The Stem

An attempt to place the wrong size or taper Teflon stopcock into the funnel. This is extremely rare. Get a new funnel. 

The stem on a sep funnel can either be straight or have a ground glass joint on the end (Fig. 53). The ground glass joint fits the other jointware you may have and can be used that way as an addition funnel to add liquids or solutions 

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Fig. V-3. Schematic representation of (a) the Stem layer (b) the potential-determining ions at an oxide interface (c) the potential-determining and Stem layers together.

The interaction of an electrolyte with an adsorbent may take one of several forms. Several of these are discussed, albeit briefly, in what follows. The electrolyte may be adsorbed in toto, in which case the situation is similar to that for molecular adsorption. It is more often true, however, that ions of one sign are held more strongly, with those of the opposite sign forming a diffuse or secondary layer. The surface may be polar, with a potential l/, so that primary adsorption can be treated in terms of the Stem model (Section V-3), or the adsorption of interest may involve exchange of ions in the diffuse layer. 

Adsorption at a charged surface where both electrostatic and speciflc chemical forces are involved has been discussed to some extent in connection with various other topics. These examples are drawn together here for a brief review along with some more speciflc additional material. The Stem equation, Eq. V-19, may be put in a form more analogous to the Langmuir equation, Eq. XI-4 ... 

The adsorption appears to be into the Stem layer, as was illustrated in Fig. V-3. That is, the adsorption itself reduces the f potential of such minerals in fact, at higher surface coverages of surfactant, the potential can be reversed, indicating that chemical forces are at least comparable to electrostatic ones. The rather sudden drop in potential beyond a certain concentration suggested to... 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Figure Bl.14.8. Time course study of the arrival and accumulation of labelled sucrose in the stem of a castor bean seedling. The labelled tracer was chemically, selectively edited using CYCLCROP (cyclic cross polarization). The first image in the upper left comer was taken before the incubation of the seedlmg with enriched hexoses. The time given in each image represents the time elapsed between tire start of the incubation and the acquisition. The spectmm in the lower right comer of each image shows the total intensity...

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61]. 

Nitric acid is prepared in the laboratory by distilling equal weights of potassium nitrate and concentrated sulphuric acid using an air condenser, the stem of which dips into a flask cooled by tap water. The reaction is ... 

The main stem of the nitrometer widens into a bulb and then narrows to form a graduated tube. The usual graduation is of 8 ml. in o o2 ml. divisions. The graduations continue to the tap Tj at the top of the stem. Above Tj there is a small reservoir H to prevent splashing of the concentrated alkali when gas is expelled from the nitrometer and also to ensure that a small excess of potash is left as a liquid seal above the tap T ,. 

H. The stem of G fits through the bung J to the filter-flask as shown. A bent tube K, Imown as a Pregl filter-tube, is fitted through another bung L into the upper neck of the funnel G. 

The stem correction may be avoided by employing a thermometer with a short scale (sometimes termed an Anschutz thermometer) and of such size that the entire column of mercury is immersed in the vapour or in the liquid. In practice, it is generally more convenient to use thermometers which have been graduated for partial immersion for a short and con-... 

The liquid should be poured into the distilling flask, preferably through a funnel the stem of which extends below the side arm, and a few fragments of unglazed porous porcelain ( porous pot, see Section 1,2)... 

Filtration of corrosive liquids with suction. A strongly alkaline or acid suspension is best filtered through a sintered glass funnel. Alternatively, glass wool or asbestos may be plugged into the stem of a glass funnel or supported upon a Witt plate in a glass funnel. 

Separatory funnels. For many operations the globular form (compare Fig. II, 1, 5, c) with 14, 19 or 24 cone on the stem and 14 or 19 socket for the stopper, is convenient. For some purposes, e.g., when used on a three-necked flask or with a multiple adapter, the cylindrical... 

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