Heat Stable Salt Terminology

Heat Stable Salts
Heat Stable Salts (HSS) have received a lot of attention in the industry. HSS are acid anions with a stronger
acid strength than the acid gases that are removed from the process gas. These anions may bind to the usable
amine and then therefore make it unavailable for acid gas absorption.
Heat Stable Amine Salts
Heat Stable Amine Salts (HSAS) refers to the salt formed by a HSS (anion) and a protonated amine molecule
(cation). HSAS may also be referred to in some instances as Bound Amine (BA)
HSS ≠ HSAS
There has been much confusion about the terminology of HSS versus HSAS. It is important to understand that
these HSS anions must be bound to a cation in solution so that the solution is balanced (Mother Nature’s Rule).
One must understand what cation forms a salt with the HSS anion to understand the disposition of the anions
and their quantity in solution. As referred to earlier, the sum of cations in solution must equal the amount of
anions in solution.
Σ Cations = Σ Anions
BA + SC = HSS + LL
BA = Bound Amine (Protonated Amine Molecule)
SC = Strong Cations (Sodium or Potassium)
HSS = Heat Stable Salt Anions
LL= Residual Lean Loading (H2S or CO2)

From the above equation we can see that HSS will not equal the Bound Amine (HSAS) if there is a substantial
amount of Strong Cations present in the amine solution. This is why we recommend that the total level of HSS
anions and Strong Cations should be measured directly. Measuring the HSAS only may give a false low reading
of the level of HSS anions in solution if Strong Cations are also present in the sample.
HSS Reporting Terminology
It is important to understand that HSS anions may be reported at least three different ways, and it is important to
understand the methodology employed to avoid confusion.
1. HSS – Weight Percent of Solution
HSS Anions (Strong Acid Anions) measured as weight percent of the total solution.
2. HSS – As Weight Percent Amine
This unit of measurement assumes that the HSS anions are bound to an amine cation (also reported
as HSAS, Heat Stable Amine Salt). This number is determined by calculating the equivalent
amount of amine cations that are tied up with the HSS anion, and is expressed as weight percent of
the total solution.
3. HSS – As Percent Amine Capacity (As Percent Total Amine)
HSS expressed as weight percent amine divided by the amine strength (Free Amine or Alkalinity).
Example Sample Analysis Form
Please see the attached example sample analysis form, which shows a bulk composition material balance and
HSS anions listed the various ways.