As we have seen that the volumetric analysis essentially requires the precise and accurate measurement of weights and volumes of interacting solutions. However, the weights are measured upto the fourth place of decimal by using a manually operated good analytical balance or a single-pan electrical balance that need to be calibrated periodically with the help of a standard weight box.
In the broader sense, volumetric apparatus may be divided into two categories, namely :
(a) To deliver a definite volume of liquid, and
(b) To contain a definite volume of liquid.
The two specific volumetric apparatus meant to deliver a defnite volume of liquid are burettes and pipettes which will be discussed very briefly below :
Various official compendia specifies a standard temperature (°C) for glass volumetric apparatus as mentioned hereunder :
Pharmacopoeia of India (IP) : 27°C ;
United States Pharmacopoeia (USP) and National Formulatory (NF) : 25°C ;
National Bureau of Standards (NBS) : 20°C.
A burette is a graduated glass tube of uniform bore throughout the entire length, used for the accurate delivery and measurement of variable volumes of liquids. Burettes are graduated into millilitres (ml) and 1/10 millilitres (0.1 ml) and are made of varying capacity ranging from 1 ml to 100 ml ; however, the most common size is the 50 ml burette that is used invariably and conveniently for most volumetric titrations. They are usually closed at the bottom either by a Teflon or glass stopcock to monitor and control the outflow of liquid.
Specifications : The design, construction and capacity of volumetric glassware must be in accordance with those laid down by the Indian Standards Institution (ISI). The tolerances on capacity for burettes, as speci-fied in the relevant Indian Standards Institution, specifications are given in Table 2.1.
British Standards Institution (B.S. 846 : 1962) has laid down specifications for burettes and these are produced to either Class ‘A’ or Class ‘B’ specifications. All Class ‘A’ and a few of Class ‘B’ burettes have graduations that extend right round the barrel (or stem) of the burette to minimise errors due to parallax while taking the exact burette reading. It may be noted that Class ‘B’ burettes are normally graduated on one side only. Permitted tolerances on capacity for burettes used in common practice are stated in Table 2.2.
In fact, the tolerance actually represents the maximum error allowed at any point and also the maximum difference allowed between the errors at any two points. For instance, a tolerance of ± 0.05 ml signifies that the burette may have an error at any point by ± 0.05 ml, provided that the difference between the errors at any two given points does not exceed 0.05 ml.
Burettes calibrated at 20°C and 25°C deliver different weights of water for each 10 ml, when weighed with standard brass weights in air at 50% relative humidity (RH) at standard atmospheric pressure, as given below :
Hence, the true volume for each 10 ml segment of the burette can be calculated from the weights obtained and recorded on a convenient chart.
Leakage : A burette must be tested for any sort of leakage before putting it into operation. Teflon stopcocks are usually adjusted by a knurled nut for perfect use. Glass stopcocks may require a small quantity of a special type of grease or lubricant to allow both ease of operation and to check leakage.
Outlet Tip : From a practical point of view the outlet tip of either types of burette, i.e., having Teflon or glass stopcocks, must be of such diameter and taper as to allow the delivery of a single drop whose volume is significantly less than that which can be held between any two finest graduations of the scale with which the burette is calibrated.
Use of the Burette : The following steps are usually observed while operating a burette, namely :
(i) Burette tap is neatly lubricated with a thin-film of grease,
(ii) Rinse the burette, before putting it into operation, at least twice with small volumes of the solution (titrant), say about 5.0 ml, carefully draining out the solution between the addition of each portion,
(iii) Pour the solution into the burette until the former is little above the zero mark,
(iv) Open the burette tap slowly to fill up the tip of the burette and to expel all air bubbles,
(v) With the zero at eye-level carefully, drain out the liquid until the lower part of the meniscus is either at level or just below the Zero mark,
(vi) Remove the drop on the tip of the burette by just touching rapidly against the inner-neck of a flask or a porcelain tile,
(vii) The Class ‘B’ burettes should be read at level so as to avoid errors due to parallax,
(viii) To assist easy and accurate observation of the meniscus (lower for colourless solutions and upper for coloured solutions) it is always advisable to hold a piece of white paper behind the burette at the appropriate level,
(ix) Burette readings may be recorded to the nearest 0.02 ml, and
(x) Once a titration is completed, 15 seconds duration should be allowed to elapse before the final read-ing is made, to allow for drainage.
