MadSci Network: Chemistry

Re: can you throw acid neutralised away without causing any damage

Date: Sat Nov 27 11:11:08 1999
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Chemistry
ID: 943442527.Ch

Hello Sanne and Linda – there is no single answer to your questions – like 
so many things – it depends…

Let’s tackle your first question – “can you throw acid neutralised away 
without causing any damage?” first.

Acids are neutralised by bases. You could equally well have asked - can 
you throw base neutralised away without causing any damage? The answer 
depends on several things – like what do you mean by “throw away”? Down 
the drain? Secondly the answer will depend on what it is that you are 
throwing away – neutrality in pH (acidity/alkalinity) is only one 
parameter to consider. Any neutralisation produces a solution of salts or 
a precipitate/suspension of insoluble salts of the reaction between acid 
and base. As extreme examples – you wouldn’t throw sodium cyanide solution 
down the drain – which fits the description “acid neutralised” whereas 
sodium chloride (common salt) wouldn’t be a problem – with reasonable 
dilution and in reasonably small quantity. (Water softeners chuck this 
down the drain from households all the time.) Then – again assuming you are 
talking “down the drain” – it depends what kind of drain – and where it 
leads to – into a septic tank, into a sewer to a sewage works (domestic or 
industrial) or into a lake, a stream or the sea. Damage will depend on the 
answer to this as well as what it is and how much and how dilute….

You second question – “is it true that acid and base solutions can be 
thrown away in the environment because they neutralise each other and 
there is no heat development and large salt concentrations?” is perhaps 
easier to answer – the answer is no. Some neutralisations cause lots of 
heat -–and in any case neutralisation takes some time and mixing before it 
is complete. It also requires exact equivalence of acid and base to achieve 
neutrality. So you risk damage before neutralisation by either acid or 
base, and damage through not achieving neutrality. Also how much salt is 
formed depends on how much acid and base is involved. 

The main conclusion on all of this – never put anything down the drain (or 
into the environment generally) without checking with someone who knows! 
Your chemistry teacher should be able to tell you. You could even ask the 
question of the Dutch Department of the Environment. I bet you can find 
them on the web. Let me know if you get any different answers or if you 
want to ask me more – my e-mail is

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