indicator is any substance used to assist in the
classification of another substance. There are
many different kinds of indicators. Some common
kinds of indicators used in Living Environment/Biology will be
The pH Scale
and bases (alkalis) are common substances studied
The pH scale is used
to indicate the relative strength of an acid of
base. The pH scale goes from 0 to
14. A pH of 7.0 is considered to be
neutral. The greater the pH is than 7.0, the
more basic the substance is. The lower the pH
is below 7.0, the more acidic a substance
Stomach acid has a pH of
Some Common Indicators
paper turns red or a shade of red in acids.
Litmus turns blue or a shade of blue in bases.
It is important to place a few drops or a small amount of
the substance to be tested on the litmus paper when
testing it. Do not dip the litmus paper in the
substance to be tested. A paper which provides
a more specific indication of the pH level of a substance
is pH paper. This paper turns different
shades of various colors which may be compared to a scale
to determine the pH value.
|2. Bromthymol blue is an indicator
used to show the presence of either carbon dioxide in
solution or an acidic solution. Low levels of
carbon dioxide or acid will result in the bromthymol blue
solution remaining blue, while higher levels of carbon
dioxide or acid will result in the bromthymol solution
taking on a yellow tint. Frequently this
indicator is used in biology labs to indicate
photosynthetic activity (solution turns blue as CO2 is
used) or respiratory activity (solution turns yellow
as CO2 is added to the solution).
|3. Lugol's solution (which is
actually IKI) is a brown solution which turns black in the
presence of starches. The test tube at
the right shows Lugol's (iodine) solution mixed with a
|4. Benedict's solution is used
to detect the presence of simple sugars such as
glucose. When a simple sugar is mixed in
Benedict's solution and heated for a short period of time
in a test tube, it goes through a variety of color
changes, eventually ending as an orange-red or brick red
color. The use of Benedict's solution before
and after using it to detect the presence of the simple
sugar glucose is shown in the pictures on the right.
Very frequently it is helpful to dye certain cell structures so that
they can be seen more clearly. Chemicals that dye parts of
cells for this purpose are called stains. Two
commonly used stains in the biology laboratory are Lugol's
iodine solution and methylene blue. Lugol's
solution is a good stain to make the nuclei of plant cells
stand out more prominently. It has the unfortunate
drawback of killing the cells it used on however.
Methylene blue is often used to stain animal cells, such as
human cheek cells, to make their nuclei more
observable. It is vital dye which does not
immediately kill the specimen.
Using Stains in Biology
|These are plant cells stained with Lugol's
solution so their nuclei are visible.
|These are human cheek cells stained with
methylene blue solution making their nuclei and outlines
much more visible.
Correctly Staining Specimens
- A specimen is obtained
and placed on the slide with forceps. A cover
slip is then lowered on to the specimen from an
approximately 45 degree angle gently. This
reduces the number of air bubbles the specimen will have.
The student then places a drop or two of water on the
- The student places a drop of stain beside and
under one corner of the cover slip.
- The student places a towel on the opposite side of the
cover slip in the water beside the cover slip.
This will draw the stain through the entire specimen in a
few seconds without removing the cover slip.. This technique will also remove any
air bubbles which have formed. The stained specimen may
now be observed. Note that this technique can be
used to draw salt water or distilled water into a specimen
having a cover slip over it without removing the cover
slip as well.