Lead salts, reactions

Detector elements are prepared either by sublimation in the presence of a small partial pressure of O2 or by chemical deposition from alkaline solution containing a lead salt and thiourea or selenourea (63). Lead sulfide and lead selenide deposit from solutions as mirror-like coatings made up of cubic crystallites 0.2—1 p.m on a side. The reaction may nominally be represented by the following ... 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

Analyses of rate measurements for the decomposition of a large number of basic halides of Cd, Cu and Zn did not always identify obedience to a single kinetic expression [623—625], though in many instances a satisfactory fit to the first-order equation was found. Observations for the pyrolysis of lead salts were interpreted as indications of diffusion control. More recent work [625] has been concerned with the double salts jcM(OH)2 yMeCl2 where M is Cd or Cu and Me is Ca, Cd, Co, Cu, Mg, Mn, Ni or Zn. In the M = Cd series, with the single exception of the zinc salt, reaction was dehydroxylation with concomitant metathesis and the first-order equation was obeyed. Copper (=M) salts underwent a similar change but kinetic characteristics were more diverse and examples of obedience to the first order, the phase boundary and the Avrami—Erofe ev equations [eqns. (7) and (6)] were found for salts containing the various cations (=Me). 

When iodine is the reagent, the ratio between the reactants is very important and determines the products. A 1 1 ratio of salt to iodine gives the alkyl halide, as above. A 2 1 ratio, however, gives the ester RCOOR. This is called the Simonini reaction and is sometimes used to prepare carboxylic esters. The Simonini reaction can also be carried out with lead salts of acids." A more convenient way to perform the Hunsdiecker reaction is by use of a mixture of the acid and mercuric oxide instead of the salt, since the silver salt must be very pure and dry and such pure silver salts are often not easy to prepare. 

The reaction is the electrochemical version of the well-known transmetallation with lead salts, and becomes significant when the lead anode oxidizes. The very high current yields (Ca 170-180%) imply that formation of R4Pb by nonelectrodic reactions takes place as well ... 

Theory The offcial test is based on the conversion of traces of lead salts present in the pharmaceutical substances to lead sulphide, which is obtained in colloidal form by the addition of sodium sulphide in an alkaline medium achieved by a fairly high concentration of ammonium acetate. The reaction may be expressed as follows ... 

A different application of visible microscopy was pioneered by Gomori. In 1941 he showed that alkaline phosphatase could be specifically located by its hydrolysis of soluble phosphate esters (initially glycerophosphate). If calcium ions were present in the medium in which the sections were incubated, insoluble calcium phosphate precipitated as a result of the action of the hydrolase. The site of the precipitate could be visualized if cobalt or lead salts were subsequently added to replace calcium and the sections exposed to hydrogen sulfide. In principle many hydrolases and other enzymes could be studied using the appropriate substrates and precipitants. It was important to ensure that the products of the enzyme reactions did not diffuse from the sites where the enzymes were located. It was also essential that the reagents could reach the enzyme site. 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Trinitrophenol (4), commonly known as picric acid (VOD 7350 m/s, d = 1.71 g/cm ), was once used as a military explosive although its highly acidic nature enables it to readily corrode metals. This kind of reaction has led to many fatal accidents, a consequence of some metal picrates being very sensitive primary explosives. The lead salt of picric acid is a dangerous explosive and should be avoided at all cost. In contrast, the ammonium (Explosive D, VOD 7050 m/s, d = 1.60 g/cm ) and guanidine salts of picric acid are unusually insensitive to impact and have been used in armour piercing munitions. 

Alkynes may also be hydrogenated, initially to alkenes, and then further to alkanes. By suitable modification of the catalyst, it has proved possible to stop the reaction at the intermediate alkene. Typically, platinum or palladium catalysts partially deactivated (poisoned) with lead salts are fonnd to be suitable for reduction of alkynes to alkenes. Again, syn addition is observed. 

Aqueous solution of lead dichloride also undergoes double decomposition reactions with several metal salts, precipitating insoluble lead salts. 

Diazotization of 2- and 4-aminopyridines leads to the corresponding diazonium salts which, as they cannot be stabilized by mesomeric donation of electrons from the heterocyclic ring, tend to be highly unstable. Under normal aqueous diazotization conditions, therefore, the major products tend to be 2-pyridones (equation 85). 2-Chloro- and 2-bromo-pyridines can be obtained in moderate to good yield if the diazotization is carried out in concentrated hydrochloric or hydrobromic acid. In contrast to this situation which pertains with simple aminopyridines, the diazonium salts obtained on diazotization of 2- and 4-aminopyridine N-oxides are stabilized by resonance and therefore undergo normal diazonium salt reactions (equation 86). 

The second separation step leading to the fully separated Ion pair requires more energy. The behavior for the barium bolaforo salts (reaction 12) Is similar except for n 2 (31). For... 

Prepare a lead sulphide precipitate by reacting solutions of a lead salt and ammonium sulphide. Treat a small amount of the precipitate, washed with water by decantation, with a hydrogen peroxide solution. Why does the colour of the precipitate change Write the equations of the reactions. What properties does hydrogen peroxide exhibit in this and the preceding reactions ... 

Pour potassium chromate and dichromate solutions into separate test tubes and add a solution of a barium salt to each of them. What is the composition of the precipitates See how the obtained salts react with dilute acids. Perform similar experiments with a lead salt. Write the equations of the reactions. 

Arsenate. Lead arsenate. CAS 10031-13-71 nrsenaieoflead Pirn A 04).. while precipitate, formed by reaction of soluble lead salt solution and sodium arsenate solution L scd as an insecticide. Banned or tightly controlled in some countries. 

Carbonates. Lead carbonate PbCOo white precipitate, formed by-reaction of soluble lead sail solution and sodium carbonate solution in ihe cold basic lead carbonate, formed hv reaction nl (II soluble lead salt solution and hoi sodium carbonate solution, and (2i lead sheets, carbon dioxide and aeccic acid, and pigment, the quality depending largely upon the conditions of the reaction. 

Chromates. Lead eliminate, "chrome yellow" PhCrOj. yellow precipitate. by reaction of soluble lead salt solution and sodium dieltromaie or chromate solution, inching point ol lead chromate S44 C. Used as a pigment basic lead chromate, red solid, insoluble, formed by healing lead chromate and sodium hydroxide solution. 

Nitrates. Lead nitrate. (CAS 1009 )-74-ft. PhtNOil . white crystals, soluble, furnied by reaction of lead oxide and nitric acid, and then crystallization, decomposes on heating leaving lead oxide residue. Used to furnish a soluble lead salt basic lead nitrate, formed by reaction of lead nitrate solution and lead oxide. 

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