Dear Ngo Chan

The paper did not say that 660 nm is the point of maximum absorption.

The concentration's are calibrated at 660 nm.

It may not be the point of maximum but it is clear that the absorbance varies with concentration of silicon in the solution.

It may be that 660 nm is used because of the availability of cheap source at this wavelength.

Best wishes

Dear Ngo Chan

The paper did not say that 660 nm is the point of maximum absorption.

The concentration's are calibrated at 660 nm.

It may not be the point of maximum but it is clear that the absorbance varies with concentration of silicon in the solution.

It may be that 660 nm is used because of the availability of cheap source at this wavelength.

Best wishes

Dear all, please check the attached files and the references therein. My Regards

Dear Ngo Chan

The method attached by Abdelkader BOUAZIZ is very detailed and clear. Hard to believe it's wrong, especially when published by the AOAC.

I assume your no-maximum curves are your Si standards. If so, either you have a very large error in preparation (like 100x?) and have practically no silicon (or practically no molybdate) in these solutions, or your spectrophotometer has a serious problem.

You should have seen the blue/purple colors the paper mentions in the treated standards. If you don't, you have an error in preparation.

If you do see the color, your spectrophotometry doesn't work. You can test this if you wish with a Cu(2+) - ammonia solution, also blue. No hard questions here: a 1-cm light path through a visibly blue solution should give you a peak in the range you show in your plot.

Good luck!

Kindly check https://www.researchgate.net/publication/229098641_Chapter_11_Methods_for_silicon_analysis_in_plants_soils_and_fertilizers

Also check https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=3352&context=gradschool_dissertations

Thank Chinaza Godswill Awuchi and Abdelkader BOUAZIZ Your attached very useful by me and I read all of them. Thank Emanuel Cooper Emanuel Cooper I think as you that AOAC is hard to wrong. In my country, AOAC about available silicon is standard method to determine. But I try many times it is a bad result, I think I wrong something that I don't know. Si standard has Guide 34, and my equiment had certificate of calibration. thank Abdelhalim abdelnaby Zekry , think it is a good idea, may be it is right.

I try scan wavelength again, Maxium absorption is 815 nm. I will increase length of the cuvette from 1cm to 5 cm. I hope it's Ok

Dear Ngo Chan

You can use longer cells but this is not the main problem. Test with a visibly blue solution - anything blue (copper, cobalt, edible color...).eidble . If your spectrophotometer doesn't show you a peak in the 600 +/- 100 nm range, the beam is just not going through th, or the instrument doesn't "see" ite system. You have to figure out why, or call a technician for help.

Conversely, if even your more concentrated standards are not visibly blue then you have a basic problem with the preparation.

Mr Emanuel Cooper I tried with CuSO4 solution, maximum absorption of solution is 812 nm. The CuSO4 solution absorb strongly from approximately 600 to 800 nm. I don't think my equipment have a problem, because I have determined of phosphorus, boron, biuret by this equiment. The result is good. I have attached image of silicon standard solution. It has blue color.

Dear Ngo Chan

Did you try (again) to get the spectrum of the blue standard you show in the picture? It should show an absorbance peak in the right range. If it does, it means something was wrong instrumentally in the previous attempts.