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Can't make a borate solution. Ideas? - (May/03/2006 )

Hey guys --

I am trying to set up to perform a reaction to assay for GlcNAc concentration using an assay variously known as the Morgan-Elson, DMAB, or Reissig assay, but I am having all kinds of trouble in preparing the required borate reagent. There are many papers out there that use this reaction (the original paper is from 1955), and almost all of them say they use a 0.8 M borate solution, sometimes made from potassium tetraborate tetrahydrate (K2B4O7.4H2O, MW 305.5), and sometimes made from sodium tetraborate decahydrate (Na2B4O7.10H2O, FW 381.37), sometimes with pH adjustment to 9.1, and sometimes using just the unadjusted solution (pH ~10.5 for the potassium salt at 0.8 M).

One paper (Anal Biochem 1997 254(2):240-8) even spells out quite explictly that they used "0.8 M tetraborate (244.4 g of K2B4O7.4H2O (MW, 305.5) per liter, adjusted to pH 9.1 – 9.2 with HCl)".

My problem is that I cannot make a solution of that molarity, as it will not dissolve -- even after pH adjustment to 9.1 with 10 N HCl, or with stirring overnight.

Information I have found indicates that the solubility of potassium tetraborate tetrahydrate is 15.8 g/100 ml at 20°C, thus the 24.4 g/100 ml I'd need to make a 0.8 M solution has no hope of dissolving, at least at room temperature in water.

I've had no luck making 0.8 M solutions using other borate salts either -- I'd need 30.5 g/100 ml to prepare a 0.8 M solution of the sodium decahydrate salt, but this again far exceeds this chemical's solubility in water (5.1 g/100 ml at 20°C). The anhydrous sodium salt (Na2B4O7, FW 201.22) is even worse -- I'd need 16.1 g/100 ml needed again exceeds the solubility in water (2.56 g/100 ml at 20°C).

None of these will go into solution, even after adjusting the pH as indicated. What obvious thing am I missing here? Do I need to heat the solution? If I do that, won't it just precipitate out as it cools?


i haven't tried it but if you heat it you may be able to make a supersaturated solution. it may be able to stay in solution until it is disturbed or it may be more soluble at the adjusted pH (after it is all in solution).


it's maybe a silly point, but when i'm preparing TBE 5X for electrophoresis from a powder pouch, i need to let it overnight agitating with magnetic device, and heat it to 50°...


my old lab chemistry table say that the solubility of Na2B4O7 (waterfree) at 20°C is 2.56 g, at 50°C 9.78 g, at 80°C 25 g and at 100°C 39.5g, all per 100 g water. So there is a good relation between solubility and temperature. But I don't know if you can use an about 80°C warm solution...



Well, I gave the potassium tetraborate tetrahydrate a bit of heat (on a hot plate) and it went right into solution (24.4 g in 80 ml dH2O). I left it on my bench to cool to room temperature, and it seems to remain in solution. I was thus able to pH the solution at the correct temp, and adjust final volume without issue.

Now, it's been sitting on my bench overnight -- I'll see when I get in this morning whether it's precipitated out...