Chemistry Department Pacific University

Appendix-Volumetric Glassware 1

Using Volumetric Glassware

Cleaning glassware Volumetric glassware (to deliver) must be clean to give good measurements. Even a slight amount of oily film will cause water solutions to bead on the sides of the glassware. Volumetric glassware can be cleaned with detergent solution, followed by rinsing with tap water and a final rinse with a small portion of deionized water. Significant errors result from these unaccounted-for droplets. Water will drain with a fine sheeting action off clean glass surfaces. Reading Volumes When taking a reading on any volumetric glassware, you need to be careful that the container is perfectly vertical and that your eye is at the same level as the liquid. The liquid level reading is always taken by comparing the bottom of the curved liquid surface with the calibration lines. Usually it is a good idea to gently tap the barrel of the glassware to make sure the meniscus has settled.

meniscus

Your instructor will demonstrate proper pipetting technique. You should practice several times with deionized water before attempting to pipet with your carefully made standard solutions. Accuracy of Volumetric Glassware The glassware you will be using is the highest grade certified by the National Bureau of Standards. Below is a table of the manufacturing tolerances for the pieces you will be using.

Type of Glassware Error (±mL) 10 mL pipet 0.02 25 mL pipet 0.03

50 mL vol. flask 0.05 100 mL vol. flask 0.08

What this means is that with perfect technique, a 10 mL pipet will deliver as much at 10.02 mL or as little as 9.98 mL (10.00±0.02 mL). The accuracy and precision of volumetric glassware, however, is highly dependent on the technique of the person using

Chemistry Department Pacific University

Appendix-Volumetric Glassware 2

it. If you are sloppy, the best glassware will still result in inaccurate volumes. We have seen students use this same glassware with a precision of 10±1 mL. Graduated Cylinders Graduated cylinders are great for measuring volumes when a moderate degree of accuracy is required. Using them is fast and convenient but the cost is additional error in your volume measurement. As a general rule, graduated cylinders are accurate to the smallest division on the volume scale on the cylinder. Below is a table summarizing this information for some of the cylinders in our stockroom.

Total volume Smallest division 10 mL 0.2 mL 25 mL 0.2 mL 50 mL 1 mL 100 mL 1 mL

Beakers and Erlenmeyer Flasks Beakers and Erlenmeyer flasks are not considered volumetric glassware and should never be used to measure volumes if accuracy is desired. The markings on the side of a beaker are a general guideline but have errors as high as 25%. Using a pipet When using pipets, make sure to rinse the pipet twice or thrice with the solution that is to be transferred using the pipet bulb (NEVER THE MOUTH!), then fill the pipet and transfer the specified volume. To fill the pipet, apply suction to the pipet bulb and fill the pipet slightly over the calibration mark. Then quickly remove the pipet bulb, and replace it with the tip of your forefinger. Wipe off the outside of the pipet. Using the thumb and forefingers, rotate the barrel of the pipet, using the forefinger to control the amount of liquid that is released. Lower the bottom of the meniscus to the calibration mark. Touch off drops hanging from the pipet tip and move the pipet to the receiving container. Allow the liquid to flow out vertically. When it stops flowing, hold the tip against the inner wall of the receiving container for a few seconds while the remaining drops exit. Do NOT blow out any remaining liquid. Volumetric pipets are "to deliver" (TD) and have been calibrated to account for any remaining liquid left behind in the pipet.

Chemistry Department Pacific University

Appendix-Volumetric Glassware 3

Using pipets

Chemistry Department Pacific University

Appendix-Volumetric Glassware 4

Using a buret

Chemistry Department Pacific University

Appendix-Volumetric Glassware 5

Using a Volumetric Flask Volumetric flasks are calibrated to contain a specified volume (5mL to 5L) when filled to the line etched on the neck. Volumetric flasks are used to prepare solutions in two general ways. The direct preparation of a standard solution requiring that a known mass of solute is dissolved to make a solution of a definite volume or the quantitative dilution of another solution. If using a solid, insert a funnel into the neck of the flask. This minimizes the possibility of loss of the solid during transfer. Wash the solute off the funnel into the flask with the solvent. When making a quantitative dilution of another solution, begin by using a volumetric pipet to transfer an accurately measured sample of the solution into the volumetric flask. After the solute (solid or liquid) is added to the flask, fill the flask half full with solvent and swirl the contents to help dissolving. Add more solvent and mix again; stopper the flask and thoroughly mix the solution by inverting the flask end over end ten times. Bring the liquid level almost to the mark and allow time for the solvent to drain off the neck above the etched mark. Then use a dropper or pasteur pipet to add liquid to the etched mark, aligning the bottom of the meniscus with the etched or calibration mark. Firmly stopper the flask, inverting to assure uniform mixing. Volumetric Flasks should be maintained at the temperature indicated on the flask. Heating them will change the volume. Solutions should never be stored in a volumetric flask; they are used only to make precise solutions.

Chemistry Department Pacific University

Appendix-Volumetric Glassware 6

Using a volumetric flask