Question

3. What is the purpose of washing a precipitate? Briefly explain how this is done. 4. In addition to the step-by-step instructions, the flow chart found on the previous page in quite useful in keeping track of the cations during the experiment. Sketch the flow chart so that the scheme is familiar before beginning the exercise.

Answer 1

3. The purpose  to wash the precipitate is to remove these adsorbed ions or other such impurities.When a substance gets precipitated, some ions that combine to form the precipitate get adsorbed/occluded/included on/inside the surface of the precipitate.

Three cause of impurities during precipitation of analyte are

1.Adsorption 2.inclusion 3 occlusion
Adsoreffion The surface of appt may have a primary adsanked lager of the lattice on in exces This result in sweface adeouptron, Aflew Barcicum piphale e complekely Apt, the lattice con in excess cueill be Butt land This cicil foye Hii pieimary layere. You couen les coir avi és a foreigin auid say Noz: The alt effeer is an adsoufed Led layer of laver BalN05), o ball Sweface odsoublion maximuin pleci - gelatencous ppt and least fou exycha streslive. the hom wuch co moat Shastigly ad ser bed ữ that colica femmeCeast soluble beer Thes on spaveingly soluble Sciphale, jeg und liat Calt. Louis Poulse adecubid' meca Ileana tez right cone as calcium salts are less soluble a than Mg-sau . fimulatly Ag adornbed Ag CAC more than Ag Nos sucie me former is more solable. Becides the smaller on dissociation A the adsorked compound, the art greater is adsorphion . Thus. Has a weak electrolyte is strongly adswebed dues many metallic sulphides 1 Month this impurities can easily be removed by washing . Digee how reduced the surf dee area, and hence the amount of adsoreflow : a less CamScanner

Inclusion It occurs when ione generally of similar size and charge all entrapped within crycãe lattice when one con can replace accortion in a enyelae resulting is a formation of a mixed Drugsfel. Thié is called. Pero morpliowe reflasemena. 9. In precipitation of Monta Pon), at having really equal size and charge to Ntext can easily replace occurs are forming rug [klog]. The appt's in which it rarely applied analytically.... ... The unipuuties appearing due to occlusion and ů etusioni cau bel removed by washing Digestion may help some brit has to not completed effective. Reprecopilation is the only way out. Purity of the ppt will be quite low when large cuptate m eustale are formed a rapidly se especially when iso morphoul Substances are present and solid solution are : Ici Scanned with touued canner

Occlusion In ne process mê material which is not part of crystal structure is rather within the cugetal for eg:- Anali metal nitrates . are are excluded excluded wellher witner Basog crystals of halis is very easily occluded within many crystals and This can easily be druen of by meeting the cryst i mercado e emyslali But the puoblem is that it win beaue une sol mbwrities Ics Scanned with, CamScanner .. walan nihate willing with which is

gelateneous precipitate require more washing than crystalline precipitate

The washing of precipitate can be done by two ways

a.A precipitate may be washed directly on the filter by using wash water which will contain one ion common to the precipitate which would en hence the solubility of the impurities but at the same time should decrease the solubility of the precipitate .sometime organic impurities are washed with organic solvent

b)A precipitate may be washed by decantation .In decantation, the precipitate is allowed to settle, and the supernatant liquid is poured off ensuring all impurities are removed in the supernatent liquid.Multiples times decantation is done to ensure complete removal of the impurities.