Understanding Blanks
Using Spectrophotometers and
Balances
Institute for Systems Biology &
Bellevue School District

What are the steps you take to find mass of 2 tablespoons of salt using a beaker, balance and salt?

1. Put the beaker on the balance.
2. Zero your balance
3. Add salt
4. Read your balance to find mass of salt.
0.0 g
Why do you have to zero your balance after putting your beaker on it?
salt

Your beaker has a certain mass. You want to find mass of salt alone, therefore you will have to zero your balance to find mass of salt alone.
In other words, you are ignoring mass of the beaker.

You have to do the same thing when using a spectrophotometer.
You will first have to use a blank to zero your spectrophotometer ( use water for the food coloring lab)
Since still some light will be absorbed by your blank, you will have to set your spectrophotometer to zero.
Your final reading of your samples indicates the amount of light difference that is absorbed between your sample and the blank.

Light shines at sample in cuvette
Some light is absorbed by the glass cuvette and water
Light inside spectrophotometer Cuvette
(Blank)
0.01
O.D.
Spectrophotometer measures how much light was absorbed
You will then have to change your reading to zero.

Light shines at sample in cuvette
Light inside spectrophotometer
Some light is absorbed by the glass cuvette and water AND THE
FOOD COLORING
0.34
O.D.
Spectrophotometer measures how much light was absorbed

Answer the following question in your journal.
Compare using blanks in spectrophotometers with using blanks while finding mass of a substance .

Important to Remember
• While using spectrophotometer, always use blank between your different samples to zero your reading.