Silver chloride, ionic states

A reference book states that the solubility of silver sulfate is 0.57 g in 100 mL of cold water. You decide to check this by measuring the mass of a silver salt precipitated from a known volume of saturated silver sulfate solution. Solubility data show that silver chloride is much less soluble than silver nitrate. Explain why you should not use barium chloride to precipitate the silver ions. Suggest a different reagent, and write the net ionic equation for the reaction. 

Since it is useful to know what state each reagent is in, we often designate the state in the equation. The designations are (s) for solid, (1) for liquid, (g) for gas, and (aq) for aqueous. Thus, a reaction of two ionic compounds, silver nitrate with sodium chloride in aqueous solution, yielding solid silver chloride and aqueous sodium nitrate, may be written as... 

Above 140 K, the situation is quite different. Ionic relaxation, either in the form of Frenkel pair generation adjacent to the impurity or silver ion diffusion from the space-charge layer, provides an alternative decay pathway for the shallowly trapped electron state. The resultant Agj+ is reduced by electron transfer from the impurity trap, a process that has been followed by EPR in particular detail in silver chloride crystals and emulsions. For Pb2 + in AgCl, this ionic decay mechanism has an activation energy of 0.36 + 0.05 eV [85,105], approximately the sum of the formation and diffusion energies for a silver ion interstitial in AgCl [42]. This result supports the Frenkel pair mechanism for the annihilation of the electron shallowly trapped at Pb2+. Similar results have been obtained from EPR studies of Cd2 +-doped AgCl. 

The derivation of the Debye-Hiickel equations is not included here, but only the end results. A complete discussion is given by Bull (1964), Chapter 3. There are, however, several basic phenomena that we need to examine. There are three mechanisms which produce ions in water solutions. These are (1) solution of an ionic crystal, (2) oxidation of a metal or reduction of a nonmetal, and (3) ionization of a neutral molecule. Most metals when they ionize give up electrons to an electronegative element so that both acquire the electronic structure of a rare gas. Exceptions of interest in electrode work are iron, copper, silver, mercury, and zinc. In the ionized state, these metals do not acquire the completed outer shell structure of a rare gas and do have residual valences. They are then somewhat unstable and complex with various molecules more easily than do the stable ionized metals with completed shells. This accounts for the poisoning of silver-silver chloride electrodes and p02-measuring electrodes when used in high-protein environments such as blood. 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

The test identifies the substance to be examined as a salt of silver (Ag+). At the present silver is referenced in only one monograph, silver nitrate. This salt is sold in sticks, called Lapis or Lapis lunaris, and is used for the treatment of warts. Silver can exist both as silver(l) and silver(ll), but since the latter is less stable, silver(I) dominates. It is a noble metal meaning that it is most stable in oxidation state 0, its metallic form. So, contrary to most metals, it does not have at tendency to be oxidized to its ionic form and thereby corrode. Silver nitrate and fluoride are soluble, while silver nitrate, acetate, and sulfate have limited solubility. All other salts are insoluble but are, however, capable of forming many soluble complexes. The insolubility of the chloride salt and its ability to form a soluble nitrate complex are useful in identification. 

The ammonia molecules and chloride ions inside the brackets satisfy the coordination number of cobalt. The chlorides in the coordination sphere do double duty, also helping to satisfy the 3+ oxidation state of the cobalt. The chlorides outside the brackets, sometimes called counterions, help satisfy only the oxidation state. They are the only ionic chlorides available to be precipitated by silver nitrate. For example, if compound (2) is placed in water and treated with aqueous silver ions, the resulting reaction would be that represented by Equation (2.2) ... 

Since silver hydroxide actually appears as silver oxide when attempts are made to isolate it, we cannot be positive that h coordinate bond is formed between silver and hydroxyl ions. However, there are two reasons for believing that one is formed (1) other hydroxides of similar nature (small positive ion) are often not ionic even in the solid state (2) silver iodide does not crystallize into an ionic lattic although the chloride and bromide do. 

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