Classification of filter paper
Laboratory filter paper can be divided into two categories: qualitative filter paper and quantitative filter paper.
In qualitative, quantitative, and chromatographic separation experiments, filter paper is often used. The filter paper has 70, 90, 110, 125, 150, and 180 mm and other specifications and different pore sizes. The flow rate is divided into three types: fast, medium, and slow, which are marked on the filter paper box. The pore size of fast filter paper is generally 16~30μm, the pore size of medium flow rate filter paper is generally 8~15μm, and the pore size of slow flow rate filter paper is 1~3μm.
Qualitative filter paper
The fiber of qualitative filter paper contains high silicon content, and the ash weight after burning is large, so it is not suitable for gravimetric analysis. It can be used for filtration and separation of inorganic precipitates and filtration of organic matter recrystallization.
Quantitative filter paper
The pulp of quantitative filter paper is made by digesting hydrochloric acid and hydrofluoric acid. The content of iron, aluminum, silicon, etc. in the paper is very low, and the ash weight of the quantitative filter paper after burning is <0.007 mg/sheet. In quantitative analysis, this weight can be ignored, so it is often called “ashless filter paper”.
Quality inspection method of quantitative ashless filter paper:
1. Ash test:
Accurately weigh 10 sheets of ashless filter paper, tear two of them into pieces, and put them into two accurately weighed platinum crucibles. They are heated on a warm fire in a high-temperature furnace, and then high heat is added. After cooling, the crucible was cooled to room temperature in a desiccator and weighed, and the average ash weight per filter paper was obtained by subtraction.
2. Test of barium sulfate retention:
Heat 20 ml of sulfuric acid to boiling, drop into 20 ml of barium chloride solution with a certain concentration at a speed of 1 mL/s and continue to heat after dripping. After 30 seconds, filter the mixture with filter paper and observe whether the filtrate has a white precipitate, which can indicate whether the filter paper can block barium sulfate.
3. Starch test:
If starch is present in filter paper, it cannot be used for carbohydrate analysis. The method to test whether there is starch in the filter paper is as follows: tear the filter paper, azeotrope it with distilled water, and drop 0.01N dilute iodine solution into it. If it appears blue, it means that there is starch.
4. Test of iron in filter paper:
Add a drop of papery ammonium solution and a drop of hydrochloric acid to the filter paper, if there is no iron, there will be no brown spots.
