Potential of Hydrogen (pH)
- by Jon Miller PhD [Chemical Engineering]
Potential of Hydrogen
Potential
hydrogen [: in
chemistry, pH is a measure of the activity of the (solvated) hydrogen ion.
p[H], which measures the hydrogen ion concentration is closely related to, and
is often written as, pH .Pure water has a pH very close to 7 at 25°C. Solutions
with a pH less than 7 are said to be acidic and solutions with a pH greater
than 7 are basic or alkaline]
Simply
put the potential of hydrogen (pH) scale is a set of numbers between 0 and 14
where 0 (battery fluid) is the most acidic and 14 (lye) is the most alkaline
(caustic) pH is a characteristic of water solutions only. There must be water
present to have a pH.
Unfortunately the scale is logarithmic, for
every integer that the scale decreases the material is 10 times stronger. Those
of us in earthquake country know all too well the consequences of a change of
from 6 to 7 on the logarithmic, Richter scale. The difference in the pH scale
is just as dramatic and therefore just as misleading.
For
example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10
times 10) more acidic than a pH of 6. The same holds true for pH values above
7, each of which is ten times more alkaline than the next lower whole value.
For example, a pH of 10 is ten times more alkaline than a pH of 9.
pH is
a scale of measurement of the relationship between hydrogen and hydroxyl ions,
to determine if the material is neutral, basic (alkaline) or acidic (reactive),
if there are more hydrogen than hydroxyl ions you have an acid the reverse will
give you an alkali (another way to say basic]
Pure
water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to
7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a
pH greater than 7 are said to be basic or alkaline. Each whole pH value below 7
is ten times more acidic than the next higher value.
Alkalinity and pH
Are
distinctly different from each other, although their definitions and functions
can be easily confused; alkalinity is essentially a measurement of water's
ability to neutralize acids. It is a measure of the buffering capacity of a
system while pH is basically the measurement of the concentration of hydrogen
ions in water, in terms of acidity or alkalinity.
Acidic and basic
Two
extremes that describe chemicals just like hot and cold are two extremes that
describe temperature. Mixing acids and bases can cancel out their extreme
effects; much like mixing hot and cold water can even out the water
temperature. Chemicals that are very basic or very acidic are called
"reactive." A very common application of hydrochloric acid is to
regulate the alkalinity (pH) of solutions.
These
chemicals can cause severe burns. Automobile battery acid is an acidic chemical
that is reactive, automobile batteries contain a stronger form of some of the
same acid that is in acid rain. Household drain cleaners often contain lye, a
very alkaline chemical that is reactive
|
pH is calculated
using the following formula:
pH = -log10[H+] |
pOH is calculated
using the following formula:
pOH = -log10[OH-] |
Reactive
0.0–1.0
Strong
acid Hydrochloric acid
1.0-2.9
Acidic
Citric acid (lemon)
3.0-4.9
Weak
Acid Vinegar (Acetic acid)
Oxalic
5.0-6.5 Very
weak acid, Acid rain
6-7.3
Neutral
Distilled water
Alkaline
7.3-9.0
Very
weak alkali
9.1-10.9
Car
wash concentrates
11.0-12.9
Weak
alkali Baking soda, Seawater
13.0-14.0
Alkaline
Bleach, Ammonia
Strong
alkali Sodium Hydroxide (Lye)
A few
examples of maximum acid strength might be helpful:
3.5-4.0.
Acid
rain has a pH of 3.5-4.0.
3.0-4.5 Bird
excrement contain highly acidic concentrates of uric acid
7.75
to 8.25 Sea
water
5.5
to 6.5 Tap
water
Organic acids
Citric acid, found in citrus fruits has a pH
of 2.0
Oxalic
acid, found in spinach has a pH of 1.8
Vinegar
has a pH of 2.0
‘Weak’ mineral acids Hydrofluoric acid
has a pH of 3.14 Phosphoric acid has pH of 2.2
‘Strong’
mineral acids Hydrochloric acid, aka Muriatic acid has a pH of 0.1 @ 1N
(indicates concentration) Sulphuric acid, battery acid, has a pH of 0 .32 @ 1N
A
couple of things to note from this list are
a) how very strong
"strong acids" are
b) How relatively weak yet
dangerous hydrofluoric acid is and
c) And how small and
misleading the difference is between organic acids and weak mineral acids. As
an example, hydrofluoric acid (HF), one of the most highly regulated, most
dangerous acids and the only acid that will dissolve glass is a weak acid.
Also, these numbers are maximums, the pH increases when the solution is diluted
or changes when there are other substances are dissolved in the solution.
Just
to complicate things a bit more, mixing some strong mineral acids together with
other relatively weak acids can create a solution of "super acid".
This can result in acid strengths of up to 12 times higher than either acid by
itself and are not measured accurately by the normal pH scale. In theory, this
is the result obtained by adding HF to other mineral acids in wheel cleaners.
Or mixing several weak acids together can have a synergistic effect where
together these acids do more than just strong solutions of each one separately.
In
practical applications, a formulator can get identical pH values by using
various quantities of different acids. For example very small amounts of very
strong acids or larger amounts of weaker acids can result in the same pH. But
since pH is not a very good indicator of the strength of the acid in every
system, this approach won't always produce the desired results. Each acid has
properties that make it most useful for certain jobs. For example Citric acid
is quite good at picking up Calcium ions in solution so acts as a good water
softener where the same pH of a Sulphuric acid solution would be worthless for
that application. My point here is that, it is not possible to judge how well a
product will do the job it is designed for just by measuring the pH and
stronger is not always better.
