Slow stirring

Bj 23.3 g of 2,6-dihydroxymethylpyrldine-bis(N-methylcarbamate), prepared as described above, are dissolved in a boiling mixture of 46.6 ml of methanol and 46.6 ml of water. When the dissolution is complete, the solution is allowed to cool under slow stirring, without applying any external cooling means. The crystals start to separate at 48°C to 50°C. When the temperature of the mixture falls spontaneously below 35°C, it is cooled externally to 0°C to 5°C, and allowed to stand at this temperature for about 8 hours. Theseparated substance isfiltered off and dried at 50°C to 100°C. 22.65 g of 2,6-dihydroxymethylpyridine-bis(N-methylcarba-mate) are obtained. The quality of the product meets pharmaceutical requirements. 

The polymerization reaction is conducted at the desired temperature with a slow stirring regime for a certain period. A typical recipe for the emulsion polymerization of styrene is exemplified in Table 1 [40]. As seen here, potassium persulfate and sodium dodecyl sulfate were used as the initiator and the stabilizer, respectively. This recipe provides uniform polystyrene particles 0.22 /Lim in size. 

A. o-Bromobenzenediazonium hexafluorophosphate. A solution of 95 ml. of 12N hydrocMoric acid in 650 ml. of water is added with stirring to 60 g. of o-bromoaniline (0.35 mole Note 1) in a 2-1. three-necked flask equipped with stirrer and thermometer. Solution is effected by heating the mixture on a steam bath (Note 2). A solution of 29 g. (0.42 mole) of sodium nitrite in 75 ml. of water is added with stirring while the mixture is maintained at — 5° to —10° by means of a bath of ice and salt or of dry ice and acetone. At the end of the addition there is an excess of nitrous acid, which can be detected with starch iodide paper. Seventy-four milliliters (134 g., 0.60 mole) of 65% hexafluorophosphoric acid (Note 3) is added in one portion, with vigorous stirring, to the cold solution of the diazonium salt. Cooling and slow stirring are continued for an additional 30 minutes, and the precipitated diazonium hexafluorophosphate is then collected on a Bilchner funnel. The diazonium salt is washed on the funnel with 300 ml. of cold water and with a solution of 80 ml. of methanol in 320 ml. 

GPC traces of products of the reductive coupling of hexylmethyl-dichlorosilane with Na in refluxing toluene. A, with rapid stirring B, with slow stirring. 

The reaction is allowed to continue for 24 hours with slow stirring. 

Effect of Stirring. The rather small effect of tube flow and slow stirring on the specific resistance results in Figure 5 was unexpected and has been checked by a series of trials in which a kaolin suspension was subjected to various periods of slow stirring, following polymer addition and rapid mixing. 

In this procedure, visible floes became apparent after about 5 minutes of slow stirring and the floe size continued to increase up to about 30 minutes. After long stirring times, floes appeared to be quite strong and survived transfer to the syringe without much break-up. 

However, during the formation of a filter cake, floes must be subject to considerable disruptive forces and large aggregates may not survive. Scanning electron micrographs of filter cakes formed after various periods of slow stirring have shown no obvious differences. 

Figure 7. Effect of slow stirring period on specific resistance of flocculated kaolin (0.5% polymer A).

De Bruijn, J., Busser, G., Seinen, W., Hermens, J. (1989) Determination of octanol/water partition coefficient for hydrophobic organic chemicals with the slow-stirring method. Environ. Toxicol. Chem. 8, 499-512. 

HPLC-k correlation ODS column Diol column quoted lit. average, Helweg et al. 1997) 3.33 (range 3.24-3.40) (slow stirring method-HPLC/fluo., De Maagd et al. 1998)... 

HPLC-RV correlation, different mobile phases, Brooke et al. 1986) 5.40 (shake flask/slow stirring-GC, Brooke et al. 1986)... 

Good stirring is essential. Slow stirring results in low yields. 

Roller PS. Chemical activity and particle size. II. The rate of solution at slow stirring of anhydrite and gypsum. J Phys Chem 1932 36 1202-1231. 

An aggregate average value of 6.24 at 20 to 25 °C was reported based on 8 independent laboratory determinations (slow stirring-GC, Tolls et al, 2003)... 

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