What is a secondary standard solution?
A secondary standard solution is a solution which must be standardised first against a primary standard and afterwards it will be stable enough for titrimetric work. Potassium permanganate is an example of a secondary standard. It has to be standardised first, but then it can be used for quantitative analysis,
A primary standard substance will not always be used in standardisation; this is because primary standard is a reagent which is very pure, representative of the number of moles the substance contains and easily weighed. For example sodium chloride is used as a primary standard for silver nitrate reaction. For example potassium hydrogen phthalate is used as a primary standard but because weak acid, wouldn’t be used to standardise a weak alkaline, like ammonia solution.
Sodium thiosulphate (Na2S2O3) is an easy titrant for many redox titration purposes. Na2S2O3+HClNaCl+H2O+SO2+S, but it concentration must be checked each time, it is used because it isn’t stable over a period of time. A secondary standard will be used instead. Consequence of not having a standard substance.
In a reaction between sulphate acid and sodium hydroxide, it is important to know the concentration of one of the reactants in order to determine the concentration of the unknown , base also on mole ration.
NaOH+H2SO4Na2SO4+2H2O
If the volume of sodium hydroxide is known = 25xm3, titre value of acid = 12.80cm3, concentration of sodium hydroxide = 0.5 mole.
Number of mole of NaOH = (0.5X25)/1000 =0.0125 moles.
Number of moles of H2SO4 = 0.0125/2 = 0.00625
Concentration of H2SO4 = (0.00625X1000)/12.80 = 0.48828125mol/dm3
It is important to know the standard substance by finding its concentration and the use this to find the know concentration.
by
ardit1 (student)
