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Beginners => PHREEQC basics => Topic started by: liamabrahamsen on January 14, 2019, 10:42:48 AM

Title: Defining a solution via a number of dissolved salts (in mass units)
Post by: liamabrahamsen on January 14, 2019, 10:42:48 AM
Hello,

I am trying to define a solution via a number of dissolved salts in units of g/L (or ppt). I have tried two methods, neither of which seem ideal.
Firstly I tried adding each element to a solution, using the 'as' function, but I found that I have to correct for stoichiometry and that I cannot have more than one entry for each element (the previous one gets overridden), as follows:

APPROACH 1:

SOLUTION 1 # Modified Zhilina’s medium, 1g/L NH4Cl, 0.2 g/L K2HPO4, 0.2 g/L KCl
units ppt # g/L
N(5)   1 as NH4Cl
Cl    1 as NH4Cl
K    0.4 as K2HPO4 # must correct for stoichiometry
P   0.2 as K2HPO4
K   0.2 as KCl

The benefit of approach 1 is that PHREEQC calculates the gfw for each salt, eliminating any errors in me calculating it! The downside is I cannot have more than one entry for each element, like for potassium above, the first entry is ignored.

The second approach is to use REACTION and add each salt to my pure solution. The downside here is that reactants must be added in units of moles, not mass. So I need to calculate the gfw, and number of moles, manually.

APPROACH 2:

SOLUTION 1 # Modified Zhilina’s medium, 1g/L NH4Cl, 0.2 g/L K2HPO4, 0.2 g/L KCl
REACTION 1
NH4Cl   1.87E-02
K2HPO4   1.15E-03
KCl   2.68E-03
END

Is this second approach the best, or is there a better way? Ideally, I could use the second approach but get PHREEQC to calculate the number of moles from my masses of salts.

Many thanks,
Liam.
Title: Re: Defining a solution via a number of dissolved salts (in mass units)
Post by: dlparkhurst on January 14, 2019, 05:05:30 PM
First, you are adding NO3 instead of NH4 with the first approach.

I think to be general in the first approach you will have to convert to moles (or grams) of the individual analytes, like Cl, NH4 (N(-3) or Amm, depending on the database), etc.

Similarly in the second approach, I think you will need to calculate the number of moles of the individual salts.

I think the second approach is a bit easier, although you cannot set the pH of your solution directly, if that is important.

You can use PHREEQC to calculate some of conversions.

Code: [Select]
solution
USER_PRINT
10 PRINT "NH4Cl, g/mol   ", GFW("NH4Cl")
20 PRINT "NH4Cl, mol/g   ", 1/GFW("NH4Cl")

Title: Re: Defining a solution via a number of dissolved salts (in mass units)
Post by: liamabrahamsen on January 15, 2019, 02:06:40 PM
Thanks David!