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3.1.1 Standardization of Hydrochloric Acid Standard Solution

Reference:BR> Standard Solution of Hydrochloric Acid. (936.15) Official Methods of Analysis. 1990. Association of Official Analytical Chemists. 15th Edition.

Scope:
This method is applicable for the preparation and standardization of hydrochloric acid solution.

Basic Principle:
An acidic solution is titrated with a standardized base solution to determine normality.

Equipment:
Buret, 50 mL, graduated to 0.1 mL Illuminated magnetic stirrer Volumetric pipet, 40 mL, class A

Reagents:
Hydrochloric acid, concentrated, 36.5 to 38%, reagent grade Sodium hydroxide, standard solution (see method 3.1.2) Distilled water, carbon dioxide (CO2) free prepared either:

• 1) by boiling for 20 min and cooling with soda-lime protection or
• 2) by bubbling air, freed from CO2 by passing through tower of soda lime, through water for 12 hr. Indicator, same as used in Kjeldahl method (3.1)

Safety Precautions:

• Handle acid safely: Use acid-resistant fumehood; always add acid to water unless otherwise directed in method; wear face shield and heavy rubber gloves to protect against splashes; if acids are spilled on skin, immediately wash with large amounts of water.

Procedure:
Preparation

1. Dilute appropriate volume of 36.5 to 38% HCl to 10 L with carbonate-free distilled water as indicated below and mix well:

Standardization

1. Fill a 40 mL volumetric pipet with the acid to be standardized and discard to rinse the pipet.
2. Withdraw a 40 mL aliquot of the HCl into the volumetric pipet. Wipe the tip of the pipet before transferring the acid into a 250 or 300 mL Erlenmeyer that has been rinsed with CO2-free H2O. Do this in triplicate.
3. Rinse a Schellbach buret with standardized NaOH of approximately same concentration as acid to be standardized. Fill the buret and wait for at least a minute (to allow the solution to drain down the inside walls) before taking an initial reading. Always read the buret at eye level and at the tip of the meniscus.
4. Add indicator used in Kjeldahl method to the acid solution and titrate, a drop at a time, with NaOH while stirring.
5. Titrate to orange endpoint (color change from red to orange to yellow). Use of an illuminated background is helpful. When the endpoint is reached, remove any drops from the buret tip (by touching the flask to the tip) and wash down the sides of the Erlenmeyer with CO2-free H2O to see if the color persists. If it doesn't, add another partial drop of NaOH and check the color before reading the buret.
6. Calculate normality and adjust as necessary with HCl or water. Mix thoroughly and recheck standardization as described above.
7. Record standardization in log book.

Comments

• The titration values of replicates should be within 0.05 mL of each other.
• Normality will be exact only if the same indicator is used in determination as in standardization.
• Several other standardization procedures are available. See AOAC. 1990. Official Methods of Analysis. 15th Ed. 936.15.
• Tris-hydroxymethyl-aminomethane (THAM), primary base standard, may be substituted for standardized NaOH in steps 3 and 4.

Calculations:
Normality of HCL = mL of Base Std x Normality of Base Std mL HCl in the flask
Base Std = NaOH or THAM primary base standard
If the normality is too high, dilute solution to required normality by following formula:

V1 = V2 X N2 / N1

Where N2 and V2 represent normality and volume of stock solution and V1 equals volume to which stock solution should be diluted to obtain desired normality, N1

3.1.2 Standardization of Sodium Hydroxide Standard Solution >>

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