Chemistry · Ch 3 — Some Analytical Techniques
Purification of solids
Purification of solids
Two Kinds of Impurity in a Solid
A solid substance may contain two types of impurities: (i) impurities that are soluble in the same solvent as the main substance, and (ii) impurities that are insoluble in that solvent. The insoluble kind is the easier of the two to remove — a suitable solvent is chosen that dissolves the main compound while leaving the impurity undissolved, and the two are then separated by filtration, a process similar to separating tea leaves from a decoction of tea, or straining sand out of a sand-water mixture.
Simple Filtration
A circular piece of filter paper is folded into a cone and fitted snugly into a funnel, which is fixed on a stand with a beaker placed below it to collect the liquid. The paper is first moistened, and the solution to be filtered is then poured onto it. The insoluble solid left behind on the filter paper is called the residue, and the liquid that passes through and collects in the beaker below is called the filtrate.
Filtration Under Suction …
What this figure shows. A circular piece of filter paper folded into a cone and fitted inside a filter funnel, the funnel fixed on a stand with a beaker positioned beneath its stem to catch the liquid that passes through; the mixture of water and sand is being poured into the paper cone, the insoluble sand grains stay behind on the paper as the labelled 'Residue', while the clear liquid that drips through into the beaker below is labelled the ' …
What this figure shows. A thick-walled conical (Buchner) flask with a side arm near its neck; the side arm is joined by a length of rubber tubing to an intermediate safety bottle (which stops filtrate being accidentally drawn into the pump), and the safety bottle is in turn connected onward to a vacuum/suction pump. A porous-bottomed porcelain Buchner funnel is fitted into the mouth of the conical flask using a rubber cork, with a correctly sized circular filter paper laid flat on the funnel's perforated base; once suction starts, crystals collect on this fil …