Calorimeter bucket

The calorimeter bucket contains the bomh plus a sufficient quantity of water to completely immerse the bomb in order to absorb the heal released from the combustion. A stirrer is used in Ihe bucket to facilitate bringing the bucket and its contents to thermal equilibrium. 

The jacket that contains the bucket with its bomh provides a thermal shield to control the heat transfer between the calorimeter bucket and its surroundings. In an isoperibol calorimeter, it is not necessary to prevent this transfer, as long as a means of precisely determining the amount of heat transferred during Ihe determination can be established. 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

Experimental. A Parr model 1221 oxygen bomb calorimeter was modified for isothermal operation and to ensure solution of nitrogen oxides (2). The space between the water jacket and the case was filled with vermiculite (exploded mica) to improve insulation. A flexible 1000-watt heater (Cenco No. 16565-3) was bent in the form of a circle to fit just within the jacket about 1 cm. above the bottom. Heater ends were soldered through the orifices left by removing the hot and cold water valves. A copper-constantan thermocouple and a precision platinum resistance thermometer (Minco model S37-2) were calibrated by comparison with a National Bureau of Standards-calibrated Leeds and Northrup model 8164 platinum resistance thermometer. The thermometer was used to sense the temperature within the calorimeter bucket the thermocouple sensed the jacket temperature. A mercury-in-glass thermoregulator (Philadelphia Scientific Glass model CE-712) was used to control the jacket temperature. 

Note The water equivalent factor includes the heat capacity of the ensemble which includes the calorimeter itself, water bucket with water in it, supports of the bucket, bomb, thermometer and stirring device. These values can be calculated theoretically, but much more accurate results are obtained by combustion of a sample of standard material (such as benzoic acid, supplied by the US Bureau of Standards), the heat of combustion of which has been previously accurately determined. The water equivalent values of the calorimeters and bombs supplied by the Parr Instrument Co are determined by them at the factory... 

Any oxygen bomb calorimeter consists of four essential parts (I) a bomb or vessel in which the sample is burned (2) a bucket or container which holds the bomb as well as a precisely measured quantity of water to absorb the heat released from the bomb and a stirring device to aid in achieving rapid thermal equilibrium (3) a jacket for protecting the bucket from transient thermal stresses and (4) a thermometer for measuring temperature changes within the bucket The cross section of a simple calorimeter is shown in Fig. I. 

The drop calorimeter starts a sample at a high temperature (TH = 300°C to 1600°C) and then suddenly "drops" it into a bucket at room temperature the (small) rise in thebucket temperature is related to several initial values of TH. This is practical for high-temperature measurementsbuthas relatively lowprecision. 

As mentioned earlier, we do observe that a burning candle can make water in a test tube boil in minutes while on a bucketful of water it cannot do so even in hours. Water in the bucket absorbs much more heat obviously than water in test tube without corresponding rise in temperature. So, while we have developed the concept of temperature through our senses, the same temperature is not a measure of the heat contained in or transferred to a system. For the measurement of heat we normally take the help of a calorimeter. 

The heat of solution of sodium chloride in water might be determined by use of a calorimeter similar to that shown in Figure 32-1, but provided with a central container in which water is placed, with a little bucket of salt crystals arranged in such a way as to permit the bucket to be dropped into the water during the experiment. A stirrer for the salt solution would also be needed in order to cause the salt to disserve sufficiently rapidly. When this experiment is carried out, it is found that the process of solution of 1 gfw of sodium chloride in water is accompanied by the absorption of approximately 1200 cal. The heat of the reaction depends slightly on the concentration of the solution that is produced. We may express this heat effect by the following equation ... 

FIGURE 6.6 A constant-volume bomb calorimeter. The calorimeter is filled with oxygen gas before it is placed in the bucket. 

Figure 6.8 A constant-volume bomb calorimeter. The calorimeter is filled with oxygen gas before it is placed in the bucket. The sample is ignited electrically, and the heat produced by the reaction can be accurately determined by measuring the temperature increase in the known amount of surrounding water.

Specific heats vary markedly from substance to substance, and vary to some extent with temperature for any given substance. Some approximate values under ordinary conditions are air (standard pressiure), 0,25 cal/g-deg water, 1.00 ice, 0.5 alcohol, 0.58 copper, 0,09. Heat measurements are made by mixing known amounts of reactants in a calorimeter (Fig. 5.1). The heat evolved by the reaction is equal to the heat absorbed by a known quantity of water, the metal bucket, the metal reaction chamber containing known quantities of reactants, the stirrer, and the thermometer. The heat capacity of the calorimeter is determined by putting in a known amount of energy and measuring the temperature rise. 

The bomb is placed in a bucket containing a measured amount of water, and the water is stirred until a steady or steadily changing temperature is attained, as measured by a thermometer in the water. The sample is ignited electrically, the reaction occurs, and the bomb, water, and bucket are allowed to stabilize. The amount of heat transferred to the calorimeter (bomb, water, and bucket) is calculated from knowledge of the heat capacity of the calorimeter. This heat capacity is very nearly independent of temperature so that... 

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