(2) Conical graduate (pharmaceutical graduate). The conical graduate is
the vessel most widely used to determine the volume of a liquid. Although it is slightly
less accurate than the graduated cylinder, it is easier to work with and has the
advantage of being calibrated in both the metric (milliliters) and apothecary (minims,
drams, and ounces) systems.
(3) Receiving versus delivery capacity. Since it is seldom possible to pour
all the liquid from the graduate after it has been measured, a differentiation must be
made between the receiving capacity (holding capacity) and the delivery capacity of the
graduate. Most graduates manufactured today are compensated to read delivery
capacity and many are marked "TD" meaning "to deliver."
(4) Medicine dropper. Too many factors enter into the delivery of a drop of
liquid from a dropper to accurately give it any unit of volume.
b. Technique of Measuring Liquids. The proper technique for measuring
liquids is just as important as the proper technique for weighing solids. It is wise to
learn the technique and never deviate from it for any reason.
(1) Choosing the graduate. Choose either a conical or a cylindrical
graduate. For most pharmaceutical operations, the conical graduate is better, except
where extreme accuracy is required. Choose the proper size graduate for the volume
you intend to measure. Obviously, you would select neither a 1-ounce graduate to
measure 8-ounces of liquid nor an 8-ounce graduate to measure 1-ounce. A large
graduate used to measure a small quantity of liquid will give you a large error. Choose
a graduate only a little larger than the amount of liquid you intend to measure. For
example, use a 4-ounce graduate to measure 3-1/2-ounces.
(2) Holding the graduate. To measure the liquid, hold the graduate between
your thumb and forefinger, allowing the base to rest on your bent middle finger. If you
are right-handed, you will probably find it most convenient to hold the graduate in your
left hand and left-handers will find the right hand more comfortable.
(3) Holding the bottle. Grasp the bottle to be poured from in the other hand
and remove the stopper, cap, or cork with the little finger of the hand in which you are
holding the graduate. Make sure the labeled side of the bottle is facing up so that its
contents will not drip or spill on the label and deface it.
(4) Holding graduate at eye level. Raise the graduate so that the mark at
the desired volume is at eye level. This minimizes the reading error known as error of
parallax. Optical illusion distorts the actual level and causes considerable error. Pour
the liquid slowly into the graduate until the bottom of the meniscus exactly reaches the
required mark (see figure 1-4). The meniscus is the natural curvature of the surface of