Here is a description of my titration kit.
I started out with a small beaker and a small mason jar filled with hot
water as a warming bath and had lots of fun trying to juggle stirring
and dropping titration solution into the beaker. I made a few
messes and finally got fed up with it.
What I needed was a way of warming the mixture of test oil and
isopropyl alcohol (IPA) which is required for an accurate titration,
and I thought it would be nice if agitation was included so that it
would not be necessary to stop and shake or stir the solution with a
stick while simultaneously trying to carefully control the flow of 0.1%
caustic solution used in the titration. A laboratory grade
stirring hotplate would be really nice but I could not justify the cost
of $500.00 to $1000.00. Necessity is the mother of invention they
say and I have always found that to be true. So here is what resulted
of my need.
There are two spring loaded clamps made from copper straps that are
sold for the purpose of securing copper water pipe that I have reformed
to fit the test tube which is about 25 mm in diameter. The bottom
one has a small kapton thin film heater attached to it using copper
adhesive tape which is sold for the purpose of stained glass
making. On the rear of the box not visible is a jack which fits
the plug which is found on the wall type power supply I picked up at a
local surplus electronics supply for 5 bucks. It has a 9Vdc 1 amp
output capability although the unit only draws about 300 mA. The
switch allows the stirring to be switched between high and low speeds
or
turned off completely. For titration I always use high speed and
if I were to make another of these I wouldn't bother with the switch. I
use a small magnetic stirring bar which is teflon coated and it can be
seen in the images suspended on the side of the test tube by the magnet
on the stirring motor on the other side of the wall of the box.
Inside is nothing too elaborate. The DC motor I picked up for 99
cents each and as you can see I made the rotating magnet by taking one
motor housing and removing the rotor and cutting the housing down till
the internal magnets are just exposed at one end. I then flipped
the empty housing around and inserted the shaft from a second
unmodified motor into the bushing in the end of the empty
housing. It is well enough aligned and balanced that the
vibration is negligible at even a high stirring speed. All that
was needed was a small drop of epoxy to secure the shaft to the bushing
on the empty motor housing. A two dollar magnetic stirrer which took 15
minutes to make! Since the input voltage is 9 Vdc I added some
zener diodes to drop the voltage down to either 3 volts or 4 volts for
high and low stirring speed. This is not necessary though and the
switch can be eliminated for a fixed speed if desired.
The heater is a 40 ohm kapton encapsulated
thin film resistor which is available from omega like these.
I had one in my junk box which was 2 inches in length and half an inch
wide and when run off a 9 volt source it produces about 2 watts which
is enough to heat 20 ml oil/alcohol mixture to 40 Deg. C in about 10
minutes. Higher voltage might heat faster but there is no
thermostat so I chose a voltage which would not allow things to
overheat if inadvertently left on. The heater reaches about 90
deg.C but thermal transfer to the test tube is not great and much heat
is lost to the air limiting the ultimate temperature of the
solution. There is no guard or insulation on my unit since I
wanted an unrestricted view of the test tube for titration to see the
colour change with the phenolphthalein indicator I use for titrating my
oil stock. Alternatively one might arrange something with a ceramic
power resistor for a heat source if that is what is readily
available. Use your imagination. Don't burn yourself!
Back to main page.