Porous Plate

Quick Answer: A Porous Plate is a flat plate of unglazed, porous ceramic (usually porcelain biscuit) used to dry freshly prepared crystals: the damp solid is spread and pressed on the surface, and the fine pores draw the mother liquor out by capillary action. A porous plate for drying crystals works without heat, so it suits compounds that decompose or lose water of crystallisation when warmed.

Drying Crystals by Capillary Action

After crystals are filtered off, whether from a school preparation of copper(II) sulfate or an organic recrystallisation, they are still wet with the mother liquor, which carries dissolved impurities. Warming them in an oven may melt a low-melting product, drive off water of crystallisation or cause decomposition. Pressing the crystals onto a porous plate is the traditional gentle alternative: the network of fine pores in unglazed ceramic soaks up the liquid, leaving the crystals behind on the surface.

The technique is simple. Transfer the filtered solid to the plate with a spatula, spread it thinly and press it down with a clean spatula blade or a piece of filter paper so that liquid is squeezed into the plate. After a few minutes the crystals can be scraped into a weighed container or a desiccator for final drying. Because the liquid moves into the plate, the plate itself takes up some of the impurities and must be cleaned or kept for one type of compound.

A porous plate is not the same as a desiccator plate, which is a perforated disc that holds dishes above the drying agent inside a glass desiccator. The size and thickness of this plate are not stated in the listing.

Specifications

Laboratory Uses

• Drying recrystallised organic products, such as acetanilide or aspirin, before melting-point determination.
• Pressing out the mother liquor from salt preparations such as copper(II) sulfate or potash alum in school practicals.
• Removing surface liquid from crystals whose water of crystallisation would be lost on heating.
• Blotting small quantities of wet precipitate before transfer to a desiccator.

Care & Handling

• Keep separate plates for coloured, toxic or strongly smelling compounds, since residues soak into the pores.
• To clean, scrape off solids, then soak in water or a suitable solvent and change the liquid until it comes away clean.
• Dry a washed plate thoroughly before the next use; a damp plate absorbs little.
• Unglazed ceramic is brittle, so avoid dropping it or pressing hard on an unsupported edge.

Why Choose LabEquip

Porous plates are used in school and college chemistry labs that run crystallisation practicals, and in organic teaching labs that recrystallise products before melting-point tests. LabEquip lists this plate in the General Lab Products range; ask about size and quantity on the contact page.

Frequently Asked Questions

How does a porous plate dry crystals?

The unglazed ceramic is full of fine, connected pores. When damp crystals are pressed onto it, the liquid is drawn into the pores by capillary action, leaving the solid on the surface. No heat is needed.

What is the difference between a porous plate and a desiccator plate?

A porous plate absorbs liquid directly from crystals pressed onto it. A desiccator plate is a perforated disc that supports dishes inside a desiccator, where a drying agent slowly removes moisture from the air around the sample.

Can the plate be reused for different compounds?

It can, but residues soak into the pores, so clean it thoroughly between uses. For coloured, toxic or strongly smelling compounds it is safer to keep a dedicated plate.

Why not simply dry the crystals in an oven?

Some products melt at low temperature, lose water of crystallisation or decompose on heating. A porous plate removes the mother liquor at room temperature, so crystals such as blue hydrated copper(II) sulfate keep their water of crystallisation.

Is filter paper an alternative?

Pressing crystals between filter papers works on a small scale, but paper fibres can stick to the product and the paper soon becomes saturated. A porous plate takes up more liquid and leaves a cleaner surface to scrape from.

How do I clean a used porous plate?

Scrape off any solid, then soak the plate in water or a suitable solvent, changing the liquid several times. Let it dry completely before reuse; a plate that is already damp cannot absorb much more liquid.

Last Updated: September 2026