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Re:LaMotte pH kit

>Subject: Re: LaMotte pH kit

One can buy from a chemical or biological supply company a small bottle
containing a gram two of crystalized bromthymol blue that will last you the
rest of your life.  Around 20 milligrams in about 10 cc of water will
dissolve to produce a concentrated solution.  They sell plain bromthymol
blue that produces a rather acidic solution and isn't very soluble.  To
dissolve more, you have to add some sodium hydroxide solution.  The acidic
bromthymol blue solution is deep orange in color, and you add a few drops
of a NaOH solution that turns it green or blue.  Then more of the
bromthymol blue dissolves, and the solution again becomes orange in a day
or two.  You add a few drops more of the NaOH solution, and the same thing
happens again until finally you get all of the bromthymol blue dissolved.
At this point the solution stays whatever color it was (green or blue). It
is best to try to arrive at a green color, because this is close to neutral
and should have the least influence on the pH of the solution you are
testing.   the chemical supply companies also sell pH-neutralized
bromthymol blue which is more soluble and produces a very concentrated
green solution without your having to add NaOH.

Back around 1990 I mixed up about 25 cc of the plain bromthymol blue, using
NaOH to get more into solution.  I still have about 5 cc left and I average
1 or 2 tests each day.  With a solution this concentrated, I use about half
a drop in a small test tube and add about 1 cc of water to be tested. To
get less than one drop, I touch the tip of the eyedropper to the side of
the test tube.   This gives me a color that isn't too concentrated and too
dark or too diluted and too light to read easily.

The color range of bromthymol blue goes from an intense blue-violet at pH
7.4 and higher through green at around pH 6.6 to yellow at 6.2 or lower.

Paul Krombholz, in bone dry central Mississippi