Thorium ratios

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Thorium has been recovered as a by-product of uranium production from ores of the Blind River district in Ontario in which the uranium thorium ratio is 6 1 [C5], In such thorium the Th activity is 3.1 X 6 = 18.6 times the activity of the Th. 

Extraction of uranium takes place in nine forward extraction stages and five stripping stages. The object of the stripping section is to minimize the extraction of thorium, and IN nitric acid is used. The ratio of feed solvent strip flow rates is 1 1 0-2. The second extractor backwashes the uranium into an equal volume of 0 02N nitric acid, in five stages, and the solvent is recycled back to the first extractor. The uranium-to-thorium ratio is reduced from 5 per cent to less than 1 ppm and the loss of thorium in this cycle is only a fraction of one per cent. 

The thorium nitrate-water system has been reported [22] as having considerable solubility up to about 225°C, at which point hydrolytic precipitation occurs. Further investigation [23] revealed a maximal stability for the 80w/o material (to around 255°C). Increasing the acidity of the solutions (increasing the N03 /Th ratio) suppresses hydrolysis and increases the stability of the solutions as indicated by Figure 3-16, which shows the precipitation temperatures for various solutions [24]. The intensity of vapor phase coloration at elevated temperatures (rapidly reversible) increased as the nitrate/thorium ratio was raised above 4.0. 

It would appear clear from observation of Figure 10.21 that the variation in the stability constant of Th2(OH)3 increases as the ionic strength increases. At an ionic strength of 0.5 mol kg the variation in the stability constant is about 0.5 log units, whereas at around 3.25 mol kg the variation has increased to about 1.0 log unit. This variation maybe due to all except one of the values coming from the work of Milic and co-workers and the fact that they only acquired data up to a bound hydroxide to thorium ratio of 0.4 (the ratio for the Th2(OH)3 species is 1.5) more data are required in chloride media where the bound hydroxide to thorium ratio is extended to the point ofthe onset of precipitation reactions which should enable more precise stability constants to be acquired for this, and other, species. 

Danesi et al, 1968 Hietanen and Sillen, 1968). The data from Milic and coworkers are within the combined uncertainty limits of the work of Hietanen and Sillen (1968), but the stability constant of Danesi et at. (1968) appears to be somewhat larger (see Table 10.16). There are also some data from perchlorate media (Baes, Meyer and Roberts, 1965 Hietanen and Sillen, 1968 Grenthe and Lagerman, 1991). Analysis of the data from the nitrate, chloride and perchlorate media leads to stability constants at zero ionic strength that are consistent and within the limits of the calculated uncertainties. Unfortunately, the vast majority of the experimental data from the studies of Milic and co-workers only reached a bound hydroxide to thorium ratio of 0.4, making it impossible to obtain stability constants with substantially higher ratios (they typically only postulated stability constants for Th2(OH)2 and Th2(OH)3 ). 

Thorium isotope concentrations and ratios, as well as parent and daughter isotope concentrations, are used to date and study the formation and metamorphosis of rocks and sediments. For example, has been used to date coral reef terraces (4). / Th disequiUbria and Th/ Th... 

Thorinm-232 is the only non-radiogenic thorium isotope of the U/Th decay series. Thorinm-232 enters the ocean by continental weathering and is mostly in the particulate form. Early measurements of Th were by alpha-spectrometry and required large volume samples ca. 1000 T). Not only did this make sample collection difficult, but the signal-to-noise ratio was often low and uncertain. With the development of a neutron activation analysis " and amass spectrometry method " the quality of the data greatly improved, and the required volume for mass spectrometry was reduced to less than a liter. Surface ocean waters typically have elevated concentrations of dissolved and particulate 17,3 7,62... 

Clay type identification A plot of thorium versus potassium will indicate what type of clay is present. The thorium/potassium ratio can also be used. 

U1 "1" (16), and Np1 "1" (17) (Table IV). Only in the thorium system have stability constants been determined as a function of acidity. Zebroski et al., (12) found that the 1 2 complex had the form TMSOi, at 0.5 M and 1.0 M acidity but suggested a contribution from the monoprotonated, bis-sulfato complex in 2 M acid. However, Zielen (9) found no evidence for such a species. The Kj/K2 ratio found in the latter study (n.8) compares favorably with the Am(III)-S0 data of DeCarvalho and Choppin (10)... 

Kj/K2 v 7) and suggests Zielen s interpretation is correct in the thorium sulfate system. The increasing K1/K2 ratio for U1 " " (a<15) Np + (A/24), and Pu + (a,25) is consistent with our interpretation of the existence of the complex AnSOitHSO. Why this species should become increasingly important with increasing atomic number is not understood. 

C22-0035. Determine Z, A, and N for each of the following nuclides (a) the helium nuclide with one less neutron than proton (b) the nuclide of barium whose neutron-proton ratio is 1.25 and (c) the nuclide of thorium that contains 1.5 times as many neutrons as protons. 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

However, thorium has only two naturally occurring long-lived isotopes, and all Th measurements by TIMS are limited by the absence of a well-constrained isotope ratio that can be used for internal normalization purposes to correct for instrumental mass fractionation. In this regard, one of the most important advantages of MC-ICPMS over MC-TIMS is the ability to admix two elements with overlapping mass ranges and use the... 

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