SOLUTE

Solutes in dilute
binary solutions obey
Henry's law, which is
stated as follows:
a¡= γ¡0 Xi
where, γ¡0 is a
constant, known as
the Hemy's law
constant. Deviation
from Henry's law
occurs when the
solute concentration
increases.
Therefore, activities
of dissolved
elements in liquid
steel are expressed
with reference to
Henry's law, and not
Raoult's law.
Raultian
standard
state
Región de
disoluciones
diluidas
SE CUMPLE
LA LEY DE
HENRY
Henrian
standard
state
a=N
Escala
actividades
de Henry
a=kN
(SOLUTE)
In non ideal cases and in very dilute solutions the activity is, as shown by the straight-line relationship,
proportional to the mole fraction though they are not equal. This is the region of Henry's law as shown by the
Henry's line in Fig. 2.1(a) for which a standard state has been defined,i.e., the activity coefficient of a solute, γ0, is
unity in aninfinitely dilute solution.
In ironmaking, as in the making of other metals, we deal with impurities in the metal whose concentrations are small. Therefore,
a more convenient standard state can be defined where the activity is equal to unity at one weight per cent concentration of the
solute, as shown schematically in Fig 2.1(b), which is an enlarged version of Fig. 2.1(a) in the dilute region.
The choice of this standard state is rendered mathematically feasible because at low concentrations the weight per cent of any
solute in solution ,is approximately proportional to its mole fraction .
(1,1)
Mole fraction vs wt %
1 wt % standard
Henrian vs.
1 wt % standards
Nj
j%
Therefore, activities of dissolved elements in liquid steel are expressed with reference to Henry's law, and not Raoult's law.
Since it is the intention to find values directly in weight percent, the composition scale is weight percent. With these
modifications, i is in solution with Fe as solvent in the binary Fe-i solution. If:
1.- Henry's law is obeyed by the solute in the binary Fe-i solution, then:
h¡ = aH,i = W¡ = [% i]
2.- Henry's law is not obeyed by the solute i, then
h¡ = aH,i = fiW¡ = fi[% i]
where, hi is activity, W¡ is weight percent and f¡ is activity coefficient in the so-called one weight percent standard state. This
is because, at 1 wt. %, hi = 1, if Henry's law is obeyed.
Interacciones soluto-soluto en soluciones diluidas de múltiples componentes
El acero fundido y el hierro, así como otros metales impuros (Cu, Pb, etc) en el estado diluido tienen varios solutos disueltos. Por
ejemplo, el acero tiene C, Mn, S, P, etc. Se ha encontrado que los solutos en una solución de múltiples componentes interactúan
entre sí y, por lo tanto, influyen en las actividades de otros solutos. Si Fe es el disolvente, y 1, 2, ..., i,… j, ..., k son solutos en
estado diluido, se tiene que:
k
log  fi T 1873 K ( 1600 oC )   eijW j  ei1W j  ei2W2  .....  eiiWi  ..  eijW j  ....  eikWk
j 1
eij = Coefficient describing the influence of solute j on the activity coefficient of solute i.
log  fi T 1873 K


1873 k j
1873
j
1
2
i
k

e
W

e
W

e
W

.....

e
W

..

e
W

....

e
W

i j
i 2
i i
i
j
i k
T j 1 i j
T
eij = Coefficient describing the influence of solute j on the activity coefficient of solute i.