Geochemist's Workbench Support Forum

# [OLD] Negative concentrations

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From: Henry Kerfoot

Subject: negative concentrations

When I use the following parameters for reaction of feldspars with groundwater, I get results with [H+] 0. Is this an indication of instability due to use of too large a reaction step, or some other type of effect? I also occasionally get [O2] 0, which I assume is a reflection of the same thing.... Has anyone had experience with these types of issues? Here is the input and system information ....

Basis is: H2O

1 kg solvent

NO3- (swapped for

NH3(aq)) total mg/l = .4

Na+ total mg/l = 8.4

K+ total mg/l = 3.5

Ca++ total mg/l = 13

Mg++ total mg/l = 4.3

Fe++ total mg/l = .18

HCO3- total mg/l = 72

SO4-- total mg/l = 6.9

Cl- total mg/l = 3.8 (

F- total mg/l = .2

SiO2(aq) total mg/l = 35

H+ pH = 7

O2(aq) total mg/l = 4

Al+++ total mg/l = .1

Mn++ activity = 1e-12

Zn++ total mg/l = .09

HPO4-- total mg/l = .1

show reactants

Reactants:

React .05 mol of K-Feldspar

React .05 mol of Albite

show variables

Variables set by user:

cpw = 1 cal/g C

cpr = .2 cal/g C

delxi = .01

density = 1 g/cm3

dxplot = .005

dxprint = .1

epsilon = 5e-11

inert = 0 cm3

itmax = 400 iterations

itmax0 = 999 iterations

nswap = 30 iterations

nswap0 = 200 iterations

simax = 3 molal

TDS = 148 mg/kg

theta = .6

timax = 3 molal

From: Craig Bethke

Subject: RE: negative concentrations

I think you are confusing the concentration of a species in solution with that of a component. Whereas the concentration of a species is invariably a positive number, a component's concentration may be positive or negative. There's a section in the green book titled â€œComponents with Negative Massesâ€?, on page 38. If you read this (and perhaps the preceding pages, starting on page 30), I think this point will be clear.

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