Page 272 - Materials Chemistry, Second Edition

256 4 Life Cycle Impact Assessment

Section 4.5.2.4. Therefore a simple characterisation by means of an AP is discussed
first.
4.5.2.5.1 Impact Indicator and Characterisation Factors Impact category ‘acidifi-
cation’ has been chosen in ISO 14044 (2006b, loc. cit. Figure 4.3) as an example to
describe an approach corresponding to standards:

• LCI results
−1
Example: SO ,HCl,HF, andsoon(kg fU ).
2
• LCI results assigned to impact category (classification)
Emissions with an acidifying impact, for example, NO ,SO , and so on, are
x 2
assigned to the impact category acidification.
• Category indicator, characterisation model
Release of protons (H + aq ); calculation of AP equivalents (mostly as SO -eq).
2
• Impact endpoints
Acid-related damages on aquatic ecosystems, forests, vegetation, buildings, works
of art, and so on.

Quantification by an AP as proposed by Heijungs et al. (1992, loc. cit.) starts
on top of a stressor-effect-chain and ‘counts’ protons per fU as SO equivalents,
2
occasionally also as mass or mole of protons. The impact indicator of this simple
model is the acid formation from precursor compounds and a successive entry of
acid through water whereby a total dissociation of the acid into protons and the
respective anions is presumed. This is a very good approximation for strong acids
but also weak acids can shift basic milieus (e.g. sea water) in direction of the neutral
point (pH 7). This is often called acidification in spite of being a mere reduction
of alkalinity (but even that is dangerous for many marine organisms whose shells
start to dissolve around the neutral point).
TheAPisatypical midpoint indicator which neither name nor model endpoints.
The endpoints however as entirety are considered in the interpretation because the
potential of acidification may cause numerous endpoints.
Characterisation factors are calculated according to the stoichiometry of the
formation of acid from precursors. Sulphur dioxide is a precursor of the two-base
acid H SO (sulphurous acid) formed by solution in water of the gas SO . Because
2
2
3
it can only produce one mol of protons (by dissolution + oxidation from NO ),
x
1 mol HNO thus corresponds to half a mole of sulphur dioxide, which produces
3
2 mol of protons following dissolution in water. It is not important that sulphurous
acid is a relatively weak acid since in the environment it oxidises into very strong
sulphuric acid (H SO ).
2 4
It should be noted that the weak carbonic acid, formed by dissolution of CO
2
in surface waters, especially the oceans, has never been included in the list of
acidifying substances. It actually is globally the most relevant acid, but only one
negative consequence of this gas is included in the impact category ‘climate change’
and the acidification potential is not considered.