Dr. Parkhurst,

I have a question regarding how to set up a solution file when modeling
a brine solution. I am using PHRQPITZ to speciate this solution;
however, I want to compare the results to PHREEQC.

I have a brine with a calcium concentration of 17,100 mg/L and an
estimated density of 1.164 kg/L. I am expecting PHREEQC to return the
following:

-   Ca molarity = 0.427 mol/L

-   Ca molality = 0.367 mol/kg

In the SOLUTION input, I enter the concentration of constituents in
mg/L. From the manual I understand that PHREEQC will use the input
density to calculate constituent molalities. The mass of water I enter
will be used to calculate the total number of moles.

I have tried various combinations for the input density and mass of
water (Table 1 below). None of the simulations return the expected Ca
molality of 0.367 mol/kg.

**TABLE 1**

  --------------- ----------- ------------ ------------ ------------ ------------ ------------
  **Parameter**   **Units**                **Units1**   **Units2**   **Units3**   **Units4**

  ***PHREEQC                                                                      
  Input***                                                                        

  **Solution                               mg/L         mg/L         mg/L         mg/L
  Units**                                                                         

  **Density**                 **kg/L**     1.164        1.164        1            1

  **Mass of                   **kg**       1            1.164        1.164        1
  Water**                                                                         

  ***PHREEQC                                                                      
  Output***                                                                       

  **Molality**                **mol/kg**   0.427        0.427        0.510        0.510

  **Moles**                   **mol**      0.427        0.497        0.594        0.510
  --------------- ----------- ------------ ------------ ------------ ------------ ------------

I understand how PHREEQC has calculated the moles of Ca in solution
(molality x mass of water). I an unable to determine how the molality is
calculated by PHREEQC.

Based on the results presented in Table 1, I assume that PHREEQC always
assumes 1 liter of water. This is based on the fact that solutions with
the same densities but different masses of water return the same
molality.

-   Can you please confirm whether PHREEQC always assumes a 1 liter
    solution, or if the volume of solution is determined from the
    density and mass of solution entered.

-   Does the mass of water refer to jus the mass of H~2~O (meaning that
    1 liter of any solution would have a mass of 1 kg) or the mass of
    solution?

-   What information does PHREEQC use to calculate molality? Does
    PHREEQC calculate density independent of the estimated input value?

Thank you for your time.

**[PHREEQC OUPUT]{.underline}**

***Filename -- Units 1***

SOLUTION 1

temp 25

pH 6.48

pe 4

redox pe

units mg/L

density [1.164]{.mark}

Alkalinity 28.7 as HCO3

Na 38180

K 1560

Ca 17100

Mg 3010

Ba 10.3

Sr 548

Fe 0.6

Cl 102968

S(6) 369

water [1 \# kg]{.mark}

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Beginning of initial solution calculations.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Initial solution 1.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Solution
composition\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Elements Molality Moles

Alkalinity 4.702e-004 4.702e-004

Ba 7.498e-005 7.498e-005

Ca [4.266e-001 4.266e-001]{.mark}

Cl 2.904e+000 2.904e+000

Fe 1.074e-005 1.074e-005

K 3.989e-002 3.989e-002

Mg 1.238e-001 1.238e-001

Na 1.660e+000 1.660e+000

S(6) 3.840e-003 3.840e-003

Sr 6.253e-003 6.253e-003

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Description of
solution\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

pH = 6.480

pe = 4.000

Activity of water = 0.912

Ionic strength = 3.413e+000

[Mass of water (kg) = 1.000e+000]{.mark}

Total carbon (mol/kg) = 4.866e-004

Total CO2 (mol/kg) = 4.866e-004

Temperature (deg C) = 25.000

Electrical balance (eq) = -9.826e-002

Percent error, 100\*(Cat-\|An\|)/(Cat+\|An\|) = -1.72

Iterations = 10

Total H = 1.110129e+002

Total O = 5.552304e+001

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Distribution of
species\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

***Filename -- Units 2***

SOLUTION 1

temp 25

pH 6.48

pe 4

redox pe

units mg/L

density [1.164]{.mark}

Alkalinity 28.7 as HCO3

Na 38180

K 1560

Ca 17100

Mg 3010

Ba 10.3

Sr 548

Fe 0.6

Cl 102968

S(6) 369

water [1.164]{.mark} \# kg

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Beginning of initial solution calculations.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Initial solution 1.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Solution
composition\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Elements Molality Moles

