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Litmus Paper
What Is Litmus Paper?
Understand the Litmus Test
Litmus Paper and the Litmus Test
by Anne Marie Helmenstine,Ph.D
You can make paper test
strips to determine the pH of an aqueous solution by treating filter
paper with any of the common pH indicators.
One of the first indicators used
for this purpose was litmus.
Litmus paper is paper
that has been treated with a specific indicator - a mixture of 10-15 natural
dyes obtained from lichens (mainly Roccella tinctoria) that
turns red in response to acidic conditions (pH 7).
When the pH is neutral (pH =
7) then the dye is purple.
The first known use of litmus
was around 1300 AD by Spanish alchemist Arnaldus de Villa Nova.
The blue dye has been
extracted from lichens since the 16th century.
The word "litmus"
comes from the old Norse word for "to dye or color".
While all litmus paper acts
as pH paper, the converse is untrue. It's incorrect to refer to all pH paper as
"litmus paper".
Fast Facts: Litmus Paper
· Litmus paper is a type of pH paper made
by treating paper with natural dyes from lichens.
· The litmus test is performed by placing
a small drop of sample onto the colored paper.
· Usually, litmus paper is either red or
blue. Red paper turns blue when the pH is alkaline, while blue paper turns red
when the pH turns acidic.
· While litmus paper is most often used to
test the pH of liquids, it can be used to test gases if the paper is dampened
with distilled water before exposure to the gas.
Litmus Test
To
perform the test, simple place a drop of liquid sample on a small strip of
paper or dip a piece of litmus paper in a small specimen of the sample.
Ideally, you don't dip litmus
paper in an entire container of a chemical. The reason is that the dye could
contaminate a potentially valuable sample.
The
litmus test is a quick method of determining whether a liquid or gaseous
solution is acidic or basic (alkaline).
The test can be performed
using litmus paper or an aqueous solution containing litmus dye.
Initially, litmus paper is
either red or blue. The blue paper changes color to red, indicating acidity
somewhere between the pH range of 4.5 to 8.3 (however, note 8.3 is
alkaline).
Red litmus paper can indicate
alkalinity with a color change to blue. In general, litmus paper is red below
pH of 4.5 and blue above a pH of 8.3.
If
the paper turns purple, this indicates the pH is near neutral. Red paper that
does not change color indicates the sample is an acid.
Blue paper that does not
change color indicates the sample is a base.
Remember, acids and bases
only refer to aqueous (water-based) solutions, so pH paper won't change color
in non-aqueous liquids, such as vegetable oil.
Litmus
paper may be dampened with distilled water to give a color change for a gaseous
sample.
Gases change the color of the
entire litmus strip, since the whole surface is exposed. Neutral gases, such as
oxygen and nitrogen, do not change the color of the pH paper.
Litmus
paper that has changed from red to blue can be reused as blue litmus paper.
Paper that has changed from blue to red can be reused as red litmus paper.
Limitations of the Litmus Test
The
litmus test is quick and simple, but it suffers a few limitations.
First, it's not an accurate
indicator of pH. It does not yield a numerical pH value.
Instead, it roughly indicates
whether a sample is an acid or a base.
Second, the paper can change
colors for other reasons besides an acid-base reaction.
For example, blue litmus
paper turns white in chlorine gas. This color change is due to bleaching of the
dye from hypochlorite ions, not acidity/basicity.
Alternatives to Litmus Paper
Litmus
paper is handy as a general acid-base indicator, but you can get much more
specific results if you use an indicator that has a more narrow test range or
that offers a wider color range.
Red cabbage juice, for
example, changes color in response to pH all the way from red (pH = 2) through
blue at neutral pH to greenish-yellow at pH = 12, plus you are more likely to
find cabbage at the local grocery store than lichen.
The dyes orcein and
azolitmin yield results comparable to those of litmus paper.
Anne
Marie Helmenstine, Ph.D.
· Ph.D.
in biomedical sciences from the University of Tennessee at Knoxville - Oak
Ridge National Laboratory.
· Science
educator with experience teaching chemistry, biology, astronomy, and
physics at the high school, college, and graduate levels.
· ThoughtCo
and About Education chemistry expert since 2001.
· Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
Anne
Marie Helmenstine, Ph.D.
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