Calibration of bomb calorimeter
Calibration of bomb calorimeter
PURPOSE
The purpose of this standard operating procedure is to provide the guidelines of calibration of bomb calorimeter.
SCOPE
The scope of this standard operating procedure includes the guidelines of calibration of bomb calorimeter.
PROCEDURE
The effective heat capacity of the system is determined by burning pure and dry benzoic acid weighing not less than 0.9 and not more than 1.1 gram. Determine the corrected temperature rise T, from the observed test data , also titrate the bomb washings to determine the nitric acid correction, and measure the unburnt fuse fire ,compute the energy equivalent by substitution in the following equations:-
H M + E1 + E2
W = ——————————
T
Where
W = energy equivalent of calorimeter in calories per degree centigrade.
H = calorific value of standard benzoic acid/ sample in calories per gram
M = mass of standard benzoic acid/samples in grams.
T = corrected temperature rise in degrees C.
E1 = correction for heat formation of nitric acid in calories, and
E2 = correction for heat of combusting of firing wire, in calories
CORRECTIONS TO BE MADE
- Cooling Correction: Cooling correction includes the effects of the heat interchange between the vessel and the jacket due to conduction, convection and radiation, and of the heats of stirring and evaporation. This is minimized by having the temperature of the water in the vessel below that of the water in the jacket at the time of firing and within ±1 o C, preferably ± 5oC, of it at the end of the chief period a low heat of stirring and a low loss by evaporation. The correction shall be determined as follows by the use of the
Regnault – Pfaundier formula or the Whitaker Formula which gives, for a representative series of coals, a mean correction differing by not more than one part in 1000 from that given by the Regnault – Pfaundier formula.
- Whitaker Correction: In place of the Regnault – Pfaundier cooling correction, which s cumbersome, the Whitaker correction may be adopted, provided that it has also been employed to find the water equivalent or the heat capacity of the apparatus. The heat capacity will be found by the Whitaker correction to be about to be 0.25 to 0.3 percent lower than the value given when R-Pcorrection is used – a difference of 7 to 9 in 3000.
To employ the Whitaker correction ten temperature readings are taken in the after period in order to obtain a more reliable value of the cooling rate per minute.
If this rate be denoted by the dt and the time from the instant of firing to any chosen temperature reading in the steady cooling period be x minutes, then the cooling correction is simply (X x dt)oC.
The “observed rise” is the difference between the temperature at the instant of firing and the temperature taken x minutes later in the cooling range. It is not necessary, nor is it desirable , to take the actual maximum temperature reading.
Generally , a temperature taken 2 or 3 minutes later in the steady range of cooling forms a more reliable and definite “observed maximum”. The temperature value at x, of course , depends upon the magnitude of x, but the same corrected temperature rise is obtained provided that dt is constant.
- Heat of Ignition: The heat release from the cotton and firing wire is subtracted from the total heat release. The heat release from the cotton is calculated from the weight, after drying at 105oC, of a known length of cotton thread, and using the calorific value of cellulose (4180 cal/g). Determine the weight of piece of wire equal in the length to the distance between the poles of the bomb, and calculate the heat release by allowing 335 cal /g for platinum wire.
- Heat of formation of Acids: The heat gain due to the formation of sulphuric acid and nitric acid is subtracted from the total heat released. These corrections amount to 3.60 cal/ml of 0.1 N sulphuric acid 1.43 cal/ml of 0.1 N nitric acid presents in the bomb washings and are calculated as follows: –
Sulphuric acid correction = 3.60(a+b-20) cal
Nitric acid correction = 1.43 (20-a) cal
Where
a= vol in ml of 0.1 N hydrochloric acid used
b= vol in ml of 0.1 barium hydroxide used
- Correction for Unburned Carbon: If unburned carbon is suspected, its heat equivalent on the basis of 1 mg of carbon equals 8 calories shall be added to the determined heat release. Unburned carbon is determined as the loss in weight on ignition of the residue from the crucible.
Calculation: The following example illustrates the method of calculation using the Whitaker corrections:
Calculation using Whitaker correction:
Weight of crucible + thread = 5.298 g
Weight of crucible + thread + coal = 6.301 g
Weight of air – dried coal = 1.003 g
Mean Calorimeter temperature = 26.5oC
Effectivemean heat capacity at 26.5oC = 2.863 cal / oC
Correction for sulphur in coal = 16 cal
Temperature of calorimeter jacket = 26.7 oC
Temperature of thermometer settings = 24.6 oC
Room Temperature = 26.6 oC
Moisture in coal = 1.90 %
Ash = 12.1%
Uncorrected temperature rise = 2.480oC
Add cooling correction = 0.048 oC
Add thermometer correction = 0.014 oC
Corrected temperature rise = 2.542 oC
Heat Liberated = 7.278cal
Subtract nitric acid corrected = 8 cal
Subtract correction for heat of Ignition = 21 cal
Heat from 1.003 3g coal = 7249 cal
Heat from 1.000 0 g coal = 7225 cal
Subtract sulphur correction = 16 cal
Calorific value of air dried coal = 7209 cal/g
Rounding Off– Round off the mean of the values or the single value4 to the nearest 10 cal /g
Precision– The results of duplicate determination in one laboratory should agree towithin 50 cal/ g and those between different laboratory to within 100 cal / g.
PRECAUTIONS
- Do not use too much sample .the bomb can not be expected to withstand the effects of combustible charges which liberated more than 10,000 calories. this generally limits the total weight of combustible material (sample plus gelatin firing oil or any combustion aid) does not more than 1.10 gram .Do not charge with more oxygen than is necessary and do not fire the bomb if an overcharge of oxygen should accidentally be admitted.
- Keep all parts of the bomb especially the insulated electrode assembly in good repair at all times Do not fire the bomb if gas bubbles are leaking from the bomb when it is submerged in water.
- Proceed with caution and use only a fraction of the allowable maximum sample when testing new materials, which burn rapidly, or have explosive characteristics.
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