Alkalinity 4.702e-004 5.474e-004

Ba 7.498e-005 8.728e-005

[Ca 4.266e-001 4.965e-001]{.mark}

Cl 2.904e+000 3.380e+000

Fe 1.074e-005 1.250e-005

K 3.989e-002 4.643e-002

Mg 1.238e-001 1.441e-001

Na 1.660e+000 1.933e+000

S(6) 3.840e-003 4.470e-003

Sr 6.253e-003 7.278e-003

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Description of
solution\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

pH = 6.480

pe = 4.000

Activity of water = 0.912

Ionic strength = 3.413e+000

[Mass of water (kg) = 1.164e+000]{.mark}

Total carbon (mol/kg) = 4.866e-004

Total CO2 (mol/kg) = 4.866e-004

Temperature (deg C) = 25.000

Electrical balance (eq) = -1.144e-001

Percent error, 100\*(Cat-\|An\|)/(Cat+\|An\|) = -1.72

Iterations = 10

Total H = 1.292191e+002

Total O = 6.462882e+001

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Distribution of
species\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

***Filename -- Units 3***

SOLUTION 1

temp 25

pH 6.48

pe 4

redox pe

units mg/L

[density 1]{.mark}

Alkalinity 28.7 as HCO3

Na 38180

K 1560

Ca 17100

Mg 3010

Ba 10.3

Sr 548

Fe 0.6

Cl 102968

S(6) 369

[water 1.164 \# kg]{.mark}

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Beginning of initial solution calculations.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Initial solution 1.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Solution
composition\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Elements Molality Moles

Alkalinity 5.625e-004 6.547e-004

Ba 8.968e-005 1.044e-004

[Ca 5.102e-001 5.939e-001]{.mark}

Cl 3.473e+000 4.043e+000

Fe 1.285e-005 1.495e-005

K 4.771e-002 5.553e-002

Mg 1.481e-001 1.723e-001

Na 1.986e+000 2.312e+000

S(6) 4.594e-003 5.347e-003

Sr 7.479e-003 8.706e-003

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Description of
solution\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

pH = 6.480

pe = 4.000

Activity of water = 0.895

Ionic strength = 4.082e+000

[Mass of water (kg) = 1.164e+000]{.mark}

Total carbon (mol/kg) = 5.729e-004

Total CO2 (mol/kg) = 5.729e-004

Temperature (deg C) = 25.000

Electrical balance (eq) = -1.368e-001

Percent error, 100\*(Cat-\|An\|)/(Cat+\|An\|) = -1.72

Iterations = 10

Total H = 1.292192e+002

Total O = 6.463263e+001

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Distribution of
species\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

***Filename -- Units 4***

SOLUTION 1

temp 25

pH 6.48

pe 4

redox pe

[units mg/L]{.mark}

density 1

Alkalinity 28.7 as HCO3

Na 38180

K 1560

Ca 17100

Mg 3010

Ba 10.3

Sr 548

Fe 0.6

Cl 102968

S(6) 369

[water 1 \# kg]{.mark}

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Beginning of initial solution calculations.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Initial solution 1.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Solution
composition\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Elements Molality Moles

Alkalinity 5.625e-004 5.625e-004

Ba 8.968e-005 8.968e-005

[Ca 5.102e-001 5.102e-001]{.mark}

Cl 3.473e+000 3.473e+000

Fe 1.285e-005 1.285e-005

K 4.771e-002 4.771e-002

Mg 1.481e-001 1.481e-001

Na 1.986e+000 1.986e+000

S(6) 4.594e-003 4.594e-003

Sr 7.479e-003 7.479e-003

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Description of
solution\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

pH = 6.480

pe = 4.000

Activity of water = 0.895

Ionic strength = 4.082e+000

[Mass of water (kg) = 1.000e+000]{.mark}

Total carbon (mol/kg) = 5.729e-004

Total CO2 (mol/kg) = 5.729e-004

Temperature (deg C) = 25.000

Electrical balance (eq) = -1.175e-001

Percent error, 100\*(Cat-\|An\|)/(Cat+\|An\|) = -1.72

Iterations = 10

Total H = 1.110130e+002

Total O = 5.552631e+001

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Distribution of
species\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--