Help with the effect of Sodium Hydroxide and Germinating Seeds.

Kathleen Brunkard brunkard at
Fri Nov 15 18:14:22 EST 1996

> At  1:14 AM 11/15/96 +0000, Paul Koch wrote:
> >I have been investigating seed's cellular resperation by doing a lab that
> >required putting germinating seeds in the bottom of a test tube.  Then a
> >layer of cotton was put on top of that.  On top of this, I put a thin
> >layer of Sodium Hydroxide and then another layer of cotton.  I put this
> >test tube upside down in a beaker of water and let it sit over night.  The
> >next day I found that the amount of air in the test tube had greatly
> >decreased.  I also found that the layer of Sodium Hydroxide had
> >disappeared.  I know that a germinating seed takes in oxygen to preform
> >areobic resperation and as a byproduct of this, Carbon Dioxide is
> >produced.  But, where did the carbon dioxide go and where did the sodium
> >hydroxide go?  Why didn't the carbon dioxide take up the same amount of
> >space as the air the seed took in?  I would appreciate it if you could
> >answer these questions and maybe go further in depth into these processes.

On 15 Nov 1996, Ross Koning wrote:
> > Paul,
> You are correct about the seeds doing respiration, and the
> fact that there should be a 1:1 relationship between oxygen
> consumed and carbon dioxide released.  However, you made
> the gases accessible to sodium hydroxide pellets...these are
> very alkaline (caustic!) and hygroscopic (draw water).  Now
> if you remember your chemistry well, about the equilibrium:
>      carbon dioxide <-> bicarbonate <-> carbonic acid
> then you can probably guess what happened to the carbon
> dioxide.  It was trapped in an alkaline solution developing
> on the surfaces of the sodium hydroxide pellets.  This is'
> the purpose of those pellets.  If you had not put them in,
> the oxygen tension would have dropped, the carbon dioxide
> levels would have increased, and respiration would have
> stopped.  Instead, the carbon dioxide was trapped and so
> respiration could continue as the oxygen was being used up.
> Indeed the use of that oxygen (forms water at the end of
> ETS) and the loss of carbon dioxide will reduce the gas
> volume in the container.  The pellets dissolved in the
> humid atmosphere which is why you need the cotton plugs
> to separate the living seeds from that caustic liquid!
> You can use this to your advantage now, if you plug the test
> tube with a soft! rubber stopper with one smoothly-drilled
> hole fitted snugly with a piece of heavy-walled capillary
> glass tubing.  You close the apparatus (handle as little
> as possible and avoid temperature variations in the environment!),
> allow a few minutes to equilibrate, then put a drop of 1%
> eosin y (or other dye/ink) at the free end of the tubing.
> The dye will move into the tubing at a certain rate.  This
> will estimate the volumetric consumption of oxygen gas by
> the seeds.  You can calibrate the volume consumed by weighing
> the water held in a specific length of the tubing (that
> assumes that the capillary tubing's ID is uniform...may not
> be valid).  There are a number of assumptions made here about
> pressures, volumes, and adsorption that may be, in part, invalid
> but it is nevertheless an interesting project that can lead
> students in an assessment of what they observe, what it means,
> what the pitfalls are, etc.  Be sure to use a suitable control
> with students...a separate supply of seeds that have been soaked
> but then BOILED AND COOLED to ambient temperature must be
> prepared in advance and kept separate.  I have used popcorn,
> peas, and mung beans in this project.  The boiled seeds are
> always a different color so they are easy to distinguish.  Each
> group, then, needs to have two set-ups.  It is also instructive
> to do a second run, and in mid-run HOLD the tube in your hot
> little can guess the result...ask students to come
> up with that on their own...maybe without the "hot little hint."
> Maybe a more opaque hint is PV=nRT.  That kind of approach...
> ross

We avoid the issue of calibrating the volume in a capillary tube by using
rubber stoppers with a 1 ml pipette attached.  That way changes in volume
can be determined by reading directly off of the pipette.  We also insert
a syringe into the rubber stopper so that we can adjust the position of
the drop of dye, allowing us to start monitoring dye position at 0 ml.

Kathy Brunkard
Biological Sciences Department
East Stroudsburg University
East Stroudsburg, PA

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