QC Lab - Volumetric Apparatus

burette & pipette

üUsed in delivering definite volume (open both at top and bottom)

volumetric flask & graduated cylinder

üUsed to contain definite volume (open at top and closed at bottom)

burette

oLaboratory apparatus used in quantitative chemical analysis used in the measurement of variable quantities

oClosed at the bottom by a glass or Teflon stopcock to control the outflow of liquid

- horizontal: closed

- vertical: open

- use iron stand in even surface and burette clamp firmly attached

- graduations: top (0 ml) to tip (bottom)

titer/titre

total volume of liquid added from the burette

filled until 0ml = final volume reached by liquid inside the burette

- can be used even when not filled to 0 mL

- FBR - IBR

meniscus

üCurve seen at the top of a liquid in response to its container

üCan be either concave or convex, depending on the surface tension of the liquid and its adhesion to the wall of the container

convex meniscus

- backwards meniscus

- molecules are more strongly attracted to each other than container

- mercury

concave meniscus

- occurs when molecules of liquid are more strongly attracted to their container than to each other

- most liquids (water)

colorless

- lower meniscus

highly colored/opaque

- upper meniscus (there is no lower meniscus)

burette reading

šApparatus must be placed on an even surface

šEye must be on the same level as the meniscus

šFor colorless solutions, reading should be made at the lower meniscus

šFor highly colored solutions, reading should be made at the upper meniscus

proper use of burette

šClean burettes with dishwashing liquid and burette brush

šBefore filling the burette with the VS, it should first be rinsed several times with small portions of the desired solution, discarding each wash. Use <5mL of the solution to wash the inside walls of the burette.

pipette

Small tubes that transfer liquids from one container to another in exact and measurable amounts (w/rubber aspirator)

proper use of pipette

šTo clean one, draw distilled water into the pipette and tilt it, so that the water makes contact with the inside surface of the pipette.

š

šRepeat this process twice, then rinse the entire pipette with distilled water to finish cleaning it.

volumetric flask

oType of laboratory glassware used to prepare solutions

oFlat bottomed bulb with an elongated neck calibrated to hold a set volume at a mark on the neck (1st apparatus)

direct

directly prepare solution inside the flask (swirl in infinity sign = homogenous)

indirect

use separate container

proper use of volumetric flask

šShould be clean prior to use. Do not use a brush when cleaning the inside walls of the flask.

šDissolve the desired mass of the reagent directly in the flask or dissolve first the chemical in a beaker then transfer it to the volumetric flask ensuring that every drop is transferred.

šAfter each addition of the solvent, the flask should be swirled carefully to allow even distribution of the solute and the solvent to produce a homogenous solution.

graduated cylinder

Narrow, cylindrical container marked with horizontal lines to represent units of measurement and used to precisely measure the volume of liquids

sources of error in use

1.Rinse water adhering to walls of apparatus. The apparatus may be dried, but it is more convenient and just as accurate to wash it out with small successive portions of the liquid to be used, discarding the washings.

2.Grease films and dirty apparatus cause irregularities in the delivery of liquid and distort the meniscus.

3.Parallax must be avoided to secure proper readings of the level of the meniscus.

4.Variations in temperature lead to changes in volume of vessels and liquids. All measurements therefore should be made at a temperature closely approximating that at which the apparatus was calibrated.

5. Air bubbles trapped beneath the liquid surface, especially below the stopcock in burettes, displace liquid.

6. Heat, such as supplied by hot solutions, causes calibrated volumetric apparatus to suffer a slight permanent change in volume.

7. Most salts when dissolved produce a change in temperature, with concomitant change in volume of the solution. (NaOH pellet + water = exothermic)

8. Failure to use apparatus in a manner approaching as nearly as possible that followed in calibration.

parallax

displacement of measurement due to observers POV

20 deg C

calibrated at this temperature to get accurate measurement

air bubbles

dislodge by opening burette