Chromatography Forum

Have been using a soil carbohydrate sample prep from a book chapter on soil carbohydrates by Martin H. Chantigny and Denis A. Angers - a chapter 50. Three different types of extractions are used:
1. hot water
2. mild acid (0.5M Sulfuric)
3. Strong Acid (12M sulfuric) softening for two hours.

THe procedure calls for a subsequent hydrolysis with 0.5M Sulfuric at 80C overnight (24 hours) and then a neutralization with Sodium Hydroxide 4M solution.

I noticed a brown/orangish precipitate during the neutralization.

Since I analyze with the Phenol/Sulfuric Acid colorimetric procedure, I decided to check the differences between neutralized (with NaOH) and un-neutralized hydrolyzed samples.

The un-neutralized sample yielded a much higher absorbance reading (approximately 40% higher) than the neutralized/filtered sample.

The soil is a clay soil.

Hopefully there is a paper that has addressed this. Perhaps a good inorganic chemist (or better than me) might have an answer to my question.

What complexes with this sample prep in a clay soil might contribute to the false positive in the absorbance reading?

The method blank shows no precipitate or discoloration.

Too lazy to translate, let Google works
Why do you think that, after hydrolysis, adsorption false ?!
You certainly have seen the ground and saw it as organic debris - leaves and roots of plants, dead bugs and spiders.
It is natural that after 12 hours in boiling sulfuric acid, these roots and beetles are hydrolyzed and you will have more organic matter in sample.

Regarding the orange precipitate. It's rust.
Sulfuric acid leaches iron from silicate minerals. Alkali was besieged in Fe(OH)3

____________________________

Dimitry S Petrishev
analytical sevices laboratory
d. med.sc. ;senior expert
Federal goverment state institution S
scientific expertise centre of medical products
127 542 Moscow . Russian federation
Shukinskaja st. 8

Dmitri,
Light absorption, not adsorption. Colorimetric assay with the phenol/sulfuric acid method. I found an old paper that mentioned the purpose of the neutralization of the hydrolized produt was to rid the solution of the di and trivalent cations. As you mentioned - rust. We also have high Aluminum content in our soil in North Carolina. I plan to do elemental analysis on the precipitate. Is not really necessary for the project but, with no definitive feedback off my discussion groups, I will find out for myself.
Thank you for your feedback.

I tried to explain to you that different methods of hydrolysis are obtained different results.
If you rinse with water - then you washes away the only "mobile" carbohydrates, soluble in water. If you boil the 12:00 sample with sulfuric acid-hydrolyzed, and you dissolve any organic fragments, primarily fragments of roots and soil animals, inevitably presence in the upper layers of clay, where it is bordered with a black ground.

In solution, there will be more organic matter and its optical density will naturally be greater.

These are three different methods of extraction. And they give three different results. Characterizing the soil from three different sides.
Water extraction - directly available to soil bacteria of the substance. That's what the bacteria and plants can be eaten at the moment

Extraction of 0.5 m sulfuric acid - easily hydrolyzable carbohydrates remains something of which the bacteria can ses in general.

Extraction of concentrated sulfuric acid is the total amount of carbon in the soil, from the coal fires of the ancient Indians to freshly machine oil.

Definitely aluminum dissolves in sulfuric acid. But it also dissolves in alkalis, with formation of aluminates. And the aluminum oxide is colorless.

But orange are iron oxides. Admixture which is even bokite and white clay. They give the usual clay-brown color.

Can ask the soil-scientists your local university.

I am not go asking in Moscow University and theTimiryazevskuyu Academy . Because I know it from childhood, with 10 years. On my own cottage.

Google translate.

I did three experiments in parallel for each soil sample.
1. The hot water extractable followed by a mild acid hydrolysis, neutralization, Phenol/sulfuric acid color assay. Results about 1 grams per kilogram.
2. Mild acid extractable/hydrolysis , neutralization, Phenol/sulfuric acid color assay. Results about 16 grams per kilogram.
3. strong acid softening followed by a mild acid hydrolysis, neutralization, Phenol/sulfuric acid color assay. Results about 2 grams per kilogram.


step three does not give total carbs, but something between experiments 1 and 2. If this is not to be the case, then we are doing something wrong. Imade certain the acid-slurry was added to the water. I had no standard for 2, but put glucose under 2 mL 12M sulfuric for 2 hours, then did the hydrolysis and lost about 30% of my glucose, so experiment 3 does dedstroy some of the glucose, but have no clue as to how much of the polysacchs from experiment 2 are destroyed, so I've done experiments 2 followed by experiment 3 on the same soil. I will be doing elemental analysis of the different fractions.

I wrote one letter in Russian many years ago when I was in graduate school to a Russian Scientist concerning the mirror cytopathic effect of infected animal cells - a beautiful body of work started by Gurwich (I think).

Thank you for your input. You've been very patient.

Hi
Do not bother. It normal
Concentrated sulfuric acid is a sufficiently strong oxidant, and sulfated
/ oxidizes most organic substances. But glucose in it completely oxidized .

These are three different tests that define the three different indicators of the soil. Give the customer all, his better know what he wants!