Drop-Time

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

An empirically determined relationship between drop weight and drop time does allow surface tensions to be determined for small surface ages [41],... 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

Drop time in polarography, 597, 608 Dropping mercury electrode 608, 628 Dry ashing 114 Dry box lOl Drying reagents 99 comparative efficiencies of, (T) 99 Drying of precipitates 119 Duboscq colorimeter 656 Duplication method 701... 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

Another important observation is that breakage occurs at the critical condition only if the time of imposed stresses acting on the drop, tp, exceed the response time of the drop, C the ratio of the two time scales is given by the drops time constant dp/A.1. If the imposed stresses are removed such that tp is less than tj., the drop will recover unbroken. 

A linear approximation for the velocity term, used to treat the DME problem [74], does not work for MEMED, because the concentration boundary layers tend to be much larger for MEMED due to the longer drop times employed. 

FIG. 18 Chloride concentration profile recorded by a microelectrode probe during the hydrolysis of TPMCl at a DCE drop-aqueous interface (O)- The concentration of TPMCl in the organic phase was 50 mmol dm, the drop time from formation to contact with the probe was 4.80 s, and the final drop radius was 0.55 mm. The solid lines represent theoretical time-dependent concentration profiles, from top to bottom, generated for k = 3.50 x 10 , 3.25 x 10 , and 3.00 x 10 molcm s . A value of 1.8 X 10 cm s was employed for the diffusion coefficient of chloride. (Reprinted from Ref. 73. Copyright 1997, American Chemical Society.)... 

FIG. 19 Normalized concentration profiles (solid lines) of the reactants and products in the DCE (a) or aqueous (b) receptor phase for the reaction between Fc (DCE) and IrClg (aqueous) with 0.1 M CIO4 in both DCE and the aqueous phase. In each case, the reactant concentration in the receptor phase was 1 mM, with 10 mM reactant inside the droplet. Drop times and final sizes were (a) 5.54 s and 0.96 mm, and (b) 6.32 s and 1.00 mm. The theoretical profiles (dashed lines) are for a transport-controlled reaction, with no transfer of the product ions. (Reprinted from Ref. 80. Copyright 1999, Royal Society of Chemistry.)... 

Kakiuchi and Senda [36] measured the electrocapillary curves of the ideally polarized water nitrobenzene interface by the drop time method using the electrolyte dropping electrode [37] at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetraphenylborate) electrolytes. An example of the electrocapillary curve for this system is shown in Fig. 2. The surface excess charge density Q, and the relative surface excess concentrations T " and rppg of the Li cation and the tetraphenylborate anion respectively, were evaluated from the surface tension data by using Eq. (21). The relative surface excess concentrations and of the d anion and the... 

The dependence of the interfacial tension at the W/NB interface on the interfacial potential difference [29,30] was investigated by using an aqueous solution dropping electrode [26,31]. In this investigation, the aqueous solution was forced upward dropwise in NB and the drop time of W was measured as a function of potential difference applied at the W/ NB interface. When W contained 1 MMgS04 and NB contained 4 x 10 " M Cs" TPhB ... 

The depression in the drop time-potential curves measured under the condition of constant Ucs+ was more conspicuous with an increase of acsTPhs demonstrated in Figs. 4(a)-(d), whereas the depression was indifferent to aQs+ as seen in Figs. 4(c), (e), and (f). These results suggest that the adsorbed species responsible for the maximum current is the ion pair, Cs" TPhB. ... 

Taking into account that both the voltammetric maximum and the depression in drop time-potential curves were affected by the ion pair formation equilibrium of Na DPA in LM, it is concluded that Na which has been transferred from NB to W may be adsorbed at the interface from the side of W inducing the adsorption of DPA as a counterion from the NB side. At the interface, the adsorbed Na may exist as an ion pair, which is denoted as Na DPAj, hereafter. The possibility of the interfacial ion pair formation between a hydrophobic cation (or anion) in an organic phase and a hydrophilic anion (or cation) in an aqueous phase has been proposed by Girault and Schiffrin [32], and Kakiuchi et al. [33]. 

If a is independent of the potential the logarithmic plot is still a straight line however, the wave position always shows a dependence on drop time. 

Although modern polarographs also permit the choice of short drop times (usually down to 0.5 s), the most attractive apparatus for rapid polarography is that supplied by Metrohm (Herisau, Switzerland) Wolf24, who was one of the first promoters of the technique, used the Polarecord E 261R together with the E 354 S polarography stand and its drop time controller (from 0.32 to 0.16 s in five steps) (see Fig. 3.27). Comparison of the normal and rapid (drop time 0.25 s)... 

NEWER METHODS (NON-CONVENTIONAL) OF POLAROGRAPHY (MOSTLY WITH A CONTROLLED DROP TIME)... 

In the newer Metrohm Polarecord E 506, which permits the application of almost the complete range of modem polarographic methods (see later) including the rapid technique, the drop time controller (from 6 to 0.4 s in eleven steps) has been built in. 

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