DNS reaction is run at a high pH, unless your buffer is at a way too high concentration the pH of the reaction should remain basic. Since acids are involved, if the buffer molecules can interact with these you can expect a change in color. Use the same buffer in the standards and you should be fine.

Dear Wasan,

The basic method (and the first evaluation of the effect of the difference in amounts of the buffer components on the colour development) is described in the milestone paper by Miller (cited by a staggering 25000 times):

https://s3.amazonaws.com/academia.edu.documents/40180527/Miller_G._Analytical_method_for_determination_of_reducing_sugars.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1551347984&Signature=H5EhjzWGa5COJ70%2B5vcKiiiBbCU%3D&response-content-disposition=inline%3B%20filename%3DMiller_G._Analytical_method_for_determin.pdf

See also the following paper for a critical view on the way the method works:

https://www.researchgate.net/profile/Michael_Bailey/publication/225268296_A_note_on_the_use_of_dinitrosalicylic_acid_for_determining_the_products_of_enzymatic_reactions/links/560d0fae08aec71cb48fa9be/A-note-on-the-use-of-dinitrosalicylic-acid-for-determining-the-products-of-enzymatic-reactions.pdf

Or the following references as examples of the many attempts to modify the method:

https://www.sciencedirect.com/science/article/pii/S0961953412002097

http://www.jmb.or.kr/journal/viewJournal.html?doi=10.4014/jmb.1405.05052

Hope this helps you any further.

Dear Mr. Omar Gonzalez-Ortega and Mr. Rob Keller many thanks for this information . please accept my best wishes