Primary saturation states

Primary saturation is the first state reached during the congruent dissolution of a solid-solution, for which the aqueous-solution is saturated with respect to a secondary solid-phase (JJ., J 4, Glynn and Reardon, Am. J. ScL, in press). This secondary solid will usually have a composition different from that of the dissolving solid. At primary saturation, the aqueous phase is at thermodynamic equilibrium with respect to this secondary solid but remains undersaturated with respect to the primary dissolving solid. The series of possible primary-saturation states for a given SSAS system is represented by the solutus curve on a Lippmann diagram. 

In the specific case of a "strictly congruent" dissolution process occurring in an aqueous phase with a [B+]/[C+] activity ratio equal to the B+/C+ ratio in the solid, primary-saturation can be approximately found by drawing a straight vertical line on the Lippmann diagram from the solid-phase composition to the solutus (see figure 1). For an exact calculation, the following relations may be used to determine the primary saturation state ... 

A detailed discussion of solid-solution solubilities at primary saturation states and at thermodynamic equilibrium states is given by Glynn and Reardon (Am. J. ScL, in press). The fundamental principles governing these thermodynamic states are given below. 

On the primary side of the power supply, the transistor output of the optoiso-lator will be a simple eommoii-emitter amplifier. The MOC8102 has a typieal eurreiit transfer ratio of 100 pereeiit with a +/- 25 pereeiit toleraiiee. When the TL431 is full-on, 6mA will be drawn from the transistor within the MOC8102. The transistor should be in a saturated state at that time, so its eolleetor resistor (Rl) must be... 

One of the primary concerns in a study of the geochemistry of carbonates in marine waters is the calculation of the saturation state of the seawater with respect to carbonate minerals. The saturation state of a solution with respect to a given mineral is simply the ratio of the ion activity or concentration product to the thermodynamic or stoichiometric solubility product. In seawater the latter is generally used and Qmjneral is the symbol used to represent the ratio. For example ... 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

Calculations involving mineral saturation state (D in Equations (19) and (20)) are dependent on accurate characterization of the thermodynamic states of the reactants and products and are commonly calculated using speciation codes (see Chapter 5.02 for additional information). Although the equilibrium constants for most simple primary silicates have been determined, thermodynamic data do not exist for many complex silicates or for solid solutions. 

Several thermodynamic states are of interest in the study of SSAS systems. The following sections discuss the concepts of thermodynamic equilibrium, primary saturation and stoichiometric saturation states. 

Figure 1. Lippmann diagram (with stoichiometric and pure-phase saturation curves) for the Ag(Cl,Br) - H2O system at 30° C. Calculated ao and ai values are 0.30 and -0.18 respectively. pK gci = 9.55 (16J. pK gBr = 12.05 (12). T1 and T2 give the aqueous and solid phase compositions, respectively, of a system at thermodynamic equilibrium with respect to an AgCl.sBr 5 solid. PI and P2 describe the state of a system at primary saturation with respect to the same solid. MSI gives the composition of an aqueous phase at congruent stoichiometric saturation with respect to that solid.

As shown in figure 1, stoichiometric saturation states never plot below the solutus curve. This is consistent with the fact that stoichiometric saturation can never be reached before primary saturation in a solid-solution dissolution experiment. The unique point at which a stoichiometric saturation curve (for a given solid Bj.xCxA) joins the Lippmann solutus represents the composition of an aqueous solution at thermodynamic equilibrium with respect to a solid Bx xCxA. 

The composition of a SSAS system at primary saturation or at stoichiometric saturation will be generally independent of the initial solid to aqueous-solution ratio, but will depend on the initial aqueous-solution composition existing prior to the dissolution of the solid. In contrast, the final thermodynamic equilibrium state of a SSAS system attained after a dissolution or recrystallisation process will generally depend not only on the initial composition of the system but also on the initial solid to aqueous-solution ratio (Glynn et al, submitted to Gcochim, Cosmochim. Acta). 

Lippmann phase diagrams can be used to describe and compare thermodynamic equilibrium (equations 3, 4), primary saturation (equations 9, 10), stoichiometric saturation (equation 13) and pure end-member saturation states (equations 14, 15) in binary SSAS systems. 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

The primary disadvantage of fixed-bed adsorbers arises when contaminant rates are high. Because of the unsteady-state nature of the operation, a large portion of the in-process adsorbent inventory is saturated and, therefore, inactive. 

It Is unknown whether suspended a-form crystal or 3-form crystal In the saturated solution displays a solid-solid transformation or not. The reason for above experimental results may be assumed as follows In the standpoint of Industrial crystallization. Since 3-form crystal Is less soluble than a-form crystal at high temperature above 284K. The state that a-form crystal Is suspended In the saturated solution Is considered to be supersaturated for 3-form crystal. So the state has the potential to take place primary nucleatlon and crystal growth for the 3-form. In this way, 3-form crystal may be produced In the suspension of a form crystal. Rewarding to the formation of 3-form crystal, a part of a-form crystal suspended may be dissolved. Opposite phenomena may take place at low temperature below 284K. 

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