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Measurement of Silica by Molybdosilicate Method water and waste water samples.

 

    Measurement of Silica by Molybdosilicate Method water and waste water samples.



ô PURPOSE:

                    To describe the Laboratory standard operating procedure for measurement of Silica by Molybdosilicate Method water and waste water samples.

                     

ô SCOPE:

                    Applicable to the Laboratory for analysis of water and waste water samples, where measurement of Silica is required.

 

ô RESPONSIBILITY:

Lab chemist

:

Analysis of sample for measurement of Silica.

Technical Manager

:

Review of activity

Quality Manager

:

Implementation and compliance of SOP.

                     

ô PROCEDURE:

1.     Principle:

·       Ammonium Molybdate at pH approximately 1.2 reacts with silica and any phosphate to produce heyteropoly acids. Oxalic acid is added to destroy the molybdophosphoric acid but not the molybdosilicic acid, even if phosphate is known to absent addition of oxalic acid is mandatory.The intensity of yellow color is proportional to the concentration of “molybdate-reactive” silicat.. Unreactive silica occurs in water which can be converted into reactive silica by heating or fusing with alkali.

 

2.     Instrument and Equipment:

·       Platinum dishes,

·       Colorimetric Equipment,

·       Spectrophotometer, for use at 410 nm, providind a light path of 1 cm or longer.


3.     Reagents:

·       Hydrochloric acid, HCl, 1+1.

·       Ammonium Molybdate reagent: Dissolve 10 g (NH4)6Mo7O24.H2O in water, with stirring and gentle warming, and dilute to 100 mL. Adjust to pH 7 to 8 with Silica free NH4OH or NaOH.

·       Oxalic Acid solution: Dissolve 7.5 g H2C2O4 .H2O in water and dilute to 100 mL.

·       Stock Silica Solution: Dissolve 4.73 g sodium metasilicate nonahydrate, Na2SiO3.9H2O, in water and dilute to 1000 mL.

·       Standard Silica Solution: Dilute 10.00mL stock Solution to 1000mL with water; 1.00mL = 10.0 µg SiO2.

4.     Test Method:

·       To 50.0 mL sample add in rapid succession 1.0 mL 1+1 HCl and 2.0 mL ammonium molybdate reagent. Mix by inverting at least six times and let stand for 5 to 10 min. Add 2.0 mL oxalic acid solution and mix thoroughly. Read color after 2 min but before 15 min, measuring time from addition of oxalic acid.

  1. Calculation:

·       Calculate the concentration of Silica in the sample as follows:

 

mg  SiO2 /L  =       µg SiO2 (in 55 mL final Volume)/ mL sample

         

ô EHS Instructions (Wherever applicable):

 

     1:  Use gloves while handling acids, alkali, solvents & Hazardous waste .

 

     2:  Handle all glassware with care to avoid OHS incident.

 

     3:  Use of apron wherever necessary

 

     4. Avoid spillage of chemical / waste sample

 

 5. Take precaution to avoid the fire incidents.

 

      6. Ensure the disposal of Hazardous waste sample.

 

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