M U R

M U R ( 123 )

A folution of muriated lead is readily decom-pofable by foda, either cauftic or carbonated;the explanation therefore of this feeming ano-maly on which Mr. Turner’s patent was found-ed, became an important chemical problem,and has exercifed the abilities of Ivirwan, Haf-fenfratz, Berthollet , Vauquelin, and others.By the experiments of the laft of thefe ablechemifts the difficulties have been cleared up,and the apparent contradiction has been recon-ciled to the general laws of chemical affinities. fHe mixed together, by grinding in a mortar,feven parts of litharge and one of muriated foda,adding as much water as brought the whole tothe confillence of thin pafte ; it was then ftirredfor feveral hours to facilitate the mutual actionof the ingredients. By degrees the colour ofthe litharge became paler, its volume increafedvery conliderably, and feveral additions ofwater were required to prevent it from form-ing a folid mafs: at the end of four days thechemical change appeared to be compleat, thelitharge having acquired a white colour, themixture was accordingly diluted with feven oreight parts of water and filtered. The clearliquor had an alkaline flavour, and when reducedby evaporation to T 'o of its bulk, afforded cry--ftals of carbonated foda mixed with a little mu-riat of lead, and the common fait was intirelydecompofed. The refidue on the filter whenwell waffled and dried, was of a dirty yellowcolour, and exceeded in weight the lithargeemployed by about £; by a gentle calciningheat its colour was changed to a fine lemonyellow, and its weight was diminifhed by about2.5 per cent. In pure water even when boilingit is fcarcely foluble •, but by digeftion withcauftic foda a confiderable portion was takenup, and the refidue affumed a dirty whitecolour, and was changed from a pulverulentconfidence to that of needle-form cryftals.The foda folution was but little different intafte from the pure alkali, but gave a veryabundant black precipitate with hydrofulphuretof foda, a white precipitate with muriatic acid,and alfo with nitric acid, but this laft was re-foluble in an cxcefs of acid. If the yellowrefidue was treated with dilute nitric acid in-ftead of foda, there remained an infolubleportion of a white colour and cryftalline texture,and the nitrous folution depofited by gentleevaporation cryftals of nitrat and muriat oflead.

From thefe facts it appears that litharge bythe decompofition of common fait, is converted

f Ann. Cheni. xxxi. p. 1.

into a fub-muriat, or muriat with excefs ofbafe, and therefore differing very materiallyfrom the common muriat; that this fait thoughfoluble in cauftic foda is not decompofable byit; and that the efficient caufe of the decompo-fition of common fait by litharge is the propertyof muriated lead to combine with an excels ofbafe, by which the affinity of the acid for thealkali is counterafted ; that the yellow colourwhich this fub-muriat acquires by being heated,is occafioned by the excefs of bafe, fince thecommon muriat exhibits no fuch character ;that to this excefs is alfo owing the infolubilityof this fubftance in water ; and finally that bythe addition of nitric acid the fub-muriat isdecompofed, being itfelf converted into muriat,and yielding its excefs of oxyd to the nitricacid.

In confirmation of the opinion that the oxydof lead decompofes common fait only in conse-quence of its being converted into a fub-muriat,it may further be remarked, that the white pre-cipitate depofited by the aftion of cauftic fodaon the foluble muriat is not a fimple oxyd ashas been generally fuppofed, but a fub-muriat,exactly fimilar to that which we have beendeferibing.

It lias been fuppofed by fome chemifts thatthe carbonic acid which litharge and moll ofthe other oxyds of lead contain, has a confidera-ble fhare in effc£ting the decompofition of muriatof foda, this however has been denied by otherchemifts, and particularly by Vauquelin, whoaffirms that carbonat of lead and muriat of fodahave abfolutely no adtion whatever on eachother. This being the cafe it follows that li-tharge, minium, See. are the worfe calculatedfor decompofing common fait in exadt propor-tion to the carbonic acid which they contain :hence arifes the propriety of mixing quick-limewith the litharge, in order to abforb its carbonicacid, and thus expedite its adtion, By availinghimfelf of this addition M. Curadeau » foundthat 3oparts of litharge and 4. of quick-lime, wereable to effedt a total decompofition of 10 partsof muriated foda. But even with this improve-ment and with every attention to economy, itdoes not appear that this method of feparatingthe foda from fea-falt can be attended withmuch profit on a large fcale: the demand forthe yellow fub-muriat is very limited, and thewhite fub-muriat although its colour is verygood, is not pofleffed of folidity enough whenground up with oil to be employed as a fubfti-tute for common white lead: it would there-* Ann. Chem.xiv. p. 15.