y bench dilute hydroxide is not a good primary standard?

Because sodium hydroxide NaOH react with carbon dioxide CO2 in the air to form sodium bicarbonate NaHCO3 and sodium carbonate Na2CO3
NaOH + CO2　－＞　 NaHCO3
2NaOH + CO2　－＞　 Na2CO3 + H2O
which decrease the alkaline value of the dilute hydroxide solution with time, so it is not a good primary standard.