HCl和NaOH标准溶液浓度的标定用英文书写,字数不限.急,

HCl和NaOH标准溶液浓度的标定用英文书写,字数不限.急,

Ask A Scientist
Chemistry Archive
Titration Callibration and NaOH and KOH
name Ngoc
status student
grade 9-12
location N/A
Question - In a titration experiment,how can I standardize KOH and
NaOH using Potassium Hydrogen Phthalate I know I can measure a certain
amount of KHP,dissolve it in water and roughly determine the
concentration.However,the equipments are not calibrated...their will be
lots of errors...what can I do?Can I titrate the KOH and NaOH solids to
avoid error?If so,how?I have never done titration on a solid.
You have several questions here with somewhat different,but related,
answers.
Let us start with KHP.KHP is used as a "primary" standard because it is
chemically stable,water soluble,inexpensive,and obtainable in high
purity.Certified primary reference material is available from a number of
chemical suppliers,e.g.Sigma-Aldrich.KHP has a formula weight of 204.22
daltons,so this is equivalent to 1 mol of H(+1).It is available as a solid
and a variety of concentrations for ease of handling.There is no need to
further calibrate the KHP if you handle it with care.It IS the standard.
Next,let us consider the KOH.You can directly standardize the KOH
using
a convenient amount and concentration of KHP depending upon the desired
concentration of standardized KOH solution you need/want.It is important to
standardize the KOH first because KOH does not react with atmospheric CO2
(NaOH DOES).So you can standardize the KOH using the KHP as your primary
standard.
Now use the KOH to standardize a convenient amount and concentration of
an acid like HCl,or H2SO4.Of course you could use either acid as a primary
standard as these too are readily available from most chemical suppliers --
but I will assume you cannot or do not want to do that.
Now the NaOH.The reason for introducing the intermediate reference acid
solution is that it is VERY IMPORTANT to "quench" any NaOH solution as
quickly as possible with an excess of standardized HCl or H2SO4 because NaOH
rapidly absorbs CO2 from the atmosphere.It also reacts slowly with glass,
so it needs frequent re-calibration every time you keep it for more than a
few hours or days.In fact it is very difficult to obtain "pure" NaOH.It is
almost always contaminated with varying amounts of carbonate from
atmospheric absorption.The excess acid used to "quench" the NaOH is then
back titrated with the standardized KOH.
A tale of obtaining "pure" NaOH.The only way of obtaining NaOH
uncontaminated by carbonate is to use the "boil off" from liquid nitrogen
and bubble it through CO2-free water.The "wet" nitrogen gas is then
CAREFULLY blown across solid sodium metal on a platinum screen.The reaction
of H2O and Na metal produces carbonate-free NaOH,and of course H2 gas.The
concentrated NaOH falls into a platinum collecting dish.Of course,this is
all done in a nitrogen glove box.Sounds simple,but the reaction is very
exothermic and sodium has a low melting point so great patience must be
exercised because if the Na melts the reaction with residual water becomes
explosive!
I do not recommend titrating either KOH or NaOH as a solid -- both the
heats of reaction and dilution are too exothermic to keep under control.The
key is to avoid having NaOH around as little as possible.Despite what is
taught in many high school and university courses,it is not an easy
material to handle if analytical accuracy is necessary.
Vince Calder
Because the NaOH and KOH are deliquescent and/or may have water of
hydration in them,
these solids are usually considered not good for accurate titrations.
One of the two is less deliquescent.If you use that one,and only in low
humidity,
and bake it at 150C before use,and keep it in a cloud container during
the titration,
then you might get some results.
But then there is another problem:CO2 in the air will be gathered to make
a carbonate:
KOH + CO2 -> NaHCO3
This will be irreversible for an amateur,so your only defense would be
perfectly fresh hydroxide from the manufacturer,
and to seal your standardized stock perfectly from the time you
standardize it to the time you use it to measure with.
You might use Na2CO3 or K2CO3,but those are hygroscopic too.
Bicarbonates (NaHCO3,KHCO3) are not hygroscopic and do not pick up CO2
rapidly.
But they are very weak bases,and you will have little or no pH
transition from KHP acidic to NaHCO3 basic.
CaCO3 is a possibility.If it dissolves fast enough for your titration.
Have coarse chips and fine grains handy.
I wonder if you can build a couple of "salt-shakers",from which to
dispense coarse or fine powder during titration.
Tight-closing small plastic containers with a 4-inch snout of 1/4 inch
polyethylene or DuPont's Teflon (TM) tubing,
and a convenient cap to slip over that whenever you pause.
If the tube never gets clogged by wet powder,then you are "getting away
with it".
Never squeeze the container; you are hoping that little air ever goes down
the long narrow tube.
Weigh the container before and at end-point.
If there is a small glob of wet stuff in the tube when you are done,
possibly you could estimate its volume as water and subtract it.
Or swish it into the solution regularly,if the tube is long,curved and
very water-repellent.
Or maybe this is all very impractical.
You might be better off carefully calibrating ruler-marks you make on
glass tubing,than trying to find a solid base.
You implied you have a scale...empty random quantities of water from your
marked tube into a small beaker.
Weigh the beaker and read from the markings,just as if they were accurate
in ml.
Do not dump the beaker; only add more to get to the next calibration point.
Take readings at about 5-10 points in your tube's ruler-scale,and you
will be able to build calibration table for the markings.
Then when titrating you would need to record readings in "marks",later
interpolating to "milliliters" and "moles".
Lots of arithmetic,I guess.
Weighing a small plastic squirt-bottle of liquid KOH solution is not a
terrible idea,
if you have a scale with the right range and resolution.
The amount of liquid inside should be at most twice the amount you expect
to use in one titration.
If you do all this,your titration could have resolution and accuracy
similar to a calibrated burette.
Watch out for KOH dripping from the tip,though.
Perhaps you need a cup-holder-like stand so you will not be waving the
squirt-bottle around
or putting it down in places your hand will later rest.