# Mixing of powders to make stock solutions

For a solution such as:

Luria-Bertani (LB) Broth:

10 g tryptone
5 g yeast extract
10 g of NaCl ($M = 58.44\ \mathrm{g/mol}$)

q.s. to 1 l, pH to 7.2, autoclave.

I would like to make a bulk amount of premixed powder, i.e. combine 1 kg tryptone, 500 g yeast extract and 1 kg NaCl in a big bottle, shake it and keep it on shelf. Whenever I need to make LB now I just weight 25 g of this premix and add to 1 l of water.

I’m concerned about a couple things thought. First how thoroughly it gets mixed, and second any settling of different chemicals over time. Either would cause many different batches to be messed up.

I’m wondering if anyone has experience with this and ways to avoid those two problems. I’m interested in doing this for other solutions as well. Do manufacturers who make these premixed powders re pulverize them or process them in a certain way to make it work?

Companies that sell such mixtures usually make them by dissolving everything into a solution, lyophilizing the solution, and then grinding the lyophilized residue. This guarantees that the size of particulates in the mixture will not be related to the chemical identity of the particulates.

If you have to mix powders, I wouldn't recommend making such a large batch. Why not make a smaller batch, enough for 10 - 20 bottles of LB, mix thoroughly by shaking (which will still not be as good as lyophilizing), and then immediately aliquot 25 g portions into a series of dry 1 L bottles? Then you can store the dry bottles until you want to make some LB, and you don't have to worry about the powder settling etc. in each bottle once it has its own charge of powder.

Once the powders are mixed, it is unlikely that they will separate unless the container is subjected to vibrations.

As for mixing, companies that need to mix batches of material use a mixer to make their batches. The second photo in the article is similar to what I have seen.

• I'm still hesitant, there's vibration everywhere, this mix will prob last 5 years and I don't have one of those awesome mixers. Especially with solutions where some components are very hydroscopic. I guess I'm stuck with aliquoting a single premix – rhill45 Sep 7 '14 at 1:15
• It doesn't need to be so "awesome". Even a rolling bottle will eventually mix the material. – LDC3 Sep 7 '14 at 1:20
• There is no guarantee that the equilibrium distribution of particles in the mixture is uniform. Mixing powders is generally a bad idea. – Curt F. May 15 '15 at 18:50
• @CurtF. Every tablet was a mixture of powders before it was compressed into a tablet. Are you saying that there is usually a large variation in the dose between tablets? – LDC3 May 16 '15 at 1:00
• Yes, that is what I am saying. It is actually a major problem in the drug industry and in the USA at least, the FDA is tightening quality control standards. Pill to pill variation in dosage can be 10% for some products. – Curt F. May 16 '15 at 3:36