Anhydrous vs hydrous dr chemicals, what is the difference?

Hi,
I am chemistry illiterate so I dont know the difference between these 2 in powder form. My simple brain says, if it is dry powder it should be anhydrous as there is no water molecule but I am certain I am wrong.

The reason I am asking is, sometimes it is hard to find anhydrous version of the same compound such as Sodium Metabilsulphite. Normal version is commonly available though.

BW,
Fatih

Sodium metabisulfite only exists in anhydrous form. Not all substances have different (stable) hydration forms; popular examples include sodium carbonate. But sodium metabisulfite is always anhydrous. If it contains any water, it'll cake very badly and/or turn into a sludge if there's a lot of water involved.

You can safely assume the sodium metabisulfite you buy from any reputable vendor will be anhydrous and water-free.

Fatih Ayoglu said:

My simple brain says, if it is dry powder it should be anhydrous as there is no water molecule but I am certain I am wrong.

Click to expand...

Not necessarily; e.g. potassium carbonate monohydrate often comes in the form of pellets, but can easily be milled into a powder. So the appearance as such is not always a good indicator of the presence of encapsulated water.

I don't know what would I do without this forum and helpful people like you @koraks thank you so much.

You have cleared another mystery in my brain!

If only all mysteries would resolve so easily! You're very welcome.

Fatih Ayoglu said:

Hi,
I am chemistry illiterate so I dont know the difference between these 2 in powder form. My simple brain says, if it is dry powder it should be anhydrous as there is no water molecule but I am certain I am wrong.

The reason I am asking is, sometimes it is hard to find anhydrous version of the same compound such as Sodium Metabilsulphite. Normal version is commonly available though.

BW,
Fatih

Click to expand...

Some (not all) chemicals used in photography can be anhydrous, which means it has no water molecules bound in the compound, or it can be hydrous, meaning that it has one or more water molecules bound. Both forms will appear "dry", so you cannot visually sort one from the other. The photochemical significance of the difference is that when water is bound in the molecule, the molecule weighs more than when anhydrous. So, when mixing a working formula, say a developer, you have to know which type of chemical you are using, so you weigh out the required amount. There are probably dozens of handbooks (usually older ones) which have conversion tables, allowing you to use what you have on hand when the formula calls for another form of the chemical. The one commonly at hand these days is The Darkroom Cookbook (any edition).