Question

Dilution question In many of the experiments, you will be asked to prepare a standard solution by diluting a stock solution. You will be specifically asked how you will do the dilution steps during the review before you start work. Here is a homework question to get you to think about this process. You have a 1000 ug/mL stock solution of lead (Pb2+). Describe how you would prepare a 100 ppb standard solution from this stock solution. Rules: No single dilution step can be greater than 1:50 You have 25.00 mL volumetric flasks and 10.00 mL volumetric flasks. You have access to three pipet dispensers. One dispenser delivers between 1 and 5 mL, the second dispenser delivers between 100 and 1000 , and the third dispenser delivers between 20 and 200 HL

Answer 1

We could do two 1:50 dilutions and one 1:4 dilutions.

Step-by-step explanation:

Hi there!

A solution that is 1000 ug/ ml (or 1000 mg / l) is 1000 ppm.

Knowing that 1 ppm = 1000 ppb, 100 ppb is 0.1 ppm.

Then, we have to dilute the stock solution (1000 ppm / 0.1 ppm) 10000 times.

We could do two 1:50 dilutions and one 1:4 dilutions (50 · 50 · 4 = 10000). Since the first dilution is 1:50, you will use the smallest quantity of the stock solution (if we use the 10.00 ml flask):

First step (1:50 dilution):

Take 0.2 ml of the stock solution using the third dispenser (20 - 200 ul), and pour it in the 10.00 ml flask. Fill with water to the mark (concentration : 1000 ppm / 50 = 20 ppm).

Step 2 (1:50 dilution):

Take 0.2 ml of the solution made in step 1 and pour it in another 10.00 ml flask. Fill with water to the mark. Concentration 20 ppm/ 50 = 0.4 ppm)

Step 3 (1:4 dilution):

Take 2.5 ml of the solution made in step 3 (using the first dispenser 1 - 5 ml) and pour it in a 10.00 ml flask. Fill with water to the mark. Concentration 0.4 ppm / 4 = 0.1 ppm = 100 ppb.