Dear Yang,

Humic substances (HSs) are a general category of naturally occurring,

biogenic, heterogeneous organic substances that can be generally characterized as yellow to black in colour, of high molecular weight, and refractory to degradation

(Stevenson, 1994). They consist of several groups of substances that, depending

on their solubility, can be grouped as follows: humin is the HS fraction that is

insoluble in water at any pH; humic acid (HA) is the fraction insoluble in water

under acidic conditions (pH < 2), but is soluble at greater pH; and fulvic acid is

the fraction soluble in water at all pH values (Stevenson, 1994).

According to the above mentioned text; you can dissolve it with the aid of a base such as NaOH, however, when bringing down the pH you should maintain it above pH 2 to prevent any precipitation that might occur under pH 2.

For more information, please use the following link:

http://www.lu.lv/fileadmin/user_upload/lu_portal/projekti/vpp/kuudra/Miresandpeat/mires-peat_175-197.pdf

Hoping this will be helpful,

Rafik

In our lab we dissolve Humic Acids at a pH of about 9 (adjusted by NaOH) on a magnetic stirrer over night. Thereafter we filter it to remove anything that is not dissolved (some alteration of properties of HA may have occurred during processing or storage) and cross-check/verify the total C concentration.

Thereafter we bring it to the desired pH by HCl addition, but as Rafik mentioned not adjusting to low pH as Humic acids will precipitate at low pH. Commercial HA flocculate at pH well above 2 I would say according to our experience. So you need again to verify the concentration after acidification.

Please go through :

Humic acids : Their detergent qualities and potential uses in pollution remediation  By Ray von Wandruszka, Geochem., Trans., 2000, 2

This is available on line. 

Dear Klaus-Holger,

I'm very curious about "some alteration of properties of HA" you mentioned in your answer. Do you have any idea what kind of alternation would happen to humic acid? Also, from the procedure in your lab, will the C concentration be the same before and after treatment?

Thanks,

Yang

Dear Yang Wu,

we mostly did not see much of a difference in concentrations. However, when we go to lower pH (6, 5, 4, 3), concentrations in the final solution tend to be lower than in the pH 9 stock solution. One may imagine that weakly acidic functional groups are getting protonated and cannot be held in solution any more. Depending on what you intend to do with this humic acid solution you need to check if this loss of HA influences your result. E.g. imagin that weakly acidic phenolic groups deprotonate only at high pH while carboxylic groups may be more acidic, you could preferentially loose one of these fractions if at lower pH a fraction of HA will not remain in solution. Which properties this may affect really depends on what you intend to do with your HA solution: e.g. complexation of metals might be affected, as well as electrochemical properties, while concentrations or absorbance may be still quite similar.

The other point is that theoretically all HA should dissolve at pH 9, but usually we see that it does not fully dissolve and a small fraction remains in the filter. Thus during storage and isolation/drying of HA, molecules may have clumped together, which influences their solubility. Ultrasonification may help to overcome this.

Dear Klaus-Holger,

Thanks for your suggestion and answer. They are pretty useful. For my experiment, I need to make a HA solution at pH 7, so I won't bring down the pH too much.

I also heard from someone else that they dissolved HA at pH 11, is there any problem with that high pH value?

Thanks,

Yang

Dear Yang,

There is no problem at all using high pH. Strong basic conditions enhance the solubility of HA.

Rafik

Thanks Rafik.

Excuse me , i dissolved the Humic acid in NH4OH  it is completely dissolved, I used ammonium hydroxide instead sodium hydroxide because it is easily evaporated by lyophilizer  than sodium hydroxide...but when I measured  the binding of humic acid  to mycotoxin at PH 7 it dissolved and i wanna to precipitate it.....can anybody tell me what can I do? 

Can humic acid be dissolved in acetone?

Thank you for the information

We disolve the material in alkaline solution, stirring overnight and then filter. The pH is adjusted to what we want to work with, usually 7.4, and the cocentration is determined at that point. The source of the humic acid and the lot from the same manufacturer makes one do this on a one-by-one basis. Don't assume they are all the same! Best advice is to go to the International Humic Substance Society for your research questions and advice. Get their analyzed reference materials if you are serious. These scientists are the experts.

I want to check the humic acid absorption to bentonite clay. I may have to go for lower pH like 4, 5 and 6. How do I minimize the alteration of HA to reach the target??

In theory, humic acid precipitation occurs at pH below 2. To track changes in HA concentration as a function of the pH, you can measure the carbon concentration at pH 4, 5, and 6, and consider any carbon lost in your final analysis.

Alternatively, if the carbon concentrations are significantly different (which I doubt), you can prepare higher carbon concentrations until you achieved the final concentration you need at a given pH.

Thank you very much Farith Diaz, The answer is very useful

This helpful! Thanks, all!

This is quite tricky thing. Any change of pH cause fractionation of humic acid, changing electric charge on the macromolecules of humic acids. I would use fluorescence, UV-Vis spectroscopy, may be Raman spectroscopy to control it.

Can anyone tell/know/estimate the maximum solubility of HA in MQ water? thnx

can anybody tell me which is the best product economically to dissolve humic acid in peat soil and also has less environmental impact?thanks in advance

Hi everyone, I need to prepare a Standard solution of Humic and Fulvic Acid to check their adsorption. We are thinking of 5 mg/l concentration for both. Can i do this simply with water? We are not looking for any pH specifically.

Thank you.

Mithila

Farith Diaz which characterization instrument should I use to measure carbon concentration. Thanks

Abdelmalek Bellal Are you measuring carbon concentrations in water? I used a Shimadzu TOC analyzer

Farith Diaz Thank you. Yes, my solvent is potassium carbonate in water. Do you have idea on how I can measure the concentration of potassium carbonate also?

Abdelmalek Bellal I think you can measure potassium using a flame photometry, although an ICP-MS could work too. Regarding CO3, you can use a TOC analyzer to measure CO2 from bicarbonates and carbonate. Check this out: https://www.researchgate.net/post/How_to_analyse_carbonates_present_in_the_water_How_it_differs_from_carbonate_alkalinity

Farith Diaz I appreciate your recommendations. Thank you

Abdelmalek Bellal You are welcome.