Preparation and calibration of sodium hydroxide standard solution

Preparation and calibration

I. method and principle

NaOH has a strong water absorption capacity and absorbs CO2 from the air. Therefore, NaOH on the market often contains Na2CO3.

Reaction equation: 2NaOH+CO2→Na2CO3+H2O

The presence of sodium carbonate has great influence on the use of indicators, so it should be removed.

The most common method to remove Na2CO3 is to mix NaOH into a saturated solution (about 52%, W/W). Since Na2CO3 is almost insoluble in saturated NaOH solution, it will slowly precipitate out. Therefore, Na2CO3 free NaOH solution can be prepared by saturated sodium hydroxide solution.After Na2CO3 precipitation, a certain amount of supernatant can be absorbed and diluted to the desired concentration.In addition, the distilled water used to prepare NaOH solution should also be heated, boiled and cooled to remove CO2.

There are many reference substances used to calibrate alkali solution, such as oxalic acid (H2C2O4·2H2O), benzoic acid (C6H5COOH) and potassium hydrogen phthalate (C6H4COOHCOOK).The most commonly used is potassium hydrogen phthalate, which is titrated as follows: C6H4COOHCOOK+NaOH→C6H4COONaCOOK+H2O

※ phenolphthalein should be used as an indicator because the solution is weakly alkaline due to the hydrolysis of weak acid salts at metering points.

Ii. Instruments and reagents

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

Reagent: potassium hydrogen phthalate (reference reagent), sodium hydroxide solid (A.R), 10g/L phenolphthalein indicator: 1g phenolphthalein was dissolved in appropriate ethanol and diluted to 100mL.

Three, operation steps

Preparation of standard solution of 1.0.1mol/L NaOH

Weigh 120g solid NaOH with a small beaker on a platform scale, add 100mL water, vibrate it to dissolve into a saturated solution, cool it and then pour it into a polyethylene plastic bottle.

Accurately absorb 5.6mL to 1000 mL of co2 free distilled water from the supernatant of the above solution, shake well and label.

Calibration of 0.1mol/L NaOH standard solution

Adding benchmark potassium hydrogen phthalate dry inside the weighing bottle from 105 ~ 110 ℃ drying to constant weight, by using the reduction method accurately said potassium hydrogen phthalate is about 0.6000 grams, put in 250 ml conical flask, add 50 ml without CO2 distilled water, warm to dissolve, cooling, add 2 ~ 3 drops of phenolphthalein indicator, used for calibration of 0.1 mol/LNaOH solution titration, until solution are pink, half a minute do not fade, do blank test at the same time, asked to do three parallel samples.