Anhydrous solvent + 4A molecular sieve ｜ Finally know what it is for!

Those who do chemical experiments must know anhydrous solvents. I believe many people also know how to prepare anhydrous solvents.
Give a few examples:
Tetrahydrofuran (THF): Pre-dry with potassium hydroxide, and then use sodium metal (benzophenone as indicator), and heat to reflux until the solution turns blue to obtain anhydrous tetrahydrofuran.
Toluene: Use sodium metal (benzophenone as an indicator) and heat to reflux until the solution turns blue to obtain anhydrous toluene.
Dichloromethane: Use calcium hydride and reflux for several hours to obtain anhydrous dichloromethane. Note: Halogenated hydrocarbon solvents must not be dried with sodium, otherwise there will be a danger of explosion!
Triethylamine (TEA), acetonitrile (ACN), pyridine (Py): pre-dried with potassium hydroxide, dried with calcium hydride, and re-evaporated to obtain anhydrous solvent.
Acetone: use calcium hydride or diboron trioxide to dry
Methanol (MeOH), Ethanol (EtOH): Use calcium hydride to dry, or use metal Mg+I2 to dry
Diethyl ether: pre-dried with calcium chloride, using sodium metal (benzophenone as indicator), and reflux until it is blue
Remarks: Diethyl ether and tetrahydrofuran are easy to form explosive peroxides. They need to be tested by starch-KI test paper before use to ensure that there is no peroxide before they can be used.
Of course, some laboratories have purchased a solvent treatment system in order to obtain the required anhydrous solvents. One system can process a variety of anhydrous solvents, which can be easily accessed at any time.
However, for those that do not often need anhydrous solvents, purchasing commercially available bottled anhydrous solvents is a more convenient and economical solution. It is often found that 4A molecular sieves are usually added to commercially available bottled anhydrous solvents, so why is this?

In 2010, an article entitled "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants" published in JOC showed us the drying ability of different desiccants to different solvents. From it, we can get a glimpse
image
The author uses various desiccants to dry the aqueous solvents, and then titrates them in a glove box (without oxygen and water) by Karl Fishcher titration to evaluate the ability of various desiccants to dry the solvent.
Karl Fischer Titration

Karl Fischer method is the most commonly used method to determine water content by titration, the earliest
It was publicly reported by the German petrochemist Karl Fishcher in 1935. It is mainly used for the determination of trace moisture in inorganic and organic substances. Now it has become a very well-known method and is widely applicable to various samples. application.
The reagent prepared according to the molar ratio of iodine: sulfur dioxide: pyridine=1:3:10 and anhydrous methanol is called Karl Fischer reagent. The principle of water content determination is based on the reaction described by R. W. Bunsen:
I2 + SO2+ 2H2O → 2HI + H2SO4
Karl Fischer discovered that this reaction can measure the water content in an anhydrous system containing excess sulfur dioxide. Experiments have shown that methanol is a suitable solvent. In order for the reaction equilibrium to proceed to the right, the acids (HI and H2SO4) produced during the reaction must be neutralized. Karl Fischer used pyridine to achieve this goal. Smith, Bryanz and Mitchell clarified a two-step reaction:
1. I2 + SO2 + 3 Py +H2O → 2 Py-H+I– + Py•SO3
2. Py•SO3 + CH3OH → PyH-CH3SO4