The pipette is the second volumetric apparatus that is meant to deliver a definite volume of liquid. Pipettes are of two types, namely :
(i) Transfer Pipettes : They have only one specific mark engraved on them and are specifically employed to deliver (or transfer) a definite volume of liquid under certain specified conditions, and
(ii) Graduated Pipettes : They have graduated stems and are used to deliver different small volumes as needed. However, they are not normally used for measuring very exact volumes of liquids.
The tolerances on capacity for pipettes, as specified by the Indian Standards Institution (ISI), are stated in Tables 2.3 and 2.4.
The British Standards Institution (BSI) has laid down the permitted tolerances and delivery times for commonly used bulb transfer pipettes as shown in Table 2.5.
The USP specifies the following tolerances accepted by the National Bureau of Standards for transfer pipettes :
The salient features of single-graduation mark transfer pipettes are :
(a) Capacity, temperature at which it was graduated (Ex) and reference to delivery time in seconds is stated on the bulb e.g., BOROSIL 1552 25 secs ‘A’ Ex 20 ml 20°C BS 1583.
(b) Class ‘A’ pipettes do mention the delivery time,
(c) Drainage time is specified, though an additional waiting time of 3 seconds after apparent cessation of flow is still important.
Note : The stated times apply only for water and aqueous solutions.
Use of the Transfer Pipette : The following steps mentioned sequentially must be followed while making use of a transfer pipette :
· Always rinse the pipette with DW before use and allow it to drain as completely as possible,
· Droplets of water remaining in the tip must be removed by touching against filter paper; and at the same time wipe out the outer surface of the pipette to prevent dilution of the solution to be pipetted,
· Rinse the pipette with 2 to 3 small portions (5 ml) of the solution and drain out the liquid completely,
· Gently suck the liquid up into the pipette a little above the single graduation mark and quickly shut the upper end of the pipette with the tip of the index finger. Now, remove the pipette from the stock solution and carefully wipe out the outer surface of the stem free from any liquid adhering to it. Hold the pipette vertically and keeping the graduation mark at the eye-level, slowly release the pressure on the index finger until the bottom of the meniscus just coincides with the graduation mark. Maintain sufficient pressure on the index-fnger so as to check any escape of liquid from the pipette, and quickly get rid of the drop attached to the tip by gently touching against a porcelain tile. Put the pipette into the receiving container, and permit the liquid to drain out with the tip of the pipette touching the inside of the container at an angle of 60°, taking care that the tip must not be dipping into the deliv-ered liquid. After all the solution has drained out, hold the pipette in this position for at least 3 seconds (waiting time), and then remove the pipette.
· The National Physical Laboratory (NPL) describes a method of reading meniscus in graduated glassware, viz., a dark horizontal line on a white background is placed 1 mm below the meniscus. A slight adjustment of the position of the dark line causes the meniscus to stand out sharply against the white background,
· The small drop of liquid that remains in the tip of the emptied pipette is taken into account while doing the calibration, and hence, it must not be added to the delivered liquid by blowing down the pipette.
· Liquids having more viscosity and much larger surface tension than water must be provided with adequate draining time e.g., strong solution of iodine.
· Presently, many analysts make use of pipette filler for sucking in and draining out of liquids from the transfer pipettes for obvious reasons.
Automatic Pipettes (Transfer Pipettes) : Automatic pipettes are always preferred to ordinary transfer pipettes because of their ability to handle corrosive and toxic liquids in routine analytical laboratories, e.g., determination of Iodine Value in edible oils by iodine-monochloride (ICl) solution.
The automatic pipette (Figure 2.1) dispenses a stated volume of liquid when filled with liquid used in the assay from tip (B) to tip (C) and is allowed to drain out in the normal manner. D is connected to an aspirator which is placed above the pipette so as to enable the solution to flow under gravity.
Operation of the Automatic Pipette : The automatic pipette may be operated by observing the following steps in a sequential manner :
· Turn the two-way tap clockwise to open so that the solution starts flowing into the pipette.
· After about 5.0 ml has run into the pipette, turn A clockwise through 180°, so that solution now flows from the pipette to fill the delivery tube B.
· As soon as B is full upto the tip, again turn A clockwise through 180°, so that the body of the pipette is filled completely to the top-tip.
· Close the tap A by turning clockwise through 90° when the solution starts to overflow at C.
