Sodium Sulfite, different?

[JPD]

The Sodium sulfite I have used for the last ten years is a very fine powder that easily dissolves in water (MERCK, and a very pure grade). I needed more but couldn't get the one from MERCK, so I bought a few kilograms from german eBay. The purity grade this time is "Reinst" ("purest"). This powder is coarser and cakes at the bottom of the beaker. It takes much longer to get into solution.

I was suspicious and compared the pH of the old and new sulfite (1 teaspoon in 120 ml water), and it was the same, ~8,5. So it probably is Sodium sulfite. Is the differences between the powders normal? Or do I have the heptahydrate? The seller said it was anhydrous.

 

[Vlad Soare]

This is the normal behaviour of anhydrous sodium sulfite, especially in cold water.
My guess is that the old sulfite was hydrated, at least partially. Crystalline forms tend to dissolve easier and to clump less than anhydrous ones.

 

[JPD]

This is the normal behaviour of anhydrous sodium sulfite, especially in cold water.
My guess is that the old sulfite was hydrated, at least partially. Crystalline forms tend to dissolve easier and to clump less than anhydrous ones.

I dissolve it in 20ºC water when I mix up "HCA" before use.

The older sulfite is anhydrous, but a much finer powder. Could that be the reason for it to dissolve more quickly?

 

[Vlad Soare]

At 20 degrees Centigrade anhydrous sodium sulfite has a tendency to form some clumps at the bottom of the beaker, which are quite hard and difficult to break with the glass rod, though after a few minutes of stirring they eventually dissolve.
It helps if you add the sulfite to the water little by little while stirring vigorously. The trick is to keep all of the sulfite moving all the time, not to allow any crystal to "rest", not even for one second.

This is normal behaviour.

Could it be that the older sulfite was initially anhydrous, but then gathered some water during the ten years of use?
Hydrated forms tend to dissolve easier. Those water molecules attached to the crystals seem to help with the hydrolysis, though I don't know the exact mechanism.

 

[Nicholas Lindan]

I think the mechanism is the anyhdrous form promptly forms large clumps of super-hydrated crystals when it hits the water with granules in contact during crystalization/hydration. If poured slowly with stirring the Sulfite granules crystalize solo and thereafter don't clump.

 

[JPD]

Thanks. I will mix a developer with the new sulfite and try it on a test roll some day.

At 20 degrees Centigrade anhydrous sodium sulfite has a tendency to form some clumps at the bottom of the beaker, which are quite hard and difficult to break with the glass rod, though after a few minutes of stirring they eventually dissolve.

I use a plexi rod, so the clumps will break before the rod does.

Could it be that the older sulfite was initially anhydrous, but then gathered some water during the ten years of use?

No, the old sulfite was easy to dissolve already when I bought it fresh from MERCK in a sealed 5 kg container.

I'm leaning to the explanation that finer powder dissolves faster than coarser.