem because they cannot replace the function of the lead drier in the first place.
💡 Study Tips for Paint Driers
- The Big Three Driers: The most common metallic driers in paints are based on Lead, Cobalt, and Manganese. Remember these three.
- Lead's Weakness is Sulfur: Associate Lead driers with the specific problem of turning black when exposed to sulfur.
- Manganese is the "Safe" Drier: Think of Manganese as the reliable alternative that does the same job as lead without the risk of discoloration.
- Distinguish Driers from Fillers: Don't confuse driers (catalysts) with fillers/extenders (bulk materials). Calcium and Magnesium compounds often fall into the filler category.