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.
This method is applicable for the preparation and standardization
of hydrochloric acid solution.
An acidic solution is titrated with a standardized base solution
to determine normality.
Buret, 50 mL, graduated to 0.1 mL Illuminated magnetic stirrer
Volumetric pipet, 40 mL, class A
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
- 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)
- 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.
- Dilute appropriate volume of 36.5 to 38% HCl to 10 L with
carbonate-free distilled water as indicated below and mix
|mLHCl to dilute to 10 L
- Fill a 40 mL volumetric pipet with the acid to be
standardized and discard to rinse the pipet.
- 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.
- 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.
- Add indicator used in Kjeldahl method to the acid
solution and titrate, a drop at a time, with NaOH while
- 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
- Calculate normality and adjust as necessary with HCl or
water. Mix thoroughly and recheck standardization as
- Record standardization in log book.
- 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.
- Tris-hydroxymethyl-aminomethane (THAM), primary base
standard, may be substituted for standardized NaOH in
steps 3 and 4.
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