Re: How and why does alkali's affects the heart rate of Daphnia?

     Hi Dan, excellent question!  Your project sounds very interesting, and 
Daphnia are fun little creatures to work with!  Your question regarding why 
the aspirin solution for your Daphnia was made more alkaline in order to 
dissolve the aspirin has to do with the chemical structure of aspirin.  
Lets look:
     Aspirin, also known as acetylsalicylic acid, or ASA, is a weak acid 
comprised of three main structures:  an aromatic ring, an ester, and a 
carboxylic acid.  Aspirin comes in tablets designed to be swallowed and 
processed by our stomach, whose very acidic environment maintains a pH of 
around 1-3 .  For aspirin to be absorbed into the body, it must be 
fat-soluble, or permeable to the gastric mucosa, in order to diffuse across 
the stomach lining and into the cells.  Aspirin is fat soluble in the 
acidic environment of the stomach due to its carboxyl group, which is 
uncharged (non-ionized).  Because of this uncharged carboxyl group, 
however, aspirin is insoluble in water.  This is good for stomach 
absorption, but bad if you're trying to dissolve it for your Daphnia, which 
live in water!  How do we get aspirin to dissolve in water, then?   Place 
the aspirin in a more alkaline (more basic, higher pH) solution!  The 
structure of aspirin changes --- it loses a hydrogen atom and becomes 
negatively charged, or ionized.  This form of aspirin is now water soluble. 
     In regards to your Daphnia:  Aspirin, as a weak acid, maintains a pH 
of approximately 5.  Introducing a solution of this pH to the environment 
of your Daphnia would most likely kill them.  Daphnia enjoy a pH between 
7.2 and 8.5 --- just slightly alkaline.  Making the solution more alkaline 
not only allows the aspirin to become water-soluble, but it brings the 
overall pH of the solution to a range where your Daphnia would be happiest.
     To answer the other part of your question on the effects an alkaline 
solution would have on heart rate:  Overall, an alkaline solution should 
not have direct effects on heart rate, unless the solution has a pH higher 
than what the Daphnia live at (higher than 8.5).  In this case, their heart 
rate may increase due to the stress of their environment.
     On an interesting note, humans can cause their blood to become more 
alkaline by hyperventilating.  Hyperventilation, which increases arterial 
oxygen pressure, unfortunately decreases blood carbon dioxide levels, 
causing the blood pH to increase from its baseline value of around 
7.35-7.45.  At high elevations, where the atmospheric pressure is low and 
oxygen becomes less available in the air, there is less oxygen per breath, 
thus, people will breathe faster in order to maintain the same oxygen 
intake.  Climbers of Mt. Everest (not using oxygen tanks) hyperventilate to 
such an extent that their blood pH becomes more alkaline and rises to 
around 7.70 --- a near-fatal value!   It would be very bad for the body if 
the heart rate were to increase in this situation.  Why?  An increase in 
heart rate means that the heart muscle is working more --- this in turn 
means that the heart muscle needs more oxygen to maintain this pace --- 
this means that the person then needs to breathe even faster to bring in 
more oxygen to feed the heart --- this means more carbon dioxide is "blown 
off" --- this means blood pH rises even further... a deadly chain of 
events.  Instead of the heart rate increasing to improve oxygen supply and 
utilization, the body produces more red blood cells to transport the 
oxygen.  This spares the body all the extra work it would require to 
maintain a fast heart rate.
     I hope this information is helpful to you and your Daphnia!  Good luck 
on your research project!   
     For additional information, check out the following terrific websites:
www.rxmed.com, www.sigma-aldrich.com, www.biomedx.com, www.daphnia.com