The main test points of sodium hydroxide deterioration include: deterioration reason, test deterioration method, test deterioration degree, and get pure sodium hydroxide by removing impurities. (total metamorphism: Na2CO3, partial metamorphism: Na2CO3, NaOH)

1、氢氧化钠变质的原因：敞口放置，与空气中的二氧化碳反应，生成了碳酸钠。

1. The reason for the deterioration of sodium hydroxide: it is placed open and reacts with carbon dioxide in the air to form sodium carbonate.

原理：2NaOH + CO2 = Na2CO3 + H2O

Principle: 2naoh + CO2 = Na2CO3 + H2O

2、检验氢氧化钠是否变质的方法：(分别滴加酸、碱、盐溶液，各为一种方法)

2. Method for testing whether sodium hydroxide is deteriorated: (add acid, alkali and salt solutions dropwise respectively, one method for each)

①取少量样品于试管中，滴加足量稀盐酸(或者稀硫酸)，如果产生气泡，说明已变质。无明显现象，说明未变质。

① Take a small amount of sample into the test tube and add enough dilute hydrochloric acid (or dilute sulfuric acid). If bubbles appear, it indicates that it has deteriorated. No obvious phenomenon, indicating no deterioration.

原理：Na2CO3 + 2HCl = 2NaCl + H2O + CO2↑

Principle: Na2CO3 + 2HCl = 2nacl + H2O + CO2 ↑

②取少量样品于试管中，滴加氢氧化钙溶液(或者氢氧化钡溶液)，如果产生白色沉淀，说明已变质。

② Take a small amount of sample into the test tube and add calcium hydroxide solution (or barium hydroxide solution) dropwise. If white precipitation occurs, it indicates that it has deteriorated.

原理：Na2CO3 + Ca(OH)2 = CaCO3↓ + 2NaOH

Principle: Na2CO3 + Ca (OH) 2 = CaCO3 ↓ + 2naoh

③取少量样品于试管中，滴加氯化钙溶液(或者硝酸钙等其它可溶性钙盐、钡盐溶液。)

③ Take a small amount of sample into the test tube and add calcium chloride solution (or calcium nitrate and other soluble calcium salt and barium salt solutions) dropwise

原理：Na2CO3 + Ca Cl2 = CaCO3↓ + 2NaCl

Principle: Na2CO3 + CaCl2 = CaCO3 ↓ + 2nacl

3、检验氢氧化钠变质程度的方法：取少量样品于试管中，滴加足量的CaCl2溶液，有白色沉淀生成；过滤，向滤液中滴加少量酚酞溶液，如果溶液变红色，说明溶液部分变质；如果溶液呈无色，说明氢氧化钠完全变质。

3. Method for testing the deterioration degree of sodium hydroxide: take a small amount of sample in the test tube, drop sufficient CaCl2 solution, and white precipitation will be formed; Filter and add a small amount of phenolphthalein solution to the filtrate. If the solution turns red, it indicates that part of the solution has deteriorated; If the solution is colorless, sodium hydroxide is completely deteriorated.

原理：（1）氢氧化钠和变质后生成的碳酸钠溶液都呈碱性，滴加足量的氯化钙溶液是为了除尽生成的碳酸钠，这时会产生白色沉淀，然后滴加酚酞溶液【还可以加入：MgCl2（白色沉淀）、CuCl2（蓝色沉淀）、FeCl3（红褐色沉淀）、FeCl2（白色沉淀）、NH4Cl（刺激性气味）、紫色石蕊溶液】，如果溶液呈红色，说明溶液呈碱性，有未变质的氢氧化钠，部分变质。如果溶液不变红，说明没有氢氧化钠了，完全变质了。

Principle: (1) sodium hydroxide and sodium carbonate solution generated after metamorphism are alkaline. Add sufficient calcium chloride solution to remove the generated sodium carbonate. At this time, white precipitation will be generated, and then drop phenolphthalein solution [you can also add MgCl2 (white precipitation), CuCl2 (blue precipitation), FeCl3 (reddish Brown precipitation), FeCl2 (white precipitation), NH4Cl (pungent smell), purple litmus solution], If the solution is alkaline, it indicates that there is no deterioration of sodium hydroxide solution. If the solution remains red, it indicates that there is no sodium hydroxide and it has completely deteriorated.

Na2CO3 + CaCl2 = CaCO3↓ + 2NaCl(注意：除去碳酸钠时，不能用氢氧化钙，因为氢氧化钙与碳酸钠反应会生成新的氢氧化钠，会干扰对氢氧化钠的检验。

Na2CO3 + CaCl2 = CaCO3 ↓ + 2nacl (Note: calcium hydroxide cannot be used when removing sodium carbonate, because the reaction between calcium hydroxide and sodium carbonate will produce new sodium hydroxide, which will interfere with the inspection of sodium hydroxide.

原理：Na2CO3 + Ca(OH)2 = CaCO3↓ + 2NaOH )

Principle: Na2CO3 + Ca (OH) 2 = CaCO3 ↓ + 2naoh)

4、如何除去部分变质的氢氧化钠中的碳酸钠。

4. How to remove sodium carbonate from partially deteriorated sodium hydroxide.

滴加适量氢氧化钙溶液，然后过滤，就得到氢氧化钠溶液，再蒸发，就得到氢氧化钠固体。

Add an appropriate amount of calcium hydroxide solution dropwise, then filter to obtain sodium hydroxide solution, and then evaporate to obtain sodium hydroxide solid.

原理：Na2CO3 + Ca(OH)2 = CaCO3↓ + 2NaOH

Principle: Na2CO3 + Ca (OH) 2 = CaCO3 ↓ + 2naoh

(注意：①不能选用盐酸或硫酸等酸液，因为酸与碳酸钠反应的同时也会与氢氧化钠反应，而且会生成NaCl或Na2SO4，引入了新的杂质。②不能用氯化钙等盐溶液，因为它们会碳酸钠反应生成了NaCl，引入了新的杂质。)

(Note: ① acid solutions such as hydrochloric acid or sulfuric acid cannot be selected, because the acid will react with sodium hydroxide while reacting with sodium carbonate, and will generate NaCl or Na2SO4, which introduces new impurities. ② salt solutions such as calcium chloride cannot be used, because they will react with sodium carbonate to generate NaCl and introduce new impurities.)