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CGUESS AND CTOT IN TOUGHREACT

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I do not quite understand what the CGUESS and CTOT mean when setting initial water component in TOUGHREACT.In user's manual,CTOT:total dissolved component concentration. CGUESS:initial guess for the concentration of the primary species,but I do understand the difference between them. Can anyone tell me what they stand for and their difference?

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It is my understanding that these definitions have changes slightly with the release of version 1.2 of TOUGHREACT. The TOUGHREACT 1.2 manual goes into more detail that the original TOUGHREACT users guide about these. The following is from the v1.2 User's Guide:

Record-24. Composition of aqueous solution

Variable: NAPRI ICON CGUESS CTOT NAMEQ QKSAT

Format: A20, I, F, F, A, F

Remark: Repeat Record-24 as many times as the number of primary species NAPRI: name of the primary aqueous species. The name of the species must match exactly those previously listed as primary species in the definition of the system, although the order may change. Names must be included between 'single quotes' such as ‘h+’. A record starting with '*' indicates the end of the list.

ICON: flag indicating the type of constraint controlling the input concentration of the aqueous species:

ICON=1: input values of CTOT represent total amounts (in moles) for aqueous species, and total kilograms for liquid H2O. Thus, for inputting total molalities, set CTOT = 1 for H2O.

ICON=2: the total concentration of the species will be adjusted such that the saturation index (log(Q/K)) of mineral or gas NAMEQ equals QKSAT. Therefore, for equilibrium with a mineral, use this option with QKSAT = 0.0, and for equilibrium with a gas at a given fugacity, use this option 32 with QKSAT = log(fugacity). With this option, input CTOT values are irrelevant and discarded.

ICON =3: input values of CTOT represent the known activity of the specific species (i.e., not total concentration). For example, to input a known pH value, use this option and set CTOT = 10-pH for H+ activity.

ICON=4: the total concentration of the species is adjusted to yield charge balance. Use only with a charged species. If non convergence occurs, choose a species with an opposite charge. With this option, input CTOT values are irrelevant and discarded.

CGUESS: initial guess for the concentration of the individual primary species (not total concentration), in moles/kg H2O (molal) for species other than H2O and in kg for H2O. Input values of CGUESS do not affect results of speciation calculations, but could affect the number of chemical iterations required during initial speciation computations.

CTOT: if ICON=1, CTOT is total moles of aqueous species, and total amount (in kg) of liquid water for H2O. Molalities are then internally computed as CTOTi H2O /CTOTH2O . If ICON > 1, refer to the discussion of ICON above for the meaning of CTOT.

NAMEQ: name of mineral or gas to use with option ICON=2. Names must be included between 'single quotes', and match exactly those previously listed as minerals or gases in the definition of the chemical system. If ICON_2, this entry is ignored, but cannot be omitted and should be entered as one of more characters between single quotes (suggested ‘ ’ or ‘*’ ).

QKSAT: desired value of mineral log(Q/K) or gas log(fugacity) when option ICON=2 is used. For equilibrium with mineral NAMEQ use QKSAT=0.0, and for equilibrium with gas NAMEQ at a given fugacity use QKSAT = log(fugacity). If ICON_2, this entry is ignored, but cannot be omitted and should be entered as a real number (suggested 0.0).

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Guest LenWo_Germany

CTOT is the sum (total amount) of one primary species (e.g. HCO3-) and all of its associated secondary species (CO3-2, CO2(aq)...). This amount is speciated over all associated species, such that the stabil concentration of the primary species itself is lower and need to be approximated with a first guess (CGUESS). If this first guess is to far from the real, stable concentration, initial speciation calculation might fail. This is especially the case for O2(aq), but also for SO2(aq) or other, highly coupled chemical species.

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