Precipitation diagrams

Precipitation diagram. Choose the cation row and read across to the anion column. If the block is blank, no precipitate will form. If the block is colored, a precipitate will form from dilute solution. Where a formula is given, that is the only cation-anion combination in that block that will precipitate. 

The precipitation diagram shown in Figure 4.3 enables you to determine whether or not a precipitate will form when dilute solutions of two ionic solutes are mixed. If a cation in solution 1 mixes with an anion in solution 2 to form an insoluble compound (colored squares), that compound will precipitate. Cation-anion combinations that lead to the formation of a soluble compound (white squares) will not give a precipitate. For example, if solutions of NiCl2 (Ni2+, Cl- ions) and NaOH (Na+, OH- ions) are mixed (Figure 4.4)—... 

Using the precipitation diagram (Figure 4.3), predict what will happen when the following pairs of aqueous solutions are mixed. 

Precipitate A solid that forms when two solutions are mixed, 78 Precipitation diagram, 78 Precipitation reaction Formation of an insoluble solid when two electrolyte solutions are mixed, 78,95-96q diagram, 78... 

The equation above is the basis of the curves shown in the hydroxide precipitation diagram (Figure 5.29). It is seen from the above equation that each line in Figure 5.29 has a gradient of- , where n is defined by the equation appeared earlier (M(OH) =f M2++ n OH-). The precipitation process is dependent on both the metal concentration and the solution pH. As in the case of the metal sulfides, hydroxide precipitation will occur in solutions whose compositions lie to the right of the lines on this diagram. The diagram demonstrates the... 

Figure 5.29 Precipitation diagram for metal hydroxides (based on data for 25 °C).

A. J. Monhemius, Precipitation Diagrams for Metal Hydroxides, Sulfides, Arsenates and Phosphates, Trans. IMM, Vol. 86, P. C202,1977. 

Bachra, B. N., Trautz, O., and Simon, S. L. Precipitation of calcium carbonates and phosphates. II. A precipitation diagram for the system calcium-carbonate-phosphate and the heterogeneous nucleation of solids in the metastability region. Adv. Arch. Fluorine Res. Dental Caries Prev. 3,101-118 (1965). 

The precipitation boundary may or may not coincide with the solubility curve since its position in the diagram depends on the time and method of detection of the onset of precipitation. In effect, it establishes the metastable zone for the given system. If a stable precipitate is formed at low levels of supersaturation the precipitation boundary and solubility curve may be assumed to be virtually coincident. In cases where they do not coincide, the composition of the critical nuclei may be determined from the slopes of the precipitation boundary while the compositions of the corresponding bulk equilibrium solid phases may be obtained from the solubility curve. Hence, comparison of the two curves can yield information as to whether the composition of both nuclei and precipitate is the same or if the bulk solid phase is formed by a solid-state transformation from a metastable precursor. A useful account of the significance of the zones on a precipitation diagram has also been given by Nielsen (1979). 

Another type of precipitation diagram has been proposed for assessing hydrometallurgical processes involving the selective precipitation of metal hydroxides and salts (Monhemius, 1977). Each diagram illustrates the relative solubilities of a particular hydroxide or salt of a range of metals and enables estimation of theoretical solubilities at any acidic pH. 

The sulphide precipitation diagram in Figure 8.9 is essentially a plot of cation activity against sulphide ion activity. pH is also recorded for systems containing H2S at atmospheric pressure. 

Figure 8.9. Sulphide precipitation diagram at 25 °C. (H2S at atmospheric pressure). After Monhemius, 1977)...

Monhemius, A.J. (1977) Precipitation diagrams for metal hydroxides, sulphides, arsenates and phosphates. Transactions of the Institution of Mining Metallurgy, 86, C202-C206. 

Follow the schematic diagram in Figure 4.3. 2. Use the precipitation diagram (Figure 4.2) to determine whether or not the possible precipitates are soluble. ... 

Precipitation diagram revised to include more exceptions... 

Using these K values calculated for cerium oxides, sulfides, and oxysulfides, Wilson et al. (1974, 1976) have constructed an inclusion precipitation diagram for the Fe-Ce-O-S system (fig. 4), where hct, ho and hs represent the Henrian activities of cerium, oxygen and sulfur, respectively. This composite precipitation diagram gives a clear, qualitative picture of the conditions of phase formations and transformations. 

It appears then that these differences in behavior arise from the fact that we have different DNA conformations. To make this point clear we studied the temperature dependence of the same system. For the temperature studies we used a fixed concentration of NaBr, 10 M, and performed the studies at 4, 25 and 50°C. Under these conditions we have at 4 °C double-helix DNA solutions and for the two higher temperatures single-stranded DNA molecules. The precipitation diagram is presented in Figure 10.8. 

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