Silver iodide, solubility

Silver Iodide. Silver iodide, Agl, precipitates as a yellow soHd when iodide ion is added to a solution of silver nitrate. It dissolves in the presence of excess iodide ion, forming an Agl2 complex however, silver iodide is only slightly soluble in ammonia and dissolves slowly in thiosulfate and cyanide solutions. 

Similar measurements were made for the heat of precipitation of silver iodide,5 which is even less soluble in water than silver chloride. As shown in Table 33 in Sec. 102, a saturated solution of Agl at 25°C contains only 9.08 X 10-9 molcs/liter, as compared with 1.34 X 10-6 for AgCl. By calorimetric measurement the heat of precipitation of Agl at 25°C was found to be 1.16 electron-volts per ion pair, or 20,710 cal/mole. 

We have seen in Experiment 8 that silver chloride has low solubility in water. This is also true for silver bromide and silver iodide. In fact, these low solubilities provide a sensitive test for the presence of chloride ions, bromide ions, and iodide ions in aqueous solutions. If silver nitrate... 

That some silver does dissolve to form Ag+ can be verified experimentally by adding a little KI to the solution. Silver iodide has an even lower solubility than does silver chloride. The experiment shows that the amount of silver that dissolves is sufficient to cause a visible precipitate of Agl but not of AgCl. This places the Ag+ ion concentration below 10-10 M but above 10-17 M. Either of these concentrations is so small that we can consider our prediction for the standard state to be applicable here too—silver metal does not dissolve appreciably in 1 M HC1. In general, the question of whether a prediction based upon the standard state will apply to other conditions depends upon how large is the magnitude of °. If ° for the overall reaction is only one- or two-tenths volt (either positive or negative), then deviations from standard conditions may invalidate predictions that do not take into account these deviations. 

It is evident that silver iodide, being less soluble, will be precipitated first since its solubility product will be first exceeded. Silver chloride will be precipitated when the Ag+ ion concentration is greater than... 

Iodides can also be determined by this method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocyanate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects. The dilute iodide solution (ca 300 mL), acidified with dilute nitric acid, is treated very slowly and with vigorous stirring or shaking with standard 0.1 M silver nitrate until the yellow precipitate coagulates and the supernatant liquid appears colourless. Silver nitrate is then present in excess. One millilitre of iron(III) indicator solution is added, and the residual silver nitrate is titrated with standard 0.1M ammonium or potassium thiocyanate. 

There are two methods for overcoming these disadvantages. In the first the precipitation of silver cyanoargentate at the end point can be avoided by the addition of ammonia solution, in which it is readily soluble, and if a little potassium iodide solution is added before the titration is commenced, sparingly soluble silver iodide, which is insoluble in ammonia solution, will be precipitated at the end point. The precipitation is best seen by viewing against a black background. 

Determination of iodide as silver iodide Discussion. This anion is usually determined by precipitation as silver iodide, Agl. Silver iodide is the least soluble of the silver halides 1 litre of water dissolves 0.0035 mg at 21 °C. Co-precipitation and similar errors are more likely to occur with iodide than with the other halides. 

Silver iodide is very insoluble in water. A common method for increasing its solubility is to increase the temperature of the solution containing the solid. Estimate the solubility of Agl at 85°C. 

Silver iodide also has other anomalous properties it is physically soft - it can even be beaten into a sheet, unlike the overwhelming majority of ionic compounds. More unusual still, it is slightly soluble in ethanol. Clearly, silver iodide is not a straightforward ionic compound. In fact, its properties appear to overlap between covalent (see Table 2.6) and ionic (see Table 2.7). 

Out of the three compounds, silver chloride is the most soluble and silver iodide is the least soluble. (You can compare the solubilities of the compounds based on their solubility products because they are all the same type. Each formula unit contains two ions.)... 

These compounds are all unstable and lose ammonia on heating. Silver iodide is readily soluble in liquid ammonia, and on evaporating the solution at —40° to —10° C. white lamellar crystals of monammino-silver iodide separate.1... 

Those submicron particulates which enter the alveolar sacs may undergo various degrees of absorption, depending upon the solubility of their components, or are transported to the base of the ciliated bronchiolar epithelium (54). Alveolar absorptive efficiency for most trace elements is 50-80% (50). Retention or absorption is not necessarily a simple function of solubility. Silver iodide, for example, is rapidly absorbed from the lungs even though it is weakly soluble in water (56). Likewise, insoluble elemental lead deposited in the respiratory passages is absorbed, but the mechanism involved remains to be elucidated (49). Vanadium probably accumulates in human lungs in insoluble forms... 

A second area in which polarization effects show up is the solubility of salts in polar solvents such as water. For example, consider the silver halides, in which we have a polarizing cation and increasingly polarizable anions. Silver fluoride, which is quite ionic, is soluble in water, but the less ionic silver chloride is soluble only with the inducement ofcomplexing ammonia. Silver bromide is only slightly soluble and silver iodide is insoluble even with the addition of ammonia. Increasing covalency from fluoride to iodide is expected and decreased solubility in water is observed. 

All metallic iodides except silver iodide, mercurous iodide, mercuric iodide, lead iodide, cuprous nidide. thallium iodide, and palladium iodide, are soluble. The iodides of antimony, bismuth, tin require a little free acid to keep them in solulion. 

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