Conceptual Models > Database selection and modification

Can't find lanthanum arsenate in database

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elbertnigri:
Dear All,
First of all, I want to show my appreciation for this community.

I am carrying out studies of arsenic removal by precipitation adding salts like Na2HAsO4ˇ7H2O and La(NO3)3.6H2 e varying the concentration of both and pH of solution. I expected to find the La(AsO3)3 species for example, but I didn't find any species containing arsenic and lanthanum together. I checked the database and didn't find it either. What can I do to resolve this? I would like to construct a diagram varying the pH to determine the species formed containing As and La.

Thank you in advance.
Elbert Nigri

dlparkhurst:
llnl.dat is the most complete database, and it has thermodynamic data for both As and La, but no La-As minerals.

You can add La-As minerals with a PHASES data block in your input file. You need a balanced dissociation reaction for a mineral and the associated log K. For temperature variation you need either a delta H of reaction or an expression for log K as a function of temperature. Adding a mineral is fairly easy, finding the equilibrium constant(s) is the difficult part. You will have to search the literature.

elbertnigri:
Thanks for the answer. I will do this

elbertnigri:
Hello again. I got the data, but I don't know where and how to add it to the database. Can you help me or show me a step-by-step? Thanks.

LaAsO4 = La(+3a) + AsO4(-3a)


T (ēC)   ΔH (KJ)    ΔS (J/K)    ΔG (KJ)           K            Log (k)
20        -36.462   -535.650   120.563   3.280E-022   -21.484
40        -51.021   -583.732   131.774   1.041E-022   -21.982
60        -64.351   -625.005   143.869   2.759E-023   -22.559

LaAsO4 (277.825 g/mol); La(+3a) (138,905 g/mol); AsO4(-3a) (38.99 g/mol)

dlparkhurst:
Here is a tutorial with log K fit with Excel.

--- Code: ---PHASES
LaAsO4
LaAsO4 = La+3 + AsO4-3
log_k -21.602
-analytical -23.25 3.497e-2 0 0 0 -9.875e-5
SOLUTION
EQUILIBRIUM_PHASES
LaAsO4 0 10
REACTION_TEMPERATURE
20 25 40 60
USER_PRINT
10 PRINT TC LK_PHASE("LaAsO4")
USER_GRAPH 1
    -headings               TC Log10(K) La
    -axis_titles            "Temperature, C" "Log10 K, LaAsO4(s)" "La, Molality"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TC
20 GRAPH_Y LK_PHASE("LaAsO4")
30 GRAPH_SY TOT("La")
  -end

--- End code ---

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