Test Method For Sodium Carbonate ( Soda Ash) Purity

General Consideration:
Soda ash is a technical grade sodium carbonate, Na2CO3 which decomposes to sodium oxide and carbon dioxide at elevated temperatures. However, chloride and sulphate of
sodium in addition to

iron (iii) oxide could be found as traces in the material. It is the main source of sodium used as fluxes, in addition to cullet in glass production, a builder in detergent making, pH adjuster in water treatment etc.

Method of analysis is by carbonate titration using dilute hydrochloric acid of 0.1 or 0.2N concentration.
1.00   Preparation of Sample for Analysis
1.10   Collect small portions from the bulk of the material large enough to yield 1000g, mix together and scoop about 5g into a weighing bottle as in other cases of raw materials, dry in the oven at 110 OC for 1 hour, and cool to room temperature in the desiccator.
2.00    Na2CO3 determination.
2.10    Reagents Preparation
2.11    Hcl (0.2N) – Pipette 16.26ml of the concentrated acid into a 1L volumetric flask containing about 800ml of boiled water, dilute to mark with more water and mix.
 2.12   Sodium Hydroxide (0.1N) – Weigh 4 g of the pellets and dissolve in about 700ml of water, transfer to 1L volumetric flask, dilute to mark and mix.
2.13     Methyl Orange Indicator – Dissolve 0.1 g of the solid reagent in 100ml of water.
2.20   Standardization of 0.2N Hcl – Dry a primary reagent grade sodium carbonate in the oven at 110 OC for 1 hour, cool and weigh 0.5g into 250ml conical flask, dissolve in 50ml of boiled water, add 3 drops of methyl orange indicator and titrate with the acid from a 50ml burette until color change is from yellow to first perceptible red. Boil for 2 to 3 minutes, cool and complete the titration. Record the volume of acid titer.

 2.30   Standardization of 0.1N NaOH – Transfer 25ml of the base into a 250ml conical flask, dilute with 50ml of water, add 2 to 3 drops of methyl orange and titrate with the 0.2N Hcl from a 25ml burette from yellow to the first perceptible red. 1ml of the acid = 2.13ml of the base.
2.40    Indicator Correction – Weigh 2.9221 g of dry sodium chloride and dissolve in 1L of water. Transfer 100ml into a 250ml conical flask, add 3 drops of methyl orange and           titrate with the acid until color change goes from yellow to red. Subtract the volume of       acid used up in the blank titration from that consumed in the titration of the carbonate standard.
2.50     Example:  If in the titration of the primary standard Na2CO3, of 99.5% purity, 0.5g was neutralized by 45.6ml of 0.2N Hcl; Therefore, 1 mole (105.99g) will be neutralized by:

                                                                45.6ml x 105.99g

12.0pt;">                                                                          0.5g

% Purity (99.5%)    =    ml Hcl x f x 105.99g

Where f, is a factor   =   0.50 x 0.995     = 1.0294 x 10 – 4
                                                                     45.6 x 105.99

2.60      Procedure: Boil about 500ml of distilled water and cool to room temperature.
             Weigh 0.5g of the dry sample each into two, 250ml conical flasks, add 50ml of the boiled water, 3 drops of methyl orange indicator and titrate with 0.2N Hcl  solution from a 50ml burette until the solution just begin to change from yellow to red. Boil the titrated solution for 2 to 3 minutes to expel carbon dioxide and cool; the color may change back to yellow indicating that the excess acid has been consumed; add more acid until the first perceptible red is observed. If the color change goes to red directly after cooling, carry out a back-titration with standard 0.1N NaOH until solution changes back to yellow. Subtract the milliliter equivalent of the acid in terms of the base from the milliliter of the total acid used up in the titration. In either case record the value of the true titer and calculate the percent concentration of the carbonate present.
%Na2CO3    =    ml Hcl x f x 105.99 g     x    100
                                                                               0.5 g
            Where, f = factor for acid standardization.
3.00     Iron (iii) Oxide Determination
3.10    Procedure: Weigh 2.0g of the dry sample into a 100ml beaker, add 20ml of water and 7ml of Hcl (1+1); when the reaction subside, heat to boiling for 5 to 7minutes to expel CO2, cool and filter into a 100ml volumetric flask.
3.11     Add in sequence, 1ml of NH2OH.Hcl, 3ml of 1, 10 Phenanthroline and 2.8ml of   sodium            acetate to adjust PH of the solution to 3.2. Also prepare a reference standard of 0.25 mg/L iron and a blank with zero iron containing all reagents specified.      
3.12                 After 5 to 10 minutes, measure off the absorbance or concentration of the sample solution at 508 nm in 1-cm cell; convert readout to milligram of iron by means of a standard curve as in limestone.
%Fe2O3   =        mg Fe2O3       x   100
                                                                       2g x 1000mg

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