Correcting Volumetric Readings

Volumetric readings can be made two ways. The easiest and most common is simply reading the volume directly from a piece of volumetric ware. Alternatively, you can weigh a sample and, if you know the molecular weight of the material, you can calculate the volume. Each approach can be affected by barometric pressure, humidity, and temperature. The calculations and tables needed to obtain true volume from observed volume or calculated weight are not difficult to use but should only be used when necessary—that is, when accuracy or precision demand their use. 

There are two different approaches for properly correcting volumetric readings caused by environmental variations because there are two approaches to making volumetric readings those done by reading volume directly from volumetric ware, and those made indirectly by weight. 

The simplest corrections are made when reading directly from volumetric ware. As the volumetric container (and the liquid contained) expands and contracts by temperature variations from 20°C, volumetric corrections are required. These corrections can be found on Table 2.10. 

For example, say you had a 100-mL borosilicate volumetric pipette whose liquid was measured at 24°C. Table 2.10 shows -0.09 for these conditions, which 

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 

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Corrections required when weighing volumetric flasks are somewhat different than straight volumetric readings. Both single- and 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. 

It may take a bit of practice to properly see the correct part of the meniscus line for accurate measurement. Fortunately, there are tricks and devices to facilitate the reading. For instance, if the graduation lines on the volumetric ware mostly encircle the tube, it is easy to line up your vision so that you can avoid parallax problems (see Fig. 2.16). 

Three or four aliquots of the sample solution are transferred to volumetric flasks and known, different amounts of a standard solution of the analyte element are added to each of these except one. The solutions in the flasks are then made up to volume. These solutions plus a reagent blank are then run in the electrothermal atomizer. Each reading should have the blank subtracted before plotting. The sample result is then itself blank-corrected. 

The values of h and — Ie (the peak height) were measured with a ruler and the temperature read from a thermometer next to the detector in a metal heating block. The column and injector temperatures were kept constant. The response was corrected for the small change in the standing current by multiplying by Imax/h = 21.2/7/,. The volumetric flow rate was corrected by multiplying by 77300 K. 

Steroid Hormones and Other Steroidal Synthetics Sublimation Pressure for Organic Compounds Surface Tension of Liquid Elements Temperature Correction for Barometer Readings Temperature Correction for Glass Volumetric Apparatus Temperature Correction for Volumetric Solutions Temperature Correction, Glass Scale... 

A stock solntion (1.0 x 10 " moldm ) of NaCl was prepared. Standards were made by adding 2.0, 3.0, 4.0, 6.0 or 10.0 cm of the stock solution to a 100 cm volumetric flask 5cm of MeCN were added to each flask which was then filled to the 100 cm mark with deionized water. A blank was also prepared. The sample for analysis was prepared by dissolving 8.90 mg of [X][PF6l2 xNalPFgl in 2.5 cm of MeCN in a 50 cm volumetric flask. This was filled to the mark with deionized water. This solution was diluted 10-fold with a 5% (by volume) MeCN/water solution. Using a sodinm hollow cathode lamp (A = 589nm), AAS was nsed to determine the absorbance of each standard. Each absorbance reading was corrected for the absorbance of the blank and the data are tabulated below ... 

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