So,
acid strength is always relative to the system you’re measuring and what
materials are in danger of being dissolved or attacked by the acid pH is a
measure of the relative strength of an acid but the key word is relative.
Ways to determine pH values -
·
Litmus Paper - acidic, the paper will turn red, alkaline, the
paper turns blue.
·
Electronic pH Meters - Provide an accurate quantitative reading
·
It is sometimes listed on a chemicals material data sheet (MSDS)
Corrosion and pH
There
is one other factor relative to pH that needs to be explained, and it has to do
with the corrosiveness of a compound. Both acids and alkali's have the
capability of being corrosive, although one would have a pH range of 0 (acid),
while the other would range in the area of 14 (alkali). Sodium hydroxide, a
very strong and corrosive alkali would have the same damaging effect on human
tissue as sulphuric acid. If a 25% concentration of sulphuric acid and
phosphoric acid were measured for pH, both would range in the area of 0.
However,
if sulphuric acid were allowed to contact human tissue, severe burns would
result, while the average person would not detect even a burning sensation from
contact with the phosphoric acid.
Why?
The answer lies in the corrosive nature of some acids over others. Certain
characteristics have been observed in the reactions of acids and alkalis, and
were assigned classifications accordingly.
One of the classifications is
corrosiveness. Therefore to classify a product or compound as being corrosive
means that it would have the potential to erode materials, in some cases very
rapidly, and it would have the capability of being harmful to objects such as
structural components of a vehicle and human tissue. It is important to understand
that some cleaning products are corrosive by their very nature and should be
handled according to the directions on the container. All corrosive products
are labelled as such and must follow strict government guidelines. (See also
Alkalinity, Acid and pH)
As a
chemical Engineer I would like to help detailers reach an intelligent and
logical understanding of the many chemicals used in detailing. Improperly used
chemicals can cause damage to the surface they are applied to (sometimes
irreparably) because the detailer was not educated regarding the chemical
make-up of the products they used.
Conversely
I also want to show you that it's a waste of time to dedicate too much time and
attention to them. All that is required is that you learn some basic chemistry
i.e. what pH values mean, and what detailing chemicals to avoid or the
precaution to take if you do choose to use them.
Material data sheets (MSDS)
Material Safety Data Sheet (MSDS) is
required under the OSHA Hazard Communication Standard. The MSDS is a detailed
informational document prepared by the manufacturer or importer of a hazardous
chemical. It describes the physical and
chemical properties of the product. MSDS’s contain useful information such as
flash point, toxicity, procedures for spills and leaks, and storage guidelines.
Information included in a Material
Safety Data Sheet aids in the selection of safe products, helps you understand
the potential health and physical hazards of a chemical and describes how to
respond effectively to exposure situations. Although there is an effort
currently underway to standardize MSDS’s the quality of individual MSDS’s vary.
A MSDS may be useful but it cannot substitute for prudent practices and
comprehensive risk management.
An MSDS should be available
for every chemical you use. Read these and follow the recommendations for safe
use and disposal of the material. The target audience for information in a MSDS
is the occupation worker who may be exposed to chemicals at work. However, much
of the information is also relevant to consumers.
Read the manufacturers
application instructions and then obtain and read the MSDS sheet to ascertain
the chemicals used. Although it should be said that an MSDS is a document that
contains details of the hazards associated with a particular chemical and
provides information regarding its safe use. The MSDS is required to state the
chemical's risks, safety and impact on the environment.
An MSDS is a fact sheet
developed by manufacturers describing the chemical properties of a product.
Material Safety Data Sheets include brand-specific information such as physical
data (solid, liquid, colour, melting point, flash point, etc.), health effects,
first aid, reactivity, storage, handling, disposal, personal protection and
spill/leak procedures.
How to Read a Material
Safety Data Sheet (MSDS) http://www.cmhc-schl.gc.ca/en/co/maho/gemare/gemare_012.cfm
Chemical Information (MSDS) A-Z- http://www.ehso.com/msds.php
Definitions of Terms Used in
Material Safety Data Sheets (MSDS) - http://www.ehso.com/msdsdefs.php
Chemical Abstracts Service
(CAS)
Each
CAS registry number (often referred to as CAS #) is a unique numeric identifier
that designates only one substance; it has no specific chemical significance,
but is a link to a wealth of information about a specific chemical substance.
Since CAS Registry Numbers are not dependent upon any system of chemical
nomenclature, they can provide a reliable common link between the various
nomenclatures terms used to describe substances. And serve as an international
resource for chemical substance identifiers used by scientists, industry, and
regulatory bodies
I
would like to think that these articles become an asset to anyone who is new to
detailing and to professionals alike, as well as industry experts who seek to
advance their knowledge.
I hope the
above article was informative. By having some understanding of the ‘What’ and
‘Why’ as well as the ‘How’ along with a little science to help you understand
how the chemicals we use react, you can achieve the results you desire.
I would appreciate it if you would share
this article as it helps other detailers further their knowledge.
Questions and/ or constructive comments
are always appreciated.
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