Preparation and calibration

　　1、Method principle

　　NaOH has strong water absorption and the ability to absorb CO2 from the air,therefore,commercially available NaOH often contains Na2CO3.

　　Reaction equation:2NaOH+CO2→Na2CO3+H2O

　　Due to the presence of sodium carbonate,it has a significant impact on the use of indicators,and efforts should be made to remove it.

　　The most common method to remove Na2CO3 is to first prepare NaOH into a saturated solution(about 52%,W/W).Since Na2CO3 is almost insoluble in saturated NaOH solution and will slowly precipitate out,a saturated sodium hydroxide solution can be used to prepare a NaOH solution without Na2CO3.After the precipitation of Na2CO3,a certain amount of supernatant can be taken and diluted to the desired concentration.In addition,distilled water used to prepare NaOH solution should also be heated,boiled,and cooled to remove CO2.

　　There are many reference materials for calibrating alkaline solutions,including oxalic acid(H2C2O4·2H2O),benzoic acid(C6H5COOH),and potassium hydrogen phthalate(C6H4COOHCOOK).The most commonly used is potassium hydrogen phthalate,and the titration reaction is as follows:C6H4COOHCOOK+NaOH→C6H4COONaCOOK+H2O

　　When measuring points,due to the hydrolysis of weak acid salts,the solution is weakly alkaline,and phenolphthalein should be used as an indicator.

　　2、Instruments and reagents

　　Instruments:Alkaline burette(50mL),volumetric flask,conical flask,analytical balance,platform scale.

　　Reagents:Potassium hydrogen phthalate(reference reagent),solid sodium hydroxide(A.R.),10g/L phenolphthalein indicator:1g phenolphthalein is dissolved in an appropriate amount of ethanol and diluted to 100mL.

　　3、Operation steps

　　Preparation of 1.0.1mol/L NaOH Standard Solution

　　Weigh 120g of solid NaOH on a scale using a small beaker,add 100mL of water,shake to dissolve it into a saturated solution,cool it down,and inject it into a polyethylene plastic bottle,seal it,let it sit for several days,clarify it,and set it aside.

　　Accurately transfer 5.6mL of the upper clear liquid of the above solution to 1000mL of carbon dioxide free distilled water,shake well,and label.

　　Calibration of 0.1mol/L NaOH Standard Solution

　　Add the reference potassium hydrogen phthalate into a dry weighing bottle,dry at 105-110℃until constant weight,accurately weigh about 0.6000 grams of potassium hydrogen phthalate using the reduction method,place it in a 250 mL conical flask,add 50 mL of CO2 free distilled water,warm it to dissolve,cool it,add 2-3 drops of phenolphthalein indicator,titrate with the desired 0.1 mol/L NaOH solution until the solution turns pink and does not fade for half a minute,and perform a blank test.Three parallel samples are required.