The Difference between Scaling Tendency and Pre-scaling Tendency?

Introduction

What is the difference between a scaling tendency and a pre-scaling tendency?

Many users are confused by what OLI means by a scaling tendency and a pre-scaling tendency (also known as a pScale). OLI uses the scaling tendency (hereafter known as ST) to indicate how much mass needs to be be moved between phases. The pScale is used only as an indication of the amount of scale that will form.

What is a scaling tendency (ST)?

Lets consider the dissolution of a solid such as calcium carbonate. The equilibrium expression will look like this:

CaCO_3(s) = Ca²⁺ + CO₃^2-

We will define the solubility product as:

K_SP = (a_Ca2+)(a_CO3(2-))

Where "a" is the activity of the species and has the form:

a_i = γ_i[i]

We now define the scaling tendency for calcium carbonate as:

ST = γ_Ca2+[Ca²⁺]γ_CO3(2-)[CO₃^2-]/K_SP

Scaling Tendency	Result
< 1.0	The solid is under-saturated and is not expected to form
= 1.0	The solid is saturated and is expected to form
> 1.0	The solid is over-saturated and may or may not form.