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pH testing
Rapid development of highly-advanced analytical
technologies can make simple wet-chemistry
techniques seem out of favour. However, their
simple and effective testing of samples remains
highly useful. Determining quickly whether you have
an acid or an alkali will aid rapid incident response.
the acid or alkali’s concentration this may not be a
good idea! We will discuss this later.
Things to consider with pH testing of samples:
Strength and concentration are two words that are
used in the English language to describe the potency
of a material. These words are used interchangeably
in regular speech. In chemistry, however, strength
and concentration are two different things and are
used to specify certain characteristics of acids and
alkalis.
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Understanding key differences between acid
and alkaline materials
What are the difference between a strong and
weak acids?
Effect of concentrations
Likely risks associated with both acids and alkalis
Safe and effective sampling
This guide focuses on understanding the principles of
acid and alkali materials, paying attention to
sampling and testing them in a safe and effective
manner.
Remember!
Normal pH testing (with indicator paper) will only
work when your acid/alkali is in a solution of water.
Also, you cannot test the pH of a solid, you must mix
it with water first (see later on).
Key differences between acids and alkalis
Materials can either be neutral, acidic or alkaline.
Each has its own behaviour. For example, we use
their behaviours in our daily lives. We clean our
toilets with bleach (high pH) and add vinegar to our
chips (low pH). Both acids and alkaline materials are
reactive; material corrosion being the most common
hazard.
The character of acids and alkalis comes from
hydrogen ions (H+) and hydroxide ions (OH-)
respectively.
H+ + OH- → H2O
The more hydrogen ions (H+) there are in a solution
the more acidic it’ll be. The same concept can be
applied for alkaline materials.
Combing an acid and an alkali will always produce
water and salt as products. However, depending on
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Strength and concentration
Hazmat is dependent on concentration- it’s all about
the ummph!
The strength of an acid relates to the its ability to
release free hydrogen ions (H+) in solution. The
concentration of an acid refers to the percentage of
that acid in a solution (of water for example).
Each acid or alkaline solution may possess a number
of hazardous properties e.g. causing burns,
corroding metal, flammable, oxidizing etc. With high
concentrations, there will be a higher potency
(ummph!) for these hazards.
For example, brick acid (10-30% hydrochloric acid)
will cause far worse skin burns that stomach acid
(0.5% hydrochloric acid). High concentrations of
acetic acid is flammable. Lower concentrations
(approx. 6 %) of acetic acid will be used to make your
chips taste nice!
It is not easy to determine the concentration of an
acid directly from a pH test. Both acids and alkalis
will reach the extreme, of their pH scale, even at low
dilutions.
Hydrochloric acid is pH 0 at around 30 % and
remains at this pH when diluted to 4 %! These are
two different concentrations of the same substance
which have different challenges for a responder.
It is important to identify whether the acid you have
is at a high or low concentration. This is important as
they have different chemical characteristics and
safety concerns. Highly concentrated acids will react
more rapidly than weaker ones.
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A quick method to work this out is to dilute a
solution by 10 times, for example, and see if the pH
changes. If the pH hasn’t changed then it suggests
that it is a relatively high concentration.
Take a small sample of
your solid material.
Chemically, the strength of the acid comes from the
amount of splitting which occurs when these
materials are placed in water. A strong acid will split
to release more hydrogen ions (H+) than a weak acid.
Mix it with water (the
water in this picture is
coloured blue).
Acetic acid and sulphuric acid are different
strengths.
Acid
(concentrated)
Sulphuric
Acetic
Common
use
Drain
cleaner
Food
industry
Acid
strength
Strong
pH
Weak
2.4
0
Dip the tip of some pH
paper into the liquid.
Representative samples
Effective sampling strategies will make sure that a
sample takenis truly representative of the bulk
material. Practically, this can lead to a number of
challenges that should be considered and overcome.
What colour is the paper?
Safe and effective sampling
Read the ‘sampling from containers guide’ in the
‘Sampling techniques’ document to familiarize
yourself with good sampling techniques.
The majority of hazmat situations the most effective
method to sample with a very small sample size. Half
a pea size is an adequate amount or when using
liquids, less than 1 mL.
To make a solid sample into a liquid which is
available for pH testing, follow the procedure.
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FOR MORE INFORMATION AND TRAINING
ON THIS TOPIC GO TO:
WWW.TACTICALHAZMAT.COM