Temperature Effects

Temperature has an effect on three important phenomena: solubility, vapour pressure and kinetics. While increasing temperature generally increases the magnitude of each effect, there are some aspects of this principle that are particularly relevant to analytical extractions.

Effect of temperature on solubility In most cases involving organic analytes, increasing the temperature of a liquid results in increased analyte solubility. Figure 2 illustrates that for the same three amino acids as shown in Figure 1 a temperature increase from 0 to 75°C results in a three- to fourfold increase in solubility. Even marginally soluble compounds show a dramatic improvement from this simple change in conditions. Indeed, as will be seen later, this principle is used in most extraction procedures.

Effect of temperature on vapour pressure Increasing the temperature of a liquid will result in an increase in vapour pressure. Boiling occurs when the vapour pressure above the liquid equals the applied (usually atmospheric) pressure.

Figure 3 shows calculated vapour pressures for several common solvents. Note that the vapour pressure is relatively large at temperatures as much as 20°C below the boiling point of the solvent. Simple evaporation in a stream of nitrogen at room temperature uses this fact to evaporate a solvent rapidly without boiling. If the applied pressure is raised, the boiling point is also raised, so that the solvent can be maintained in its liquid state at higher temperatures. Press urized fluid extraction uses this phenomenon to advantage. Similarly, lowering the applied pressure, as in a rotary evaporator, reduces the boiling point, allowing faster evaporation at lower temperatures. Finally, at any given temperature, the relative vapour pressure of each compound above the liquid phase provides an estimate of the relative evaporation rates of the liquids. Such knowledge is essential when performing critical steps such as solvent evaporation or solvent exchange.

Effect of temperature on kinetics All chemical processes are affected by the temperature at which the process is occurring, although the exact change in reaction rate with temperature is unique for any process. However, many reaction rates will approximately double for each 10°C increase in temperature, and this rule of thumb can be a helpful guide in understanding the effects of temperature changes. These changes can be either positive or negative, depending on whether the temperature change is increasing or decreasing. For example, storing samples and solutions at low temperatures slows down evaporation and degradation. These processes are about four times slower if the solution is stored at 4°C compared to room temperature.

Solar Panel Basics

Solar Panel Basics

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