Help with preparing Congo Red TSA?

Question

So I want to make Congo Red TSA. My instructor gave some hints but I still have some questions: 1. I start by making two stock solutions: 20 mg/ml of congo red and coomassie in two different falcon tubes. To prepare 30 ml of each, would I dissolve 600 mg in sterile water? 2. The final concentation in a 500 ml TSA solution needs to be 40 μl/mL of CR and 20 μg/mL of coomassie. How do I calculate the amount of solution needed for the TSA?

Answer 1

Hey there,

You're on the right track! This is a concentration problem, so you'll want to use C₁V₁=C₂V₂

For the first step, you're absolutely right--you just take the total ml (30 ml) and multiply that by the concentration you want of your solute (20mg/ml).

(ml solvent) * (final concentration) = (mass solute)

30 ml H₂O * 20mg/ml = 600 mg of your solute needed, so congo red or coomassie.

For part 2, you'll want to set up two equations: one for the coomassie, and one for the congo red.

You have the final concentration that you want, as well as the final volume you want, so you can plug that in to C₂V₂

For coomassie, that looks like (20 micrograms/ml * 500ml) and for congo red, that looks like (40 micrograms/ml * 500ml). After doing those equations, you'll have a concentration in micrograms (μg) that you'll need to convert to mg to get the correct measurement.

For coomassie, that looks like (20μg/ml *500ml) = 10,000μg needed in 500ml, which you can then convert to mg through dividing by 1000 (mg is 1000 times μg), which gives you 10mg.

That means you'll need 10mg total of coomassie added to 500ml of sterile water to make a concentration of 20μg/ml.

So, now that we have the far side of C₁V₁=C₂V₂, we just need to do some algebra and isolate V₁, as that is the volume of our stock solution from the first part that we'll need to add.

C₁V₁=C₂V₂ becomes C₁V₁=10mg

C₁ we know from our stock solution of coomassie: C₁ = 20mg/ml

So, C₁V₁=C₂V₂ becomes (20mg/ml) * V₁ = 10mg

And then by dividing the whole equation by C₁ (20mg/ml), we get V₁ = (10mg)/(20mg/ml) = .5ml of stock to add to 500ml of sterile water to get a coomassie concentration of 20μg/ml.

You can do the same overall process to figure out the amount of Congo Red stock solution you'll need to add.

If you get tripped up in the process, here's a cheat sheet for the steps:

1) Determine C₂V₂ by taking the final concentration and multiplying by the final volume you want

2) Take your initial concentration, C₁, and divide C₂V₂ by your initial concentration--this will leave you with V₁ on one side of the equation.

3) Add V₁ of your stock solution to V₂ of your sterile water, and you have the concentration you want!

Hope that helps you or others (I have the distinct feeling you may have figured this out by now, given that this question was asked quite some time ago).

Best,

Kai