Anhydrous, Monohydrate or dihydrate???!!! - Blown apart with this querry!!!! (Jan/21/2010 )

Hi all... today my boss asked me a strange question!!! not tat i dint expect it form him!!! after all her was free!!!

can i substitute anhydrous monohydrous and dihydrate forms or any compound just by adding more or less??? if so why does these forms exist... why not in the same form... wat i managed to think was that may be to go till the anhydrous phase it requires a more sophisticated protocol so the cost will be more as hydration reduces, so we can use any but then why is there three different compounds differing in only water molecule... any special applications??!!! please give ur inputs!!

in general, you can interchange them when preparing solutions, just recalculate based on the different formula weight caused by hydration.

mdfenko on Jan 21 2010, 11:49 PM said:

yeah md.. tat s wat i understand and i replied... may be its just the purification till the anhydrous stage and so different compounds and different prices... but i just wanted to knmow if there can by any specific application of a mono di or anhydrous compound!!!! specially if i wanna make a non-infringing formulation and sumone has patented anhydrous sodium dihydrogen phosphate say... can i use monohydrate or dihydrate instread??? i hope i dint confuse u!!

The compounds usually have differing solubilities, and thus are useful for various applications that might not use water as the primary solvent. Many times you'll also find that one form or another also dissolves more easily in water. Once they're in aqueous solution though, there's no way to tell them apart -- they are the same thing -- which is why you can use them interchangeably in aqueous solutions, and also why you can't get around a patent by making a solution with a chemical that differs from the patented one only in the degree of hydration of the chemicals used.

HomeBrew on Jan 22 2010, 10:35 AM said:

yeah tat seems logical!!!
cause some formulations say one component is anhydrous and the other is dihydrate and both are the excipients of the same formulation to be dissolved in water... any ideas on why do they do this on such cases??!!!

I don't know -- can you post an example? If we're talking about anything other than hydrate, it's a whole different thing -- as you know, sodium monophosphate is different than sodium diphosphate for example; they're entirely different chemicals...

HomeBrew on Jan 22 2010, 07:01 PM said:

Each 1 mL of solution contains
40,000 Units of Epoetin alfa, 2.5 mg Albumin (Human), 1.2 mg sodium phosphate monobasic monohydrate,
1.8 mg sodium phosphate dibasic anhydrate, 0.7 mg sodium citrate, 5.8 mg sodium chloride, and 6.8
mcg citric acid in Water for Injection, USP (pH 6.9 ± 0.3). This formulation contains no preservative.


cant i add a monohydrate of NA phosphate dibasic or a anhydrous NA phosphate monobasic!!!!

You can use any degree of hydration of the monobasic sodium phosphate and the dibasic sodium phosphate so long as you convert to the correct amount to use. Monobasic sodium phosphate is usually available in anhydrous (NaH2PO4) and monohydrate (NaH2PO4.H2O) forms, while dibasic sodium phosphate is available in anhydrous (Na2HPO4), dihydrate (Na2HPO4.2H2O), heptahydrate (Na2HPO4.7H2O), and dodecahydrate (Na2HPO4.12H2O) forms.

For example, the recipe is telling you to use 1.2 mg of sodium phosphate monobasic monohydrate (FW 137.99) -- it needs to specify the hydration state because it is giving you a weight. You would not use 1.2 mg of anhydrous sodium phosphate monobasic (FW 119.98) to make the same ~8.7 mM solution, you'd use 1.04 mg.

But once the solution is made, they're indistinguishable from one another, they're both ~8.7 mM sodium phosphate monobasic solutions.

There can be differences, due to absorption of water from the air once the bottle is open. Some anhydrous chemicals do not remain that way in the open air, and some hydrated ones will lose water of hydration. Of course, sometimes the anhydrous material is intended to remove water from the air (CaCl2 e.g.).

hi hb.. the examploe i quoted is the label claim of a drug tat all use.... so the write up specifically tells to add monohydrate and anhydrous rrespectively.... what i was asking is will there be a specific reason in doing so? or its just that they are the innovators and so thjey used wat they thought is the best??!!!!

yeah phage.. exactly... tat is why i was confuised if tere are siome specific usages of these levels of hydration... using them for non-aqueos solvents is one point tat was made to which i agree... in tat case wat is the use of a mono or a dihydrate??!!!