Cations in precipitation

Table 2-3. The effect of some anions and cations in precipitating proteins141.

TABLE I. Basic cations in precipitation from several locales... 

Divalent cations must be removed from the wash water because of the negative role played by these cations in precipitating various insoluble salts, including anionic surfactant salts this boils down to consuming these surfactants while canceling their effects as surface-active agents. 

When a potential is appHed across the ceU, the sodum and other cations are transported across the membrane to the catholyte compartment. Sodium hydroxide is formed in the catholyte compartment, because of the rise in pH caused by the reduction of water. Any polyvalent cations are precipitated and removed. The purified NaOH may be combined with the sodium bicarbonate from the sodium dichromate process to produce soda ash for the roasting operation. In the anolyte compartment, the pH falls because of the oxidation of water. The increase in acidity results in the formation of chromic acid. When an appropriate concentration of the acid is obtained, the Hquid from the anolyte is sent to the crystallizer, the crystals are removed, and the mother Hquor is recycled to the anolyte compartment of the ceU. The electrolysis is not allowed to completely convert sodium dichromate to chromic acid (76). Patents have been granted for more electrolytic membrane processes for chromic acid and dichromates manufacture (86). 

Generally, the most common cations in the soil solution are potassium, sodium, magnesium and calcium. Alkali soils are high in sodium and potassium, while calcareous soils contain predominantly magnesium and calcium. Salts of all four of these elements tend to accelerate metallic corrosion by the mechanisms mentioned. The alkaline earth elements, calcium and magnesium, however, tend to form insoluble oxides and carbonates in nonacid conditions. These insoluble precipitates may result in a protective layer on the metal surface and reduced corrosive activity. 

The anionic portions of the soil solution play a role of equal importance to the cations. The anions function in the manner outlined for cations in conductivity and concentration-cell action, and have an additional action if they react with the metal cation and form insoluble salts. Thus, if the metal is lead and the predominant anion is sulphate, a layer of insoluble lead sulphate may precipitate on the metal surface and form an effective barrier against further loss of metal. 

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)—... 

One way to separate two cations in water solution is to add an anion that precipitates only one of the cations. This approach is known as selective precipitation. To see how it works, consider a simple case, a solution containing Mg2+ and Na+ ions. Referring back to Table 16.1 (p. 433), you can see that Mg2+ forms a couple of insoluble compounds MgC03 (K = 6.8 X 10-6) and Mg(OH)2 (K = 6 X 10-12). In contrast, all of the common compounds of sodium are soluble, including the carbonate and hydroxide. It follows that you could readily separate Mg2+ from Na+ by adding either C032- or OH- ions to the solution. In either case, Mg2+ will precipitate while Na+ remains in solution. 

Group I consists of the only three common cations that form insoluble chlorides Ag+, Pb2+, and Hg22+. Addition of hydrochloric acid precipitates AgCI, PbCI2, and Hg2CI2. Cations in Groups II. III. and IV remain in solution, since their chlorides are soluble. 

FIGURE 11.20 Part of a simple qualitative analysis scheme used to separate certain cations. In the first step, three cations are separated as insoluble chlorides. In the second step, cations that form highly insoluble sulfides are removed by precipitation at a low pH and, in the third step, the remaining cations are precipitated as the sulfides at a higher pH. 

Anionic and cationic products generally tend to interact with each other, usually diminishing the surface-active properties of both and often resulting in precipitation of the complex formed. Amphoteric compounds can also be incompatible with anionics in acid solution but are generally compatible with cationics and nonionics. Interaction between anionic and cationic agents can sometimes be prevented by addition of a nonionic. In some cases, if an ethoxylated sulphate or phosphate is used as the anionic component a cationic compound produces no obvious precipitation, since the oxyethylene chain acts as dispersant for any complex that may be formed. 

Calcium and magnesium are the major cations (co-)precipitating trace elements as carbonate. Trace elements are also precipitated as sulfate or phosphate. Solubility and reactions of carbonates, sulfates and phosphates of selected major and trace elements are in Table 3.9. 

Table 5.16. Applications of co-precipitating agents to the pre-concentration of cations in seawater... 

A common problem in offshore petroleum production is that sulfate scale may form when seawater is injected into the formation during waterflooding operations. The scale forms when seawater, which is rich in sulfate but relatively poor in Ca++ and nearly depleted in Sr++ and Ba++, mixes with formation fluids, many of which contain bivalent cations in relative abundance but little sulfate. The mixing causes minerals such as gypsum (CaSC>4 2H2O), anhydrite (CaSC>4), celestite (SrSOzO, and barite (BaS04, an almost insoluble salt) to become saturated and precipitate as scale. 

The fate of the bipyridinium radical cation in the mercaptan monolayer is not clear. The second reduction wave (Figure 3) possesses structure suggesting a precipitated phase similar sharp peaks are seen during the reductive precipitation of Cu.bpyMe2 ... 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

When the aluminium halide solution is added to a solution of monomer, only the aluminium present as cations can initiate (disregarding any active cations that may have been formed by reaction of the initiator with impurities in the solvent) and the unionised aluminium halide becomes complexed with monomer and thus formation of further ions from it stops or becomes at best a very slow process. This is what was called the Esso technique [1] and it was the commonest method of experimentation. If the system is sufficiently free from terminating impurities and if the propagating ions are not occluded in precipitated polymer, all the monomer should be consumed eventually, and so the bound aluminium halide should in the end become free by the shifting of equilibrium (ii). However, these conditions are generally unfavourable for the reaction going to completion, and it comes virtually to a stop at incomplete conversion. 

Since most of our observations on the reacting systems were made by means of conductivity measurements it is necessary to remember that in these systems the only factor which increases conductivity is an increase in the concentration of ions, but that a decrease of conductivity could be due to any or all of the following effects increase of size of cation by polymerisation, increase of viscosity of solvent due to polymer, occlusion of ions in precipitated polymer, trapping of polymer between the electrodes. A similar list was given by Matyska in one of the earliest applications of conductivity measurements to a cationic polymerisation, that of isoprene by aluminium bromide in toluene solvent [19]. 

Hydroxide and carbonate typically form insoluble precipitates with polyvalent cations in natural waters. The activity of both of these species increases with pH. The presence of surface functional groups that are capable of exchanging a proton creates pH dependent-charge, whereby the ionic character of the surface increases with pH [158,284,285]. 

One way to identify cations in solution is hy selectively precipitating them out of solution. As you know, cations may form soluble or insoluble ionic compounds, depending on the anions that are present. For example, copper(II) chloride, CuCl2 is soluble in water. Copper(II) sulfide, CuS, is insoluble in an acidic solution. Knowing about the relative solubility of cations when combined with various anions helps chemists identify them. 

Using precipitation reactions to identify cations was once a common part of qualitative analyses. Today, chemists usually identify and quantify unknowns using instruments such as spectrophotometers. However, examining the identification of cations through precipitation reactions allows you to see how solubility equilibria can be manipulated. It also provides an opportunity to solve a chemical mystery —what is in the unknown solution ... 

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