Story of litmus papers
Story of litmus papers
In chemistry, we
have met so many situations to consider the acidity or the basicity of solution,
or any mixture. Basically in our school classes, we used litmus papers to
consider the acidity or basicity of an aqueous solvent.
In the short form, I was used to remembering
the color change of the litmus paper as in acids blue color change into red and
in basic medium red color change into a blue.
But I was really
curious about the story of this litmus paper, showing a color change due to
this acidity and basicity. In other words, acidity and basicity are represented
as pH values. So litmus paper can represent color change according to different
pH values.
What is the pH scale?
pH is a measure
of acidity or basicity, denoting the potential of hydrogen. Acidic solutions
have lower PH values and basics have higher PH values.
“The pH
scale is logarithmic and inversely indicates
the concentration of hydrogen ions in the solution”
Pure water is
considered as neutral, pH 7. Then when the solution is acidic the value may
below 7, and if the solution is basic it may show the value 7-14. Measured pH
values will lie mostly in the range 0 to 14.
“Since pH is
a logarithmic scale, a difference of one pH unit is equivalent to a tenfold
difference in hydrogen ion concentration.”
A pH indicator
can be used to measure the pH value or the acidity and basicity of a solution.
It may show different colors according to the pH value.
“More precise
measurements are possible if the color is measured spectrophotometrically,
using a colorimeter or spectrophotometer.”
Litmus paper as a pH indicator
Litmus paper is
made of lichens which can show color changes in different pH values. But it
cannot give us a numerical value of pH. For numerical values, we have to use a
pH meter or a universal pH indicator.
A reference
color card can be used to understand the acidity or basicity of a solvent using
a litmus paper, by comparing the color change.
History of the litmus paper
The first time
litmus has used by Spanish physician Arnaldus de Villa Nova in about 1300. It also says that litmus paper was
developed in the early 1800s by a French chemist, J.L. Gay-Lussac. The word
litmus is “to dye or color”, which came from the Old Norse word.
How do litmus papers make up?
Mainly to make
litmus paper, wood pulp, lichens, and adjunct compounds are used. First, papers
are prepared using wood pulp. Then these papers are infused with a lichen
solution. The paper is running through a bath of solution for the infusion.
For the
extraction of dyes of lichens, different kinds of lichens can be used.
- · Roccella montagnei (Mozambique),
- · Roccella tinctoria (South American),
- · Roccella fuciformis (Angola and Madagascar),
- · Roccella pygmaea (Algeria)
Litmus
papers can be found in both blue and red. The original color of the litmus
paper is blue. It can be turned into red using an acidic solution when it is
made.
After the
infusion of lichens, litmus papers are allowed to ferment in the presence of
ammonia and potassium carbonate and then mixed with chalk. This paper may show
in blue color.
Red litmus
paper is made by adding a small percentage of hydrochloric acid or sulfuric
acid.
Litmus papers are then allowed to dry in the
air and paper strips are made and packaging is done.
Advantages and disadvantages of using litmus
paper
Litmus papers
can be easily used to know about the acidity and basicity of a solution. And it
can be provided at a reasonable cost. But using a litmus paper, we can’t get a
numerical pH value of a solution.
Some varieties
of lichens are becoming extinct. So it says that “manufacturers of litmus
paper may switch to synthetic materials in the future”
https://en.wikipedia.org/wiki/Litmus
written by.. Rashmi Premathilake
Comments
Post a Comment