2.3.9 Correcting Volumetric Readings

Table 2.11 % Volume Corrections for VariousSolutions"

" From the ASTM document E 542, Table 6, reprinted with permission.

means that you must subtract 0.09 mL from the stated volume to compensate for water and glass expansion. Thus you actually delivered only 99.91 mL of liquid.

You may occasionally see a soda-lime glass equivalent for Table 2.10, but I have not included it in this book. Although soda-lime glass volumetric ware was common many years ago, it is not used for any accurate volumetric purposes

Table 2.12 Corrections for Water Weight Determinations for Borosilicate Glass Using a One Pan Balance

(Nominal capacity 100 mL)

today. It is unnecessary to provide highly accurate corrections for nonaccurate glassware.

The corrections shown in Table 2.10 are only valid for distilled water. Different liquids will have different coefficients of expansions and therefore will require different corrections. Table 2.11 provides a few representative solutions at different normalities and a factor to correct Table 2.10 for volumetric discrepancies.

Corrections required when weighing volumetric flasks are somewhat different than straight volumetric readings. Both singleand double-pan balances have four common parameters which can affect the accurate weighing of liquids, but the single-pan balance has one separate parameter of its own. The common parameters are water density, glass expansion, and the buoyancy effect.

Water density varies because as materials get hot, they expand and take up more space. Despite their taking up more space, they still have the same mass and are thus less dense. One liter of hot water would therefore weigh less than one liter of cold water.

Glass expands as it gets hot. The effects of expansion of solid materials are well demonstrated with the ring-and-ball demonstration. In this demonstration, a ring is unable to get past a ball on the end of a rod. If the ring is heated just a small amount, it expands sufficiently to easily slide past the ball. In a similar fashion, a warm glass container holds more liquid than a cool glass container.

The differences in the buoyancy effect (based on Archimedes' principle) of materials in air at different barometric pressures is not as great as the buoyancy differences in water versus air, but it still exists and can affect accurate weighings.

The actual effect of these parameters on any measurement can be calculated by volumetric measurements. Then, their relative significance can be properly considered. The ASTM calculated the values shown in Table 2.13.

As Table 2.13 shows, for most laboratory work, the parameter that is likely to have the greatest effect on measurements when using a two-pan balance is water temperature. Any weight measurements made when the liquid temperature is not 20°C can be corrected by using Table 2.12. This table is used when the volumetric flask is Type I, Class A (borosilicate glass). It may also be used for Type I, Class B

Table 2.13 Relative Significance of Environmental

Parameters on Volume Measurements^

" From the ASTM document E 542, Sec. 14.2.1, reprinted with permission

100 Measurement

(aluminosilicate glass) by adding 0.0006 degree for every degree below 20°C. Likewise, subtract 0.0006 degree for every degree above 20°C.

A sample calculation (using Table 2.12) for Type I, Class A glass is as follows:

As can be seen from this example, the correction is 10 times smaller than the tolerances capable from the flask itself (0.03 mL). Thus, to make the time spent on any such correction worthwhile, you need to see that any changes caused by temperature are at least equal to, or greater than, inaccuracies of the container. Otherwise, do not bother with any correction.

It is interesting to note that for both Table 2.12 and Table 2.14, there are corrections for 20°C and 760 mm of atmospheric pressure. Because the glassware is calibrated for this temperature and pressure, one may wonder why a correction is necessary. The reason goes back to Archimedes' principle which cannot be accounted for when the glass is calibrated. The only way to avoid use of these tables at STP is to do all weighing in a vacuum to avoid the effects of air's weight.

Corrections for single-pan balances include the preceding parameters, plus a fourth one: the apparent mass of the built-in weights. The apparent mass of weights can vary somewhat from their true mass because the weights in a singlepan balance were calibrated at a specific temperature and barometric pressure. However, in a lab of different temperature and barometric pressure, you are weighing against their apparent mass.1^ Thus, when making accurate weighings with a single-pan balance, you must keep track of the temperature and barometric pressure of the room in which you are working.

Calibration correction tables for single-pan balances are provided in Table 2.14, which is used in the same manner as the previous example. Like Table 2.12, this table is used when a volumetric flask is Type I, Class A (borosilicate glass). It may

The figures in the tables are based on 100-mL sample sizes. The sample calculation is made with a 25-mL flask, therefore the calculations need to be multiplied by 0.25.

fOld single-pan balances used brass weights with a specified density (at 20°C) of 8.3909 g/cm3. Now, stainless steel weights are used with a specified density of 8.0 g/cm3. Fortunately, the differences of this change are too small for most purposes to be concerned about requiring any further calibration between the two.