• Economic and Environmental Geology
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• v.38 no.6 s.175
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• pp.657-662
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• 2005

Extractions fro fertilizer and soil samples were performed to yield the operationally defined fractions 'soluble' chromate (extractable with $NH_4NO_3$), 'exchangeable' chromate (extractable with phosphate buffer pH 7.2), and these results were compared with the data obtained by extractions with ammonium sulfate, borate buffer pH 7.2, saturated borax pH 9.6, and polyphosphate (Graham's salt). In order to maintain the pH of extractant solution about constant, the concentration of extractant buffer had to be raised to at least 0.5 M. The results strongly depended on the kind of extractant, and the solid: liquid ratio. For most of the samples investigated, the extraction efficiency increased in the order borate-sulfate-nitrate-phosphate. Whereas the recovery of $K_2CrO_4\;and\;CaCrO_4$ added to the samples of basic slags prior to the extraction was about complete, the recovery of added $PbCrO_4$ was highly variable. In soil extracts, the color reaction was interfered from co-extracted humics, which react with the chromate in weak acid solution during the time period necessary for color reaction (1 hour). However, this problem can be overcome by standard addition and subtraction of the color of the extractant solution. In soil extract of about pH < 7, organic material reduced chromate during the extraction period also, and standard addition of soluble chromate is recommended to prove recovery and the stability of chromate in the samples. In admixtures of soils and basic slags, results for hexavalent chromium were lower than from the mere basic slags. This effect was more pronounced in phosphate than in nitrate extracts. As a proficiency test, samples low in organic carbon from contaminated sites in Hungary were tested. The results from $NH_4NO_3$ extracts satisfactorily matched the results of the Hungarian labs obtained from $CalCl_2$ extractants.

• Education of Primary School Mathematics
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• v.20 no.3
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• pp.205-224
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• 2017

A model method has been known as the main characteristic of Singaporean elementary mathematics textbooks. However, little research has been conducted on how the model method is employed in the textbooks. In this study, we extracted contents related to the model method in the Singaporean elementary mathematics curriculum and then analyzed the characteristics of the model method applied to the textbooks. Specifically, this study investigated the units and lessons where the model method was employed, and explored how it was addressed for what purpose according to the numbers and operations. The results of this study showed that the model method was applied to the units and lessons related to operations and word problems, starting from whole numbers through fractions to decimals. The model method was systematically applied to addition, subtraction, multiplication, and division tailored by the grade levels. It was also explicitly applied to all stages of the problem solving process. Based on these results, this study described the implications of using a main model in the textbooks to demonstrate the structure of the given problem consistently and systematically.