Regents Prep: Living Environment: Laboratory:
Indicators and Stains

An 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 indicated below.

The pH Scale

Acids and bases (alkalis) are common substances studied in science.

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 is.

Stomach acid has a pH of approximately 2.0.


Some Common Indicators

1.  Litmus 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 starch suspension.

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

  1. 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 specimen.
  2. The student places a drop of stain beside and under one corner of the cover slip.
  3. 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.

Created by James M. Buckley, Jr.
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