Colloids at pH 3 with Organics

To understand the effect of organics on the primary particles, another set of experiments far from surface water conditions (no electrolyte solution and pH 3) was carried out. At this pH the colloids are initially stable. When the organics are added, the surface charge is reversed resulting in aggregation. This was confirmed with microfiltration, after which large aggregates were found on the membrane 

Here the colloids are prepared as in the previous paragraph and the organics are then added from the stock solution. The solution was left stirring for about 2 hours to allow completion of the adsorption process. 

---

For HA, at low pH (3.5), particulates were measured in the absence and presence of calcium. Calcium increased the measured particulate size, indicating aggregation of organic colloids. At pH 8 particles formed in the absence of calcium, which shows that there may be some undissolved HA. At pH 10, no particulates were measured and the HA was fully dissolved. Once calcium was added at pH 10, particulates were measured, possibly confirming the co-precipitation of organics with calcite. In the presence of 25 mM calcium, particles are smallest at pH 8 and largest at pH 3.5. Organic concentration did not have a measurable effect on the size of the colloids, and at 25 mM CaCl 2 particles formed at 25, 50, and 75 mgL" HA as DOC. 

The experiments were carried out at pH 3 with the two smallest colloids (see Appendix 3 for preparation and characterisation). This extreme pH was required in order to obtain stable (non-aggregating) primary colloids, rather than their aggregates in the absence of organics (see Chapter 4 for characterisation of colloidal systems). Results are shown in Figure 7.42. At pH 3 the colloids have a high positive charge. This should lead to a significant repulsion between the colloids, increased backdiffusion, and therefore lower concentration polarisation of colloids (Bhattacharjee et al. (1999)). 

It was originally thought that lead azide existed in two allotropic forms a and 13 (Vol. Ill, p. 169). Now it is accepted that tlie substance is polymorphic and exists in four forms a, / , 7 and 6. Tlie a-orthorhombic is the only one acceptable for technical application. It is the main product of precipitation with traces of the other fonns present [89]. The monoclinic /3-form is stable when dr>, but recrystallizes as the a-fomi. Tlie presence of some organic dyes (e.g. eosin) enhances the fomiation of 0-form and hydrophUe colloids inhibit its formation. Breaking a needle of 0-form may produce an explosion (contrary to the views of some authors Vol. Ill, p. 173). Tlie monoclinic y-form is less stable than a and 0 [90]. It can be obtained from pure reagents at pH 3.5- 7.0 or in the presence of vinyl alcohol. The triclinic 6-forin precipitates from pure reagents at pH values of between 3.5 and 5.5 [90]. Both forms 7 and 6 are usually-precipitated simultaneously and can be separated (with care ) by hand. 

Lead azide exists as four polymorphic forms [287] of which the orthorhombic oc-lead azide is the most stable [276]. In fact, a-lead azide is the only acceptable form for technological applications. Presently, the state of the art of making the polymorphs can be summarized as follows a-lead azide is the main product of precipitation, with traces of the other forms present [288]. The monoclinic [276] /3-form is stable when dry, but re crystallizes as the a-form [276,289]. The presence of organic dyes (eosin, neutral red) at the precipitation enhances formation of /3-lead azide [276] the presence of hydrophile colloids inhibits it [276,287]. The monoclinic [287] 7-form, apparently less stable than a and /3 [289], is obtained from pure reagents at a pH of 3.5-7 [287] or in the presence of polyvinyl alcohol [289]. The triclinic 6-form precipitates from pure reagents between pH 3.5 and 5.5 [287]. No method is presently available to yield a single polymorph exclusively, but the crystals differ sufficiently in shape to allow hand selection under the microscope [287,288]. 

---