Specifying Si in SOLUTION - Si vs SiO2

[Tom]

My understanding is that specifying Si in the SOLUTION data block defaults to Si as SiO2, since the following solutions give the same output (same molalities for Si and SiO2 in each solution).

SOLUTION 1
units ppm
Si 10

SOLUTION 2
units ppm
Si 10 as SiO2

Assuming the Si source was not SiO2, what are the options for specifying Si, but not as SiO2? I appreciate that free Si in solution is not encountered and the species is more akin to silicic acid.

With SOLUTION 3....

SOLUTION 3
units ppm
Si 10 as Si

I find:

Molality Si 3.561e-4

Which is equivalent to 0.01 g Si (molecular mass 28.08 g/mol) in 1 litre, or 10 ppm, suggesting this may be the correct approach on a first glance...

Are there any other approaches users know of? Thanks!

[dlparkhurst]

The gfw that you want to use depends solely on the way the data are reported. At least in the US, it is common to report Si as SiO2. If the lab reports Si as Si, then the second approach you outline is correct. Alternatively, you can change the gfw in SOLUTION_MASTER_SPECIES in the database file. I tend to want to keep the standard database files intact, so I would make a personal database file, or do it the input file as you have done.

Note that it really has nothing to do with the source of Si in solution, or the predominant species for Si in solution, just the reporting method from the lab.

[Tom]

Thanks for that clarification David, and the useful tip regarding modification of databases for this purpose.