· The pipette is now full from the lower-tip to the upper-tip and is ready for operation.
· Remove the drop of solution from tip B, run out and drain for 15 seconds in the usual way.
The two particular volumetric apparatus meant to contain a definite volume of liquid are volumetric flasks (also known as measuring or graduated flasks) and measuring cylinders (also known as graduated cylin-ders) which will be discussed here briefly :
Volumetric flasks are normally round or pear-shaped, flat-bottomed; having a long-neck, which possesses a single graduation mark round the neck.
Flasks bearing one graduation mark, are meant to contain specified volume of liquid at 20°C, when the lower part of the meniscus coincides with the mark and are known as volumetric flasks.
The long and narrow neck of uniform diameter affords as a measure of accurate adjustment, since the height of the liquid is sensitive enough to small variations of volume.
Litre—is defined as ‘the volume occupied by one kilogram of water at its temperature of maximum density (4°C) and subjected to normal atmospheric pressure’. The litre is considered as the standard unit of volume for all volumetric measurements.
The cubic centimetre is the volume occupied by a cube of which each side is 1 cm in length, and thus, 1 litre equals 1000.028 c.c. Therefore, it follows from here that the millilitre and cubic centimetre are not the same, though the difference is quite negligible. Hence, all volumetric apparatus is universally standardized in millilitres.
The Indian Standard Institution (ISI) has laid down the tolerances on capacity of volumetric flasks (with different capacities) calibrated at 27°C as stated in Table 2.6.
The United States Pharmacopoeia (USP) requirements for volumetric flasks calibrated to contain the indicated volume at 25°C are given in Table 2.7.
The British Standards Institution (BSI) and the National Physical Loaboratory (NPL) have laid down the tolerances in the capacity of volumetric flasks (i.e., measuring flask) at 20°C by two sets of toler-ances viz., Grade ‘A’ and Grade ‘B’ respectively, evidently to indicate the class of accuracy to which the flask has been subjected to for graduation, followed by the manufacturer’s name and finally the BS standard number. However, the permitted tolerances for volumetric flasks commonly used in analytical laboratories are de-picted in Table 2.8.
A reasonably well established analytical laboratory requires a number of standard solutions for its routine as well as specific assays. Therefore, it necessitates to know the intricacies of preparing the standard solutions as detailed in the following steps :
· Transfer the requisite quantity of the accurately weighed pharmaceutical substances or solid quantitatively into a beaker and dissolve it in either distilled water (DW) or other specified solvent,
· Pour the resulting solution quantitatively, into the funnel placed in the mouth of the volumetric flask with the help of a glass rod and a sharp jet of water from a wash-bottle by holding the beaker with the right hand and the guiding rod with the left hand,
· Wash down the contents of the beaker through the funnel by means of the glass rod and the jet of DW. Repeat the process several times till the flask is 2/3rd full,
· Remove the funnel, swirl the contents of the volumetric flask and make up the volume upto the mark,
· Final adjustment of the volume must be made with the help of a teat pipette by adding DW/solvent dropwise. In doing so, adequate care should be taken to allow suffcient time for water/solvent to drain-down the inside of the neck of the flask, and
· Finally shake the contents of the flask thoroughly for 2 to 3 minutes to obtain a perfect homogeneous solution.
(i) For precise work, the temperature of the solution must be adjusted to 20°C before making the volume upto the mark,
(ii) Standard solutions are usually stored in stock-bottles,
(iii) Ensure before any transfer is actually affected that the receiving vessel must be rinsed with at least 2 to 3 successive small quantities of the solution, and
(iv) When a standard solution is used a while after preparation, the contents of the stockbottle must be shaken thoroughly before any solution is withdrawn, thereby the condensed droplets of water collected on the inside neck of the container gets mixed with the main bulk of the solution.
The graduated cylinders are also referred to as the measuring cylinders among volumetric apparatus meant to contain a definite volume of liquid. Measuring cylinders are containers either unstoppered or stoppered having a wide range of capacities varying from 5 ml upto 2000 ml (2 Litres). In usual practice, the smaller cylinders upto 100 ml are normally graduated either in fractions of a millimitre or in millilitres. On the contrary, the large cylinders are graduated in units of 2, 5, 10, 20, or 50 ml, as per their specific size and volume. However, it is pertinent to mention here that measuring cylinders are used in a broader sense for measuring volumes of solution when only approximate volumes are needed.