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Lead Nitrites
Besides normal lead nitrite numerous basic salts have been described, as well as a compound of nitrite and nitrate, formerly known as hyponitrate. These products have been obtained by the action of finely divided lead on a solution of lead nitrate or by the hydrolysis of lead nitrite. Lorenz described fifteen of such compounds, and Peters no less than twenty-eight. No doubt many of these supposed compounds were mixtures, and, therefore, it became an important and difficult problem to decide what individual compounds really exist. Our knowledge upon this subject is due largely to the work of Chilesotti, who has given a resume of the various lead nitrites which have been described, and submitted them to a critical examination.
Normal lead nitrite, Pb(NO2)2, is prepared by mixing equivalent proportions of solutions of lead chloride and silver nitrite at 25° C., and concentrating the filtrate by freezing, followed by evaporation over sulphuric acid. The monohydrate Pb(NO2)2.H2O was thus obtained in yellow, transparent prisms, and subsequently the anhydrous salt Pb(NO2)2, mixed with a little lead oxide and nitrate. The electric conductivity of lead nitrite in concentrated solution is somewhat less than that of the nitrate or chloride; but on dilution the differences become small. It is probable, from their conductivity and their intense yellow colour, that solutions of lead nitrite contain complex anions of the type Ag(NO2)2' or Hg(NO2)4''. Lead nitrite solution slowly decomposes thus:
3Pb(NO2)2 + 2H2O = Pb(NO3)2 + 2Pb(OH)2 + 4NO,
a decomposition similar to that which nitrous acid itself undergoes; this reaction increases the conductivity of the solution. When a solution of lead nitrite is boiled and cooled, nacreous scales are deposited, having the composition
Pb(NO2)2.Pb(OH)2.H2O or Pb(NO3,NO2).Pb(OH)2.H2O;
and by further hydrolysis, 3PbO.N2O3.xH2O and 4PbO.N2O3.H2O are produced. These three, together with the salt Pb(NO2)2.Pb(OH)2, are the only basic lead nitrites recognised by Chilesotti. Pb(NO2)2.Pb(OH)2 may be regarded as a derivative of orthonitrous acid, N(OH)3, thus:
The existence of a nitrite-nitrate of lead has been investigated by Chilesotti by measurements of conductivity of mixed solutions of nitrite and nitrate. Owing to a change in the direction of the specific conductivity curve when the two salts are present in molecular proportions it is concluded that the two salts show some tendency to combine in the molecular ratio: Pb(NO2)2:Pb(NO3)2. Attempts to separate a solid salt were, however, unsuccessful. Several double nitrites of lead and the alkali metals have been obtained. 3KNO2.2Pb(NO2)2.xH2O crystallises in orange monoclinic crystals from a concentrated solution of lead acetate to which potassium nitrite has been added.
2KNO2.Pb(NO2)2.H2O is obtained when excess of KNO2 is added to lead acetate solution; according to Chilesotti, however, it is not pure, but mixed with the former salt.
CsNO2.Pb(NO2)2.H2O forms bright orange plates.
Normal lead nitrite, Pb(NO2)2, is prepared by mixing equivalent proportions of solutions of lead chloride and silver nitrite at 25° C., and concentrating the filtrate by freezing, followed by evaporation over sulphuric acid. The monohydrate Pb(NO2)2.H2O was thus obtained in yellow, transparent prisms, and subsequently the anhydrous salt Pb(NO2)2, mixed with a little lead oxide and nitrate. The electric conductivity of lead nitrite in concentrated solution is somewhat less than that of the nitrate or chloride; but on dilution the differences become small. It is probable, from their conductivity and their intense yellow colour, that solutions of lead nitrite contain complex anions of the type Ag(NO2)2' or Hg(NO2)4''. Lead nitrite solution slowly decomposes thus:
3Pb(NO2)2 + 2H2O = Pb(NO3)2 + 2Pb(OH)2 + 4NO,
a decomposition similar to that which nitrous acid itself undergoes; this reaction increases the conductivity of the solution. When a solution of lead nitrite is boiled and cooled, nacreous scales are deposited, having the composition
Pb(NO2)2.Pb(OH)2.H2O or Pb(NO3,NO2).Pb(OH)2.H2O;
and by further hydrolysis, 3PbO.N2O3.xH2O and 4PbO.N2O3.H2O are produced. These three, together with the salt Pb(NO2)2.Pb(OH)2, are the only basic lead nitrites recognised by Chilesotti. Pb(NO2)2.Pb(OH)2 may be regarded as a derivative of orthonitrous acid, N(OH)3, thus:
The existence of a nitrite-nitrate of lead has been investigated by Chilesotti by measurements of conductivity of mixed solutions of nitrite and nitrate. Owing to a change in the direction of the specific conductivity curve when the two salts are present in molecular proportions it is concluded that the two salts show some tendency to combine in the molecular ratio: Pb(NO2)2:Pb(NO3)2. Attempts to separate a solid salt were, however, unsuccessful. Several double nitrites of lead and the alkali metals have been obtained. 3KNO2.2Pb(NO2)2.xH2O crystallises in orange monoclinic crystals from a concentrated solution of lead acetate to which potassium nitrite has been added.
2KNO2.Pb(NO2)2.H2O is obtained when excess of KNO2 is added to lead acetate solution; according to Chilesotti, however, it is not pure, but mixed with the former salt.
CsNO2.Pb(NO2)2.H2O forms bright orange plates.
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