[something weird in Rxn logK]

Oops, I realized that I was mistaken. I had assigned 1 to the k-feldspar activity instead of albite. Problem solved. I apologize for the false alarm! 🙂

[something weird in Rxn logK]

Let me illustrate it by assigning activity_Na+ = 0.2 and activity_K+ = 0.3. Considering the logarithm of the ratio, we have: log K = log[(0.2*1)/(0.3*0.891)] = -0.126. Considering the relationship log K = log(0.2)+0+0.05012-log(0.3) = -0.126. On the contrary, if we consider the relationship log K = LOG(0.2)+0-0.05012-LOG(0.3) = -0.226; i.e., we get a different number if we have -0.0512.

[something weird in Rxn logK]

Hi Jia, thank you for the quick answer.

I still have a doubt, because taking your formulation:

logK = log activity(K-feldspar) + log activity(Na+) - log activity(albite) - log activity(K+)

then it would be

logK = log activity(1) + log activity(Na+) - log activity(0.891) - log activity(K+)

logK = 0 + log activity (Na+) − (−0.05012) - log activity (K+)

So:

log⁡(K)= log ⁡activity(Na+) + 0.05012 − log ⁡activity(K+))

In other words, there should be +0.05012 because we have -(-0.05012).

Thank you

 

[something weird in Rxn logK]

Dear GWB users,

I have noticed something weird in the RXN tool. Specifically, writing the reaction:

albite_low + K+ = k-feldspar + Na+

and assigning an activity of 0.891 to k-feldspar, the logK is calculated as:

logK = -0.0501 + logNa - logK

However, in reality, the logarithm of the equilibrium constant should be:

logK = +0.05012 + logNa - logK

because:

logK = log[(Na/(K*0.891)] = logNa - log(K*0.891) = logNa - logK - log(0.891) = logNa - logK + 0.0512

Is that not correct? Or perhaps I am making a mistake somewhere? I would like to specify that I am using version 12.0.9.

[Density Calculation]

Hi, I am using GWB 12.0.6. These are the results obtained by Spec8:

          Temperature =  13.9 C    Pressure =  1.013 bars
          pH =  7.997
          Ionic strength      =    0.005059 molal
          Charge imbalance    =   -0.000267 eq/kg (-3.873% error)
          Activity of water   =    0.999995
          Solvent mass        =      1.0000 kg
          Solution mass       =      1.0003 kg
          Mineral mass        =     0.00000 kg
          Solution density    =    1.021    g/cm3
          Solution viscosity  =    0.012    poise
          Chlorinity          =    0.000132 molal
          Dissolved solids    =         291 mg/kg sol'n
          Elect. conductivity =      326.08 uS/cm (or umho/cm)
          Hardness            =      164.64 mg/kg sol'n as CaCO3
            carbonate         =      164.64 mg/kg sol'n as CaCO3
            non-carbonate     =        0.00 mg/kg sol'n as CaCO3
          Carbonate alkalinity=      172.96 mg/kg sol'n as CaCO3
          Water type          =    Ca-HCO3
          Bulk volume         =        980. cm3
          Fluid volume        =        980. cm3
          Mineral volume      =       0.000 cm3
          Inert volume        =       0.000 cm3
          Porosity            =        100. %
          Permeability        =        98.7 cm2

Water is fresh (TDS = 291 mg/kg sol'n), so I am very surprised to obtain a calculated density of 1.021 g/cm3. This latter value is typical of seawater or an NaCl brine. Indeed, using PHREEQC and density calculation by Pizer.dat dataset, I have obtained Density (g/cm³)  =   0.99950.
Thank you.