Chromium predominance diagram

Chromium at oxidation state -i-III is under the form of chromic ions Cr + Chromic hydroxide Cr(OH)3 already precipitates at weakly acidic pH [Als(Cr(OH)3)= 10 °]. The predominance diagram /pH of the different species of chromium is given in Fig. 20.3. The couple used in chromimetry is Cr207 /Cr +. Hence, it is practiced only for pH <4. The half-redox reaction is... 

Figure 11.30 Predominance diagram for the speciation of the chromium(lll) ion at 25 °C. The behaviour in the region of -log [H+] >12 should be treated with caution due to changes in activity coefficients.

Polymerizations that use supported chromium (Phillips) catalysts are conducted predominantly in slurry processes (though a small portion employs the gas phase process, see below). The historical development of the Phillips process has been expertly reviewed by Hogan (5, 6) and McDaniel (7-9). The slurry process originally developed by Phillips Petroleum (now Chevron Phillips) has been called the "particle form loop slurry process" and the "slurry loop reactor process" for production of HDPE and LLDPE (10). Hexene-1 is most often used as comonomer for LLDPE in the Phillips process. A simplified process flow diagram for the Phillips loop-slurry reactor process is shown in Figure 7.3 and key operating features are summarized in Table 7.4. 

Fig. 20.3 Predominance-area diagram of different species of chromium (C= 10 mol/L, second convention)...

The stability and solubility constants derived at 25 °C for zero ionic strength have been used to create a predominance speciation diagram for chromium(lll). The diagram is illustrated in Figure 11.30. The speciation behaviour illustrated in the figure is quite similar to that indicated by Rai, Sass and Moore (1987) for an ionic strength of 0.01 moll . These authors showed that the solubiUty of Cr(OH)j(s) increased at a pH between 11 and 12 due to the formation of Cr(OH) , as well as... 

---