Increase the pH would be my first choice. NaB(OH)4 is much more soluble.

Another method might be to heat the solution to 50-80 C where boric acid is much more soluble. After cooling the supersaturated solution seems stable for some time.

Increased pH can give a stable solution with higher B concentration.

Increase temperature to 50-80 C can give you a stable supersaturated solution with higher concentration.

thanks Dr andersen ...... increasing temperature is not practical solution because i prepare a liquid fertilizer, and boric acid will undergo recrystallization after the temperature decreaseو, are there a chemical compound that can be dissolved and give me B % stable solution higher than 10%

Then go for the higher pH.

Google gave a NaB(OH)4 solubility of 55 g/L at room temperature, but 130 g/L at 40 °C.

Dissolve boric acid use dil HCl  then dilute with water.

After long hard work and tests, i got the procedure that can give me the targeted result. i can do this by using Ethanolamine. bY using Ethanolamine i can dissolve an a mount of Boric acid may reach 900 gm\L, By this procedure we can get a liquid fertilizer that has a percent content of elemental boron (B) more than 15% ....(W\V).Note that liter of this unique fertilizer composed of (boric acid +water+etanolamine)

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Dear Hatem you access the best possible solution to dissolve Boric acid in aqueous solution of Monoethanolamine to get your Boron contents.

Thank you Mr. Saeed Happy to contact

A solution containing 300 g of glycerin, 200 g of monoethanolamine and 100 g of water is prepared beforehand with vigorous stirring. When the mixture is entirely homogeneous, 400 g of disodium octaborate tetrahydrate are added and the mixture is heated at 80° C. for two hours with vigorous stirring. Return to room temperature gives a solution containing 8.4% of boron B.

Dr. Mahmoud Abdelaal, Thanks for your response

I  started my work with the method you mentioned previously, but it just give a maximum conc. 8.4% boron, the product is highly viscous, not stable (may pericipitate after a few weeks) and disodium octaborate tetrahydrate is not common.

befadl ellah , i have got the procedure that can give me 15%  or more of boron content  using Ethanolamine only and boric acid ,no heating, no glycerin.

the product is clear, very stable and with acceptable viscosity according to its content of boron 

Dr.Hatem 

i'm so happy to hear that from you and 

i would like to know more about your procedure if that available

Dear Mohammad and Hatem

The solubility of boric acid was increased if used with Borax with 4:5 weight ratio 

Dear Dr. Hatem  You can achieve 10 % or little above soluble boron by dissolving in aqueous solution of Mono Ethanol Amine (MEA) . If you take 20 g of distilled/ Deionized/ RO water and dissolve 20 g of MEA during stirring till whole of MEA is dissolved. The during stirring , slowly start adding  Boric Acid . Total amount of 60 g of Boric acid has to be dissolved to get 10 % Boron Solution. When by adding boric acid in increments is dissolve, keep it preferably over night , it will give you clear trans parent solution of 10 % Boron..  

Hi Dr. Hatem, I want to Know can we prepare Boronic Acid from Boric Acid? If yes How we make it? 

Hi Dr. Hatem , I want to ask that in method of preparing 900g/litre of boric acid solution how much water and ethanolamine is added in that one litre ?

Dear Mr Hatem Ashour I want to ask you what is the quantity of MEA and water is to be added to make 10% boron solution W/W. I have make 10% B solution w/w but after 12 hours crystal growth is seen at the bottom so how can i solve this problem of crystallization.

Dear Dr. Hatam,

I was wondering if you could tell me whether you increased solubility over 10% or not??

Solubility of boric acid in water increases with temperature (0 ºC to 100 ºC)

Hello, adding citric acid, or ascorbic acid, or glycerol, will increase the solubility of Boric Acid in water.