Second limits in boric acid coated vessels

1 SECOND LIMITS IN BORIC ACID COATED VESSELS 

The behaviour of the second limit partial pressure plots in boric acid coated vessels has already been shown in Fig. 11. Limit curves at a number of temperatures are also shown in Fig. 12. Clearly the linear relation feojPo = K no longer applies. Instead, as the oxygen concentration decreases the mixture will still explode even though the [O2 ] /[H2 ] ratio becomes very small indeed, the limit pressures increase with decreasing [O2 ] /[H2 ] ratio in this region. Egerton and Warren [24] found the limits to be described very closely by an expression of the type 

500 °C (37.2) agrees closely with the value of if = 37 found for KCl coated vessels also. 

Similar general behaviour to that described for boric acid coated vessels has also been reported by Dixon-Lewis et al. [63], who used silica vessels washed with hydrofluoric acid and distilled water, and has been found also with H3PO4 coated, HNO3 washed and HF washed Pyrex vessels [53, 62]. 

Below 500 °C the behaviour is even more complex, and Baldwin et al. [20, 21] have investigated the effects of several variations in withdrawal procedure on the limits in a 36 mm diameter vessel. The three factors which could be varied were  

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The reactions of H atoms with HO2 have been discussed by Dixon-Lewis and co-workers [159]. The numerical side of their argument is modified slightly here to accommodate new information obtained from a recent re-interpretation by Baldwin et al. [73] of their second limits in boric acid coated vessels (Sect. 4.3.3 and Table 18). This gives (ftg + Iklki 0 = 0.325 at 773 K to correspond with (feg + fe8a)/ 20 = 12.0 exp (—540/T). Two values of fe, q are available at room temperature (i) (1.8 0.2) X 10 due to Foner and Hudson. [184], and (ii) (2.2 0.3) x 10 due to Paukert and Johnston [185]. Assuming ftjo = 2 x 10 ... 

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