3.6   Ra dng c thu tinh

 

sch ca dng c thu tinh em dng c nh hng rt ln n kt qu ca mt php th hoc mt php nh lng. Cc dng c thu tinh nh cc vi c m, buret, pipet, bnh nn, bnh cu v.v... u phi tht sch, c bit l khi c dng nh lng bng phng php vi sinh vt, th ch nhit t, hoc khi dng ly mt th tch nh cht lng hay dung dch.

Mt trong nhng cht lm sch dng c thu tinh tt nht l acid nitric un nng. Hn hp acid cromic cng l tc nhn lm sch rt tt dng loi sch cht hu c khi b mt thu tinh m khng phi un nng. Hn hp ny c pha ch bng cch ho tan 200 g natri dicromat hoc kali dicromat vo khong 100 ml nc, lm lnh trong nc ri thm t t 1500 ml acid sulfuric, va thm va khuy. Vic pha ch phi c thc hin trong cc vi bng thu tinh boro - silicat v cn eo knh bo h khi thm acid. Hn hp acid cromic l cht rt n mn v ht nc, v th, phi c bo qun trong nhng bnh thu tinh c nt mi v ni an ton. Khi yn, tinh th acid cromic c th c to thnh, tch ra khi hn hp, khi cn gn loi i. Nu hn hp acid cromic c mu xanh th khng dng na.

Dng c thu tinh c x l bng hn hp acid cromic cn phi ra bng nc rt nhiu ln trnh acid cromic b ht bm ln b mt. Khng dng hn hp ny lm sch nhng bnh ng dng cho cc php o quang hc.

ra sch dng c thu tinh, ngi ta cn c th dng nhng dung dch ty ra tng hp hoc nhng ho cht ty c tnh kim nh trinatri phosphat; dng nhng cht ny cng c yu cu phi ra nhiu ln bng nc.

Tt c dng c thu tinh cui cng u c trng bng nc ct v lm kh trc khi dng.