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1.9 VOLUMETRIC APPARATUS

1.9 VOLUMETRIC APPARATUS

          Most of the volumetric apparatus used in the analytical operation is calibrated at 20º, although the temperature specified generally for Pharmacopoeial tests and assays is 25º. This discrepancy is inconsequential provided the room temperature is reasonably constant.

          Use To attain the degree of precision required in many Pharmacopoeial assays involving accurate volumetric measurements, the apparatus must be chosen and used with care. A burette should be of such size that the titrant volume represents not less than 30 per cent of the nominal volume. Where less than 10 ml of titrant is to be measured, a 10-ml burette or a microburette generally is required.

          The design of volumetric apparatus is an important factor in assuring accuracy. For example, the length of the graduated portions of graduated cylinders should be not less than five times the inside diameter, and the tips of burettes and pipettes should restrict the outflow rate to not more than 500 μL per second.

          Standard of accuracy The capacity tolerances for volumetric flasks, transfer and graduated pipettes, and burettes are those accepted by the International Organization for Standardization (ISO) as indicated in the accompanying tables.

Volumetric Flasks

Nominal Capacity (mL) Capacity Tolerances (±mL)
 5 0.025
10 0.025
25 0.040 
50 0.060 
100   0.100 
200  0.150 
250  0.150 
500  0.250 
1000    0.400 
2000    0.600 

                                                                                                                                                   From ISO: 1042:1999

           Transfer and graduated pipettes calibrated “to deliver” should be drained in a vertical position and then touched against the wall of the receiving vessel to drain the tips. Volume readings on burettes should be estimated to the nearest 0.01 ml for 25- and 50-ml burettes, and to the nearest 0.005 ml for 5- and 10-ml burettes. Pipettes calibrated “to contain” are called for in special cases, generally for measuring viscous fluids like syrups; however, a volumetric flask may be substituted for a “to contain” pipette. In such cases, the pipette or flask should be washed clean, after draining, and the washings added to the measured portion.

Transfer Pipettes

Nominal Capacity (mL) Capacity Tolerances (±mL)
1 0.008
2 0.010
5 0.015
10  0.02  
20  0.03  
25  0.03  
50  0.05  
100   0.08  

From ISO 648:2008

Burettes​

Nominal Capacity (mL) Capacity Tolerances (±mL)
10 0.02
25 0.03
50 0.05
100  0.10

 From ISO 385:2005

1.10 WEIGHTS AND BALANCES 

          Pharmacopoeial tests and assays require the use of balance of capacity and sensitivity corresponding to the degree of accuracy sought.

          When weighing quantities of 50 mg or more that are to be “accurately weighed”, an analytical balance of 100 to 200 g capacity and 0.1 mg sensitivity is required. When weighing quantities of less than 50 mg that are to be “accurately weighed”, an analytical balance of 20 g capacity and 0.001 mg sensitivity, usually called an analytical microbalance, is required.

Apparatus

          Analytical balances should possess adequate capacity and sensitivity. They may be either of the equalarm type, requiring the use of a set of calibrated weights, or of any other suitable type (for example, analytical microbalances using magnetic measurement) provided their performance is periodically checked by means of a reference set of calibrated weights.

          The analytical balance should be so constructed as to support its full capacity without developing undue stress and its sensitivity should not be altered by repeated weighing of the full-capacity load. The type of analytical balance having constant sensitivity over the whole capacity range is the constant-load, single-pan balance. It has a set of weights suspended from a counterpoised beam; in the process of weighing, these are removed from the beam by a manually operated mechanical device until equilibrium is reached.

          The analytical balance should be constructed in a proper housing with suitable openings to permit the placement of the material to be weighed and to exclude air currents. Desiccants may be placed inside the housing, such as silica gel, for the maintenance of a relatively dry atmosphere.

          Sets of calibrated weights used with balances that require manual placement of weights and sets of weights used to check the sensitivity of balances of another type should be kept in a case made of suitable material and properly lined.

Placement of Balance

          The analytical balance should be placed upon a firm foundation that is as free from mechanical vibration as possible, preferably on an antivibration table of proper design. Alternatively, it may be placed on a concrete slab resting upon piers that are either sunk into the ground or connected to the construction elements of the building; or it may be placed upon a stout table or shelf protected by shock absorbers, such as cork mats or sheet rubber.

          The balance should also be protected from humidity and acid fumes, preferably by placing it in a separate room of the laboratory. It should not be placed in a current of air or in direct sunlight.

          The balance should be equipped with a levelling device and an indicator of proper position. Proper adjustment of levelling should be frequently checked.

Checking of Sensitivity

The sensitivity of the balance should be periodically checked by a qualified expert.

Checking the Stability of the Equilibrium Position

          Before the balance is used, its equilibrium position without load should be checked several times. After each test, the balance has to be arrested. The equilibrium position of the balance under load should also be determined from time to time, for example, with one-tenth of the full load and with the full load. The difference between equilibrium positions found in two successive determinations made with equal loads should not exceed 0.1 mg for analytical balances and 0.001 mg for analytical microbalances.

Operation of the Balance 

          When the balance is not in use, the balance beam and pan supports should be raised. The doors of the housing should always be kept closed.

          To release the balance, the beam and pans should be lowered very carefully.

          Objects to be weighed must be allowed to attain the temperature of the balance before weighing is started. The object to be weighed, as well as the weights, should always be placed on the pan as centrally as possible. During a weighing or on any occasion when objects are being added to or removed from the pans, both the beam arrests and the pan supports must be raised. Substances must be weighed in suitable containers such as beakers, weighing bottles, or crucibles. Liquids and volatile or hygroscopic solids must be weighed in tightly closed vessels, such as stoppered weighing bottles. No chemicals or objects that might injure the balance pans should be placed directly upon them.

          When small quantities of a substance (for example, the sulfated ash) must be weighed in a large vessel and a fairly long period elapses between the two weighings, atmospheric pressure and temperature may alter sufficiently to affect the buoyancy and thus cause an appreciable error. In two-pan balances, this error may be eliminated by using another vessel of similar shape and weight for taring.

         The pans of the balance should be periodically lightly brushed with a camel-hair or similar brush to remove any dust that may have collected.

         The weights should be handled only by means of a pair of forceps, which should possess tips covered with suitable material.

Calibration

          If necessary, turn on the power, and allow the balance to equilibrate for at least 1 hour before proceeding with the calibration. (Microbalances may require up to 24 hours to reach equilibrium.) If the balance power has gone off and then has come back on, as in a power outage, certain types of balance may display a message indicating that the balance must be calibrated before a weighing is made. If the operator touches the balance bar, the message may be cleared and the balance may display zeros; however, the balance will not give the correct weighing until it has been calibrated. Electronic analytical balances have an internal calibration system based on an applied load. The calibration applies for the current ambient temperature.

APPENDICES • 1.9 VOLUMETRIC APPARATUS
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หมายเหตุ / Note : TP II 2011 PAGE 337-339