sodium metaborate in PMK - octa or tetra?

[splanken]

Right - finally venturing out into Pyro with the purchase of Gordon's Pyro Bible. I am mixing the stock B and find myself unable to determine if the sodium metaborate is the tetrahydrate or octahydrate. Both are commonly sold...the FW of the tetrahydrate is 137 and octahydrate is 209 (roughly...). His recipe calls for 300g...quite a lot to dissolve in 1L of water...
I have tatrahydrate...if he used octahydrate I only need to use 197 of tetrahydrate to get an equimolar amount....
I am sure someone has been through this before. Ant takers?

[BradS]

I think this is Gainer's favorite subject...waiting....

[Kirk Keyes]

MSDS for Kodalk (http://www2.itap.purdue.edu/msds/docs/9737.pdf)

See section 2.

[Shen45]

Right - finally venturing out into Pyro with the purchase of Gordon's Pyro Bible. I am mixing the stock B and find myself unable to determine if the sodium metaborate is the tetrahydrate or octahydrate. Both are commonly sold...the FW of the tetrahydrate is 137 and octahydrate is 209 (roughly...). His recipe calls for 300g...quite a lot to dissolve in 1L of water...
I have tatrahydrate...if he used octahydrate I only need to use 197 of tetrahydrate to get an equimolar amount....
I am sure someone has been through this before. Ant takers?

I checked in my darkroom and mine is Tetra. 300gms to a litre

[splanken]

OK - so Kodalk MSDS says octahydrate, so some folks use that. PF sells tetrahydrate and most folks seem to be using tetra. Looks like some confusion all round...
Does it really matter though??

[Kirk Keyes]

Does it really matter though??

A little. It affects pH and buffering capacity of the developer.

However, if you are doing tests to determine your development times, you most likely will have made adjustments to your times to counteract any change in developer activity due to the difference in pH between the two forms of metaborate.

[Sevo]

In solution, they are exactly the same. Octahydrate contains less of the anhydrous salt and more water, though, so the recipe has to be recalculated to result in the same concentration. And there may be quite strong differences regarding solubility.