Standard Solutions

1
Standard Solutions

• Definition
• Choosing a Standard Solution
• Making the Solution

2
Standard Solutions

• A standard solution one that we know the exact
  concentration of
• They are used in a volumetric analysis to help us
  calculate the concentration of another solution.
  Therefore we must be certain of the concentration
  of our standard
• Only certain chemicals can be used in the
  production of a standard solution

3
Choosing a Standard

• Before we make up a standard solution we must
  make sure of a few things first
• The standard must be chemically stable during
  storage (i.e. it cant react with air or water or
  itself)
• It must have a known formula (i.e. CuSO4.5H2O)
• It isnt allowed to absorb water from the air (it
  cant be hygroscopic)
• We must be certain of the fast, complete,
  balanced reaction with the chemical of unknown
  concentration
• It must be pure (analytical quality)

4
Making Up The Solution - Calculations

• First use c n/v to calculate the amount and
  volume required to make your standard solution.
• Then turn the amount into mass using m Mrn
• Example We want 250ml of 0.100molL-1 Na2CO3
• c 0.100molL-1, v 0.250L, n ?
• n cv 0.100 x 0.250 0.0250mol
• n 0.0250mol, Mr (2x23 12 3x16)
  106gmol-1, m ?
• m Mrn 106 x 0.0250 2.65g
• In other words we need to add 2.65g of the Na2CO3
  to 250ml of water to make a 0.100molL-1 solution

5
Making the Standard Solution The Process

• Each step of the process is design to ensure the
  right mass of the standard gets into the right
  volume of water
• Accurately weigh the mass out with a balance onto
  a piece of filter paper. Write down the exact
  mass you used.
• Use a funnel to transfer the mass to a clean,
  correct sized volumetric flask i.e. 250ml. Ensure
  that you do not drop anything.
• Wash off the paper and funnel with distilled
  water from the wash bottle. All of the mass must
  end up in the flask.
• Fill the flask ¾ full of water. Stopper and
  invert to mix until dissolved.
• When dissolved, carefully fill up to the mark
  with distilled water. Invert to mix.
• Calculate the concentration (based on the mass
  and volume you used) and write it onto the flask.