Chemistry Lab 2007 – Agenda:
Possible known waters: salt water, DI water, sugar water, water with baking soda
(NaHCO3 – sodium bicarbonate)
Possible unknown waters: well water, river water, rain water, lake water
Before lab:
-change the handout to the actual waters that I will be using
-get out all materials:
-handouts
-glassware
-gloves and goggles
-titrators, ph and conductivity meters
-clean all glassware
-calibrate all probes
-practice all experiments
**keep an example of salmon color for titration experiment for
students to compare to
-draw carbonate/pH diagram on board
-draw boxes for answers on board, get averages for the groups before trying
out the unknown samples
Purpose:
-to learn how to measure and interpret pH, conductivity, and alkalinity
-illustrate the concept that different waters have different characteristics
Background Information and Methods:
pH:
-the greater the pH, the fewer H+ ions and thus the less acidic
-remember, 0 = strong acid, 14 = strong base
-lakes and streams vary between 6-9, although exceptions exist (bogs are
acidic wetlands)
-demonstrate the pH meter and how to use it
Meters:
-do not submerge below line
-allow to stabilize before taking a reading
Conductivity:
-ability of water to pass an electric current
-related to the total amount of dissolved ions in solution (if lots of dissolved
ions, the conductivity will be high)
-demonstrate 2 kinds of meters that will be used and how they will be used
WARNING: do not immerse probe all the way, not water proof
NOTE: 1000 microsiemens = 1 millisiemens, make sure to know what you are
recording, units are important in this lab
Alkalinity:
WRITE ON BOARD: CO3 + 2H+HCO3 + H+ CO2 +H20
-also known as ANC (acid neutralizing capacity or a buffering capability,
ability of the water to neutralize a strong acid, if extra H+ is put into solution,
the CO3 sucks it up and turns into HCO3 so pH would be lowered)
(EXPLAIN USING DIAGRAM)
-a lake or stream high in alkalinity would resist a reduction in pH
-a stream low in alkalinity would drop in pH upon acid addition because it
wouldn’t have the ability to resist the change (EXAMPLE: Adirondack
mountains, low in alkalinity because of geology and the pH was quickly
reduced from the acid rain, now very few fish live in the lakes there because
they are so acidic)
-major contributor to alkalinity in Michigan is CaCO3 (calcium carbonate) from
limestone deposits and MgCO3 (magnesium carbonate) from dolomite
-to determine, do a titration experiment
-explain how to use titrator
Titrator:
-get rid of bubbles
-set to 0000
-rinse tip with DI water to make sure none extra being added to solution
When acid is added, pH is lowered and it will hit these points:
1st Endpoint = pH 8.3: solution will turn pink when phenolphalein indicator is added if
the pH is above 8.3, determine how much acid is necessary to bring the pH down to
8.3 (carbonate converted to bicarbonate and pink solution goes clear)
2nd Endpoint = pH 4.5: if pH is less than 8.3 and nothing happens when
phenolphalein is added, add bromcresol-green methol-red indicator packet,
determine how much acid is necessary to bring the pH down to 4.5 (all bicarbonate
converted to CO2, blue/green solution  grey/purple  pink/salmon color)
*at this point the water has no buffering capacity left
The amount of acid required to reach the phenolphalein alkalinity + the amount to
reach the bromcresol-green methol-red alkalinity is the total alkalinity and
represents the total buffering capacity
-recorded as mg/L CaCO3 or ppm CaCO3
The Experiment:
Safety:
-wear gloves and goggles when handling H2SO4 (sulfuric acid when doing
alkalinity)
Rinse:
-DI rinse all meters and titrator before using it on the next sample
-shake gently to dry
-do not dry with paper towel
-rinse glass with DI between samples
-acid wash the flasks before titrating next sample
Procedure:
-using 4 knowns and 4 unknowns
-record the pH, conductivity, and alkalinity of all knowns on lab handout and
on the board (we will average the known’s numbers just to see a trend but
you should use what you got as a group)
-obtain 4 unknowns and record pH, conductivity, and alkalinity
-try to identify samples based on the measurements you get
Assignment:
-ask if any questions?
-typed questions due at the beginning of lab next week (individual, not group
assignment)
*Clean glassware and wipe down tables before leaving