Substituting anhydrous for monohydrous and vice versa

[Previous Community Member]

Heyo!

I'm preparing a growth medium for mold, following the recipe below.

Dissolve the following ingredients, in the order listed, into 500ml of cold tap water: 44.0 grams Lactose Monohydrate, 25.0 grams cornstarch, 3.0 grams sodium nitrate, 0.25 grams magnesium sulfate, 0.50 grams potassium phosphate mono, 2.75 grams glucose monohydrate, 0.044 grams zinc sulfate, 0.044 grams manganese sulfate. Then add enough cold tap water to make one liter. Use hydrochloric acid to adjust the pH to between 5.0 and 5.5.

But, We're short on chemicals. We don't have lactose monohydrate, but we have lactose (anhydrous). We don't have glucose monohydrate, but we have dextrose anhydrate. We don't have anhydrous zinc sulfate, but we have zinc sulfate (monohydrous). Is it possible to substitute these chemicals in with the ones we have on hand? Is there a way to convert the chemicals on hand to their monohydrous/anhydrous form?

[RGHayter]

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[scooke]

Dear Drew,

Yes.  Because it is all going into an aqueous solution anyway, the level of hydration of the compounds doesn't really matter.  For accuracy you should adjust the amounts to similar molar ratios.  But in practice I don't think that the mass change +/- for one water molecule is going to affect the overall medium effectiveness if you just use the original mass values.  They will ALL become fully hydrated when added to the 500mL of water, so the starting form doesn't really matter.

Best regards,

Steven Cooke

[Previous Community Member]

Thanks Stephen! We corrected the amount we were using, and it seems significantly better.

The only problem now (which we also had with the previous batch) is that the particulate will collect on the bottom and leave a clear liquid on top after ~15-20 minutes. What could be causing this separation? Heating it dissolved the particulate, but it soon appears again. I assume the mixture is supposed to be uniform, not separated.

[tomflynn]

Dear Drew,

The particulate matter you are observing could result from your use of tap water rather than distilled or de-ionized water to make your medium. Your tap water may be "hard", which means that it has a high content of calcium. Calcium will combine with the phosphate ions from your potassium phosphate to form either insoluble calcium phosphate, or if the pH of your tap water is alkaline, highly insoluble calcium hydroxy phosphate (also called hydroxyapatite). Once these insoluble phosphates form, it is nearly impossible to get them back into solution without using conditions that could be detrimental to other components in your medium. So, my suggestion to you would be to try distilled or de-ionized water to make your medium and see if that solves the problem of the particulates you are observing.

Best regards,

Tom Flynn