Chapter 16 Temperature return to Table of Contents
We have refrigerators because food spoilage is retarded at low temperatures. The combustion of wood occurs at a measurable rate only at high temperatures. An egg cooks in boiling water much faster at sea level than in Leadville, Colorado (elevation 10,000 feet), where the boiling point of water is approximately 90º C. These observations and others lead us to conclude that chemical reactions speed up when the temperature is increased. Experiments have shown that virtually all rate constants show an exponential increase with absolute temperature.
The collision model describes the concept that molecules must collide in order to react chemically. The kinetic molecular theory of gases predicts that an increase in temperature raises molecular velocities and so increases the frequency of collisions between molecules.
Temperature is one of the most frequently taken tests in the water/wastewater industry. Accurate water temperature readings are important not only for historical purposes but also because of its influence on chemical reaction rates, biological growth, dissolved gas concentrations, and water stability with respect to calcium carbonate, in addition to its acceptability by consumers for drinking.
The impacts of temperature on process performance can be, and often are, of prime importance. Care should be taken when applying the results observed at one facility to another without understanding the temperature conditions. Care should also be exercised when applying temperature adjustments in process kinetic characterizations, although it should be said that there is generally good confidence in process kinetic temperature descriptions of nitrifier growth rates.
Furthermore, the solutions change volumes, most expanding when raising the temperature. Water is significantly unique in its behavior, shrinking in volume from 0 to 4º C, thereafter expanding with temperature rise. This volume variation affects the reagent concentration. Diluting most acids and bases in water is exothermic, whereas dissolving ammonium chloride is endothermic. Therefore, final volume adjustments should always be delayed until the solution’s temperature has stabilized at the initial temperature.
Changes in temperature also have an important effect on the rates of chemical reactions. In general, the rate constant k of a chemical reaction will increase with increasing temperature according to this function: , Ea is an empirical constant, the energy of activation, R is the universal gas constant, and T is the Kelvin temperature. This equation is called the Arrhenius equation. For typical values of this parameter, a 10º C rise in temperature will cause the reaction rate to increase twofold or threefold. While temperature is important to the rate of collisions, the molecular orientations during these collisions determine reaction or no reaction. The net effect of this principle applied to water treatment is that oxidation rates may be substantially retarded in colder waters.
Microbial regrowth is not only keyed to bacterial strains that quickly adjust to limited nutrient sources but also to water temperature elevations. Water temperature above 50º F (10º C) accelerates the growth of adapted organisms with slow generation times. Low water temperatures result in a precarious balance between new cell development and death of old cells. Data available from water systems located in geographical areas of pronounced seasonal temperature changes suggest that regrowth of some organisms is more pronounced than among coliforms, and abrupt surges in density may occur in summer. Nutrient accumulations in particulates during periods of minimal microbial activity in winter may be the key in summer bacterial regrowth occurrences.
The influence of temperature is often confused by conflicting observations. The confusion can be avoided by recognizing basic equilibria in water chemistry and by remembering that temperature effects are complex and depend on both the water chemistry and the surfaces containing the water mass.
Temperature significantly affects the dissolving of CaCO3. Less CaCO3 dissolves at higher temperatures, which means that CaCO3 tends to come out of solution (precipitate) more readily at high temperatures. The effect of temperature on pH is seldom recognized. For pure water, pH decreases with increasing temperature. But the degree of influence of temperature on pH is a function of the alkalinity of the water. [The molecules are moving faster, with more energy, so you may be sensing more of the ions in a time frame]. Complex and interrelated parameters can be difficult to make confident analyses, but the lab analyst can with enough reliable, recorded data.
Temperature is one of the most important factors affecting biological growth. The nitrification process occurs over a range of approximately 4 to 45º C, with about 35º C optimum for Nitrosomonas and 35 to 42º C optimum for Nitrobacter. Temperature measurements can be helpful in detecting changes in raw wastewater quality. One of the many uses of this test is to calculate the percent saturation of dissolved oxygen in the DO test.
As explained in Chapter 7, the temperature in the incubator for the BOD temperature must be held to within 1º C of the desired temperature of 20º C in the dark for 5 days. In Chapter 8, the tubes are then incubated at 35 ± 0.5º C for 24 to 48 hours. In Chapter 9, the TVS, TVSS, and TVDS fractions are determined by the difference between the TS, TSS, and TDS values, respectively, and the ash value obtained from ignition of the respective solids fraction at 550 ± 50º C for an hour. Temperature is an important variable that must be observed.
There are three types of thermometers and two scales in general, normal use. There are always special, custom types of instruments for unique purposes, usually very costly. Total immersion, partial immersion and dial thermometers are available, not only calibrated to the Celsius or Fahrenheit scales, but to differing sensitivities and ranges.
Measure the temperature. Not as simple as it sounds. Correct procedure is necessary. The sample will change temperature as you are trying to measure. The larger the sample, the slower the temperature will change. The probe should be inserted according to the manufacturer’s directions. When the thermometer reading is stable for a minute, record that reading to the nearest fraction of a degree. Then write down the time and all the rest of the pertinent information. Whichever scale the thermometer is divided, mathematical conversion makes the temperature reading convenient for the reader.
One interesting feature of the Celsius and Fahrenheit scales is that -40º F and -40º C represent the same temperature.
The temperature known as absolute zero is defined as “the cessation of all motion”. Everything vibrates. Electrons when shifting orbits, emit radiation, which we sense as light. Atoms or small molecules vibrating, we sense as temperature. Large groups of molecules moving, we sense as pressure, touch or hearing. Heat is ‘how many molecules are vibrating’, temperature is how fast some of them are vibrating. A red-hot needle is at high temperature, but doesn't hold much heat.
66º Bé Sulfuric acid freezes
@ -29º F
65¼º Bé Sulfuric acid freezes
@ 24.6º F
|25% NaOH gels @ 0º F||
65½º Bé Sulfuric acid freezes
@ 13º F
64º Bé Sulfuric acid freezes
@ 46º F
50% Sodium Hydroxide gels
@ 52º F
|HCl freezes @ -63º F|
Etymology: Antoine Baumé
: being, calibrated in accordance with, or according to either of two
arbitrary hydrometer scales for liquids lighter than water or for liquids
heavier than water that indicate specific gravity in degrees.
[I can not make the equation work, something is missing. If someone out there knows more please let me know.]
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