I think If we use high concentration of HNO3, it will  produce H3PO4 which is colorless in nature and may cause error in detection

Not quite clear at what your question is targeting: in principal its more a convention than a solid scientific based reason why different acids are used!

Can you please give some more details of methods involving HNO3 or HCl you are mentioning/referring?Is it in diacid digestion or colour /turbidity development?

Dear A S Rao,

In case of rice plant P and S determination, we use diacid (HNO3, perchloric) digestion. After digestion, we take digest in a test tube then use 2N HNO3 for P determination, my question why not other acid and why 2N ? After use 2N HNO3, we add color reagent (Ammonium vanadate and Ammonium molybdate). After color formation, we measure color intensity by spectrophotometer.

For S determination, we add 6N HCl to digest. Why HCl and 6N? For color formation we use BaCl2 (Saturated or 10%) and measure color intensity. 

I think this will help you to answering my question as well Ewald Schnug.

In case of S we measure the turbidity.

As I understand both 2N HNO3(for P estimation and 6N HCl (for S estimation) are added in the colour developing or keeping barium sulphate precipitate  in suspension.For estimating P by vanadomolybdate yellow colour method,for stable colour development(for complex formation) and to avoid interference from iron and silicon,certain strength of acidity need to be maintained in the medium .In the final solution plus reagents, the acid strength is around 0.5M.In some procedures perchloric acid(60%) 8.5 -9 ml is used) in place of HNO3.

The precipitation of BaSO4 and the resulting turbidity is a sensitive process.BaSO4 tends to precipitate if thesolution is not properly acidified but if the solution is too acidic,the precipitation of BaSO4 is low. 6N HCl is added to maintain medium pH.The acidity maintenace ,addition of stabilizing agent and BaSO4 crystals help in stabilizing turbidity.

The following two books may provide more details.

A Text Book of Soil Chemical Analysis by P.R.Hesse John Murray( Publishers),London .1971

Methods of Analysis of soils,plants,waters,fertilizers & organic manures Edited by HLS Tandon Fertilizer Development and Consultation Organization. 204-204A Bhanot Corner,1-2 Pampose Enclave NewDelhi.E-mail:[email protected] or [email protected]

Greetings again my friend,

I believe that you have a very good question. To my mind the acid extractant for P is way too strong. Do you have any meaningful calibration data for this extractant??

Cheers for now and keep well,

Mart.

I agreed with Rao's discussions. I think M Iqbal you are now clear about the reason for using HNO3 and HCl  for P and S determination.