Scan rate

Traditionally, the most common scan rate used by thermal analysts is 10°C/min, but with commercially available instruments rates can be varied between 0.001 and 500°C/min, often to significant advantage. Choice of scan rate can affect the following areas  

Temperature calibration. Calibrations need to be performed prior to analysis to ensure that the scan rate of use is calibrated. Different instruments use different approaches to calibration and manufacturer s instructions should be followed. 

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The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

In a testing context, it refers to the first detection of exothermic-activity on the thermogram. The differential scanning calorimeter (DSC) has a scan rate of I0°C/min, whereas the accelerating rate calorimeter (ARC) has a sensitivity of 0.02°C/min. Consequently, the temperature at which thermal activity is detected by the DSC can be as much as 50°C different from ARC data. 

With the advent of computerized real-time systems, the distinction between continuous and periodic condition monitoring must be modified. Though technically periodic, a scanning system operating fast enough to protect against catastrophic failure is considered continuous. Most people consider one second or faster scan rates as continuous. A scan rate of one second is defined as monitoring each point once each second. 

Figure 1 DSC curves of P7MB representing a cooling cycle (lower) starting from the isotropic melt, and the subsequent heating cycle (upper). Scanning rate 10°C/min.

Figure 4 DSC melting endotherms of P7MB after isothermal crystallization at 135°C, starting from the isotropic melt [10]. The curves correspond to 0, 3,6, and 35 min of crystallization time, from bottom to top. Scanning rate 5°C/min.

Fig. 4.21 Effect of temperature on the anodic behaviour of nickel in 0 025 m H2SO4 -I-0-025 M K2SO4 (pH 1 -3) de-aerated with H2. The curves were determined potentiokinetically at a scan rate of 2 V/h and proceeding from negative to positive (after Cowan and Staehle )...

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

Fig. 19.43 Effect of potential scan rate on the value of for Type 304 stainless steel in O-I mol dm NaCI (after Leckie )...

Now commence the voltage sweep using a scan rate of 5 mV per second, or with a manual polarograph, increase the voltage in steps of 0.05 V. The recorder plot will take the form shown in Fig. 16.4 if a manual instrument is used, then since the current oscillates as mercury drops grow and then fall away, the plot will have a saw-tooth appearance, and for measurement purposes a smooth curve must be drawn through the midpoint of the peaks of the plot. 

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