Definition Choosing a Standard Solution Making the Solution

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…

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 can’t react with air or water or itself…) It must have a known formula (i.e. CuSO 4.5H 2 O) It isn’t allowed to absorb water from the air (it can’t be hygroscopic) We must be certain of the fast, complete, balanced reaction with the chemical of unknown concentration It must be pure (analytical quality)

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 = M r n Example: We want 250ml of 0.100molL -1 Na 2 CO 3 1. c = 0.100molL -1, v = 0.250L, n = ? 2. n = cv = x = mol 3. n = mol, M r = (2x x16) = 106gmol -1, m = ? 4. m = M r n = 106 x = 2.65g  In other words we need to add 2.65g of the Na 2 CO 3 to 250ml of water to make a 0.100molL -1 solution

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 1. Accurately weigh the mass out with a balance onto a piece of filter paper or beaker. Write down the exact mass you used. 2. Use a funnel to transfer the mass to a clean, correct sized volumetric flask i.e. 250ml. Ensure that you do not drop anything. 3. Wash off the paper/beaker and funnel with distilled water from the wash bottle. All of the mass must end up in the flask.

Making the Standard Solution – The Process 4. Fill the flask ¾ full of water. Stopper and invert to mix until dissolved. 5. When dissolved, carefully fill up to the mark with distilled water (remember the flask is made up to the mark adding the last few drops of water using a dropping pipette.) 6. Invert to mix several times to ensure thorough mixing of the solution.. 7. Calculate the concentration (based on the mass and volume you used) and write it onto the flask.