Total carbon concentration

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

We now consider as an example an analysis (Table 15.1) of water from Mono Lake, California. The reported alkalinity of 34 818 mg kg-1 as CaC03 is equivalent to 700 meq of acid or 350 mmol of H2SO4. Since at this pH carbonate and bicarbonate species are present in roughly equal concentrations, we can quickly estimate the total carbonate concentration to be about 30 000 mg kg-1. We take this value as a first guess and model the titration with react... 

Ionization fractions for constant total carbonate concentration, Ct Ct = [H2CO3 ] + [HCO3-] + [C03 ] [H2CO3 ] = aoCx[HC03 ] = aiCx[C03 ] = 2Cx... 

Fig. 6. Percentage of uranyl sorbed on quartz at a total carbonate concentration of 10-3 M (series Cl) or 10 2m (series CIO) obtained after sample centrifugation and ultracentrifugation (after Mignot 2001).

The calcium ion (Ca2+) and total carbonate concentrations are shown in Figures 7 and 8, respectively. The concentration levels and the trends of Ca2+ and total carbonate profiles obtained at the three Mg2+ concentrations are very similar. 

A common OH scavenger in natural waters is alkalinity. Alkalinity is a measure of the total carbonate concentration. This is complicated by the equilibrium that exists in natural waters produced by the dissolution of atmospheric C02 to produce carbonic acid, and the dissociation of carbonic acid. 

These data show that, because the fines derive from crystalline rocks, a carbon component has been added to the fines. The total carbon concentration in Apollo 11 fines increases with decreasing particle size, indicating that there is a surface correlationS4,67 It has been suggested54 therefore, that solar wind implantation of carbon into the surfaces of the fines makes a significant contribution to the total carbon, although a meteorite contribution cannot be discounted. 

The total carbonate concentration in water at equilibrium with carbon dioxide is expressed... 

Since the value of [H+] is 5 x lO " at pH 6.3, the ratio on the right is practically equal to unity. Accordingly we have found" that at this pH the concentration of unionized carbonic acid is roughly equal to the concentration of hydrogen carbonate ion. The ratios of [CO3—], [HCOg ], and [HgCOg] have thus been found to be 1 10,000 10,000. The total carbonate concentration, in all three forms, is accordingly 20,000 times the concentration of the carbonate ion, COg . 

Figure 5 Calculated theoretical for sorption of uranyl onto a goethite substrate as a function of pH at fixed total carbon concentration in the presence of a sequestering agent (EDTA). are shown for several levels of calcium concentration. Surface area of the substrate is 100 L total carbon is fixed at 1.5 X 10 M and total uranium...

We can now compute the pH of the alkalinity titration endpoint for different total carbonate concentrations. Some results are as follows... 

Geochemical modeling programs need to have information about the total carbonate concentration (Equation (3.51)). 

In unpolluted fresh waters, the acidity in milliequivalents per liter is approximately equal to ih2co in millimoles per kilogram, and the alkalinity in milliequivalents per liter is approximately equal to mHCO- in millimoles per kilogram. The sum is the total carbonate concentration. 

The carbonate module in the present system compares values of the total dissolved carbonate concentrations from several sources to evaluate the internal consistency of data on the carbonate system. Three total carbonate concentrations may be available tdcl, the sum of the analyzed concentrations of dissolved CO2, bicarbonate and carbonate, tdc2, the analyzed total dissolved carbonate concentration, and tdc3, the value calculated from the analyzed pH and alkalinity by the geochemical modeling program. Differences among these three concentrations are used to decide the quality of the carbonate analytical data. 

Figure 1 shows how the differences among the three total carbonate concentrations lead to conclusions about the quality of the analytical data on which the carbonate concentrations are based. The three columns at the left represent the arithmetic differences between the three total carbonate concentrations. These differences can be positive ("plus" in the table), negative ("minus" in the table), or they may be smaller than some uncertainty (10% is the value used in the prototype system), in which case they are considered "good". 

The six columns on the right represent the conclusions which can be drawn about the analytical data from which the total carbonate concentrations were calculated. The entries in each column indicate whether the quality variable corresponding to the data item is to be incremented ("+"), decremented ("-"), or left unchanged ("- -") based on the differences found in the three left columns. 

Usually, alkalinity is actually the ultimate parameter that is subject to a procedure of genuinely chemical titration, in this regard as a proxy parameter for carbonate. A new spectropho-tometric method for the determination of alkalinity was proposed by Sarazin et al. (1999). Mostly, alkalinity will be, for reasons of simplification, set equal to the total carbonate concentration, although a number of other substances in pore water will contribute to the titration of alkalinity as well. Most geochemical model programs (cf Chap. 15) foresee the input of titrated alkalinity as an alternative to the input of carbonate. The model program will than calculate the proportion allo-catable to the different carbonate species. 

This thesis includes cryoscopic measurements performed under the same conditions as in [69FAU/DER], as well as additional measurements in a solution with half the total carbonate concentration. Table A-15 gives the (calculated) compositions of the solutions used in the two sets of cryoscopic measurements described in [72DER]. 

Chapter 4 of this reference is entirely devoted to carbonate complexation studies. As for the hydrolysis part, several techniques have been applied (potentiometry, NMR and Raman spectroscopy, dialysis, electrospray mass spectrometry), leading in general to mutually consistent results. Because the work was focussed on the determination of complex stoichiometry, no Zr-C03 formation constant could be selected from these data. Moreover, due to the high Zr concentrations (0.01 to 1 M), little can be said about the nature of carbonate eomplexes in dilute Zr solutions. It must also be noted that —except for the titrations— the investigated pH range was quite limited (pH 8 to 10) and that total carbonate concentrations were always largely in excess of Zr. 

Figure XI-3 Solubility of Th02(am, hyd) and speciation at / = 0.5 M (NaHCOs-Na2C03-Na0H-NaCl) and a total carbonate concentration of Ctot = 0.1 and 0.015 M [2005ALT/NEC]. The corresponding data of [19940ST/BRU] at Ctot = 0.1 and I = 0.5 M (NaC104 media) are shown for comparison. The calculations are based on the equilibrium constants and SIT coefficients selected in the present review and logio = -(47.6 + 0.5) calculated from the data in carbonate-free solutions.

Figure XI-6 Stabilitity region of Na6[Th(C03)5] 12H20(s) compared to aged ThOzCam, hyd) (logi = -47.5) at [Na ] = 2.0 M (NaHCOs-NazCOs-NaNOs) and 25°C. The regions A and B indicate the experimental conditions in [1973DER/FAU3] the dotted lines show the corresponding values of the total carbonate concentration Qot and -logio [H ] for equilibrium CO2 partial pressures of 1.0, 0.1, and 10 bar (air).

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