diluting acid

JIMMIEDEE123@YAHOO.COM · science forum beginner Joined: 29 Jun 2006 Posts: 2

Its been a long time(30+ years) since I had chemistry 101 and I need
some help making a dilute solution of HCL. My stock solution of HCL is
31.4% HCL and the rest is H2O. I need a solution having a pH between
2.2 and 2.5. Could someone please tell me how much water and stock
solution I would need to make 10 gallons of the rquired solution. In
case it matters my stock solution is muriatic acid from Home Depot. I,m
using it to remove calcium scale from copper pipe.

<lucasea@sbcglobal.net · science forum addict Joined: 25 Mar 2006 Posts: 94

You will probably want a pH near the low end of your range, since HCl will
be consumed during use, raising the pH. Back-of-the-envelope, your starting
HCl concentration is about 10 mol/L (37 % HCl, standard laboratory conc. HCl
is about 12 mol/L). pH 2.2 corresponds to ca. 0.0063 mol/L, so you need a
dilution factor of about 1:1600. 1 1/2 tablespoon of your 31 % HCl in 10
gallons would be a dilution of exactly 1.5 tablespoon in 10 gallons*(4
quarts / gallon)*(4 cups / quart)*(16 tablespoons / cup) or 1 in 1700. At
that dilution, you should end up with about 2.23.

However, at such a low concentration of HCl, it will quickly get used up
dissolving scale, dropping the pH back toward neutral (7). Might you want
to consider using a buffer instead of straight acid? If you can get your
hands on something like glycine or alanine, you could make a good
concentrated solution of it (ca. 10 %, which is upwards of 1 mol/L), and add
about 1/2 equivalent of HCl (0.5 mol/L), for a 1:1 solution of glycine and
glycine.HCl. This will give you a pH very close to the pK1 of glycine,
which is about 2.35. Then, the buffer capacity and a large reservoir of HCl
(about 0.5 mol/L, instead of the 0.005 mol/L you would get from a straight
HCl solution) will keep the pH in the desired range, while much of the HCl
is consumed doing its magic on the Ca scale.

Consult a table of organic acids to find others that have a pKa in the range
you want your pH, to see if there are others that might be more practical.
http://www.zirchrom.com/organic.htm
Ideally, you would want an amine base so you could add HCl , which will be
the most effective at dissolving Ca scale. Another one that I noticed is
PABA (4-aminobenzoic acid), pK1 = 2.5. That might be easier to find, since
it's a common ingredient in sunscreen.

Eric Lucas

<JIMMIEDEE123@YAHOO.COM> wrote in message
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Jimmie D · science forum beginner Joined: 24 Jun 2006 Posts: 8

Thanks Eric, your numbers are very close to what I came up with after about
2 weeks of research. I couldnt believe that it would take so little acid to
get what I wanted so I assumed my calculations were way off. I feel a lot
better now. Instead of using a buffer I will probably just use more acid, 1
or 2 cups/10gal for a pH near 1. This would be a concentration I feel
comforatble/safe working with.

<lucasea@sbcglobal.net> wrote in message
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Borek · science forum Guru Wannabe Joined: 02 May 2005 Posts: 157

On Fri, 30 Jun 2006 07:46:48 +0200, Jimmie D <jimmiedee@bellsouth.net>
wrote:

JIMMIEDEE123@YAHOO.COM · science forum beginner Joined: 29 Jun 2006 Posts: 2

Borek wrote:

Google