the carbon dioxide gas in the sample is substantially above or below the air equilibrium value. Errors in
the field measurement may also be caused by the
inferior equipment used.
Therefore, to eliminate these two main sources of
error in pH measurement, samples were collected and taken quickly to a field laboratory for determination on a good quality pH meter. For particular studies in the changes in pH of a given loch water the same pH meter was set up at the loch side using a portable petrol-powered generator for a power supply.
Method The samples of loch water were collected in
500 ml screw-cap jars under water, sealed to exclude
air bubbles and to prevent gaseous exchange with the atmosphere. These samples were then taken to the field,
laboratory and analysed immediately. Samples could be
collected from any location on the loch surface or up to any depth colonised by macrophytes by means of aqualung diving.
An aliquot of this sample was taken and titrated against 0«02M HC1 standard acid. This acid was freshly
made up using distilled water that had been boiled for
at least an hour and cooled with a soda lime tube to prevent any reabsorption of carbon dioxide. The
apparatus used to perform the titration is shown in
Acid/ reservoir •-Burette Electrode pH Meter Magnetic stirrer Figure 3•8
The apparatus set up in the field laboratory for
titration of loch water samples* The burette is refilled between titrations directly from the acid reservoir, the soda-lime tubes preventing entry of carbon dioxide to the system. The magnetic stirrer kept the contents of the flask mixed during the titration.
70
burette to be filled from the standard HC1 reservoir
without allowing any absorption of carbon dioxide
from the atmosphere. Thus, when the apparatus was set up many samples could be analysed rapidly and in
succession.
The magnetic follower in the titration flask
ensured that the acid added was mixed rapidly and that the pH electrode read correctly. The individual
titrations were carried out as quickly as possible to prevent exchange between atmospheric carbon dioxide and the contents of the flask becoming significant. The samples were first taken to an end point of pH 8»3, if already above this, and then titrated to an approx imate end point of about pH 5*0. An approximate value of T.A. was then calculated and knowing the initial pH
and conductivity range of the sample the approximate
total carbonic acid was calculated using Tables 11.1 and 11.2 in Appendix II after Colterman (1969). The true end point pH is then read off Table 11.3 and the
sample titration•continued to this. The volume of
standard acid required to get to this end point gives
the T.A. The total, carbonic acid is also calculated.
Results Samples were taken from four limestone lochs (L. Borralie, L. Caladail, L. Croispol and L, Lanlish) and from one peat loch (L. Meadie) in the area studied in Durness. Samples were collected at various locat ions and depths corresponding to the range of veget ation present. One such set of results for these lochs /
is given in Table 3.9. The values of pH, total
alkalinity and total carbon dioxide of the four lime
stone lochs are similar and in contrast to the non limestone loch. The pH and T.A. values of the lime stone lochs show that although these waters have a high total carbon dioxide, there will only be a small amount of this present as free carbon dioxide. This will be biologically significant ranging from several times the atmospheric concentration around pH 8 to several times less at ph 9,
Table 3*9
Analysis of water samples from selected lochs in the Durness area in May 1973» The total^ alkalinity and the total carbon dioxide are given in units of m.eq*!*”^»
Borralie Caladail Croispol Lanlish Meadie
pH 8.66 8*72 8*37 8.34 7*14
TA. 2.33 3.02 . 3.44 2 .3 6 0.012
3.3 Dissolved Carbon Dioxide
All dissolved'gases are held in solution by their
respective partial pressures in the gas phase assoc iated with the liquid phase (Henry’s law). Therefore, when carbon dioxide partitions itself between the gas
and liquid phases, by diffusion, the amount in solution
will be in equilibrium with the partial pressure (pCOg)
in the gas phase* Thus, the solubility coefficient
will be defined as
Dissolved CO^ (moles l”^) = pCOg .
However it will now be important to make the distinct ion between dissolved carbon dioxide gas and hydrated
carbon dioxide as in equation 2. The gas solubility
law will apply only to the dissolved gas.
Although there are seasonal changes in the con
centration of carbon dioxide in the atmosphere (Bisch- off, 1960), for the purpose of the present study it is reasonable to consider the atmospheric partial pressure of carbon dioxide above a loch water to be constant.
The solubility will be temperature dependent and the
variation of the solubility coefficient with tempera ture is given in Figure 3.10.
Photosynthesis and respiration by aquatic plants
will cause fluctuations in the dissolved carbon dioxide and total carbonic acid of the loch water. The diss
olved carbon dioxide changes will cause a displacement
moles/l,/Atm, xlO' 700 600 500 4oo 3500 8 4 12 16 20 o Figure 3,10
The variation of the solubility coeficient of carbon dioxide in water with temperature, (Drawn from data given in Riley and Chester, 1972,) The solubility coeficient is given
from: ■ Cone, dissolved C0_ Partial pressure CO^
C-raoles/li
equilibria. The magnitude of the shift of equilibria will be related to the 'pCOg* buffering capacity of a water type for changes in total carbonic acid
(Kanuisher, 1963). The net effect of this will be that in a water of given alkalinity the change in the
dissolved carbon dioxide, that will occur for a given
change in total carbonic acid, will be less than that of distilled water. This difference in change of diss olved carbon dioxide will be related to the alkalinity of the loch water concerned. The difference in fpCOg* buffering capacity for waters of different alkalinity has been experimentally determined (Kanwisher 1960), Figure 3.11 shows this difference in tpCOg* buffering capacity for fresh distilled water and sea water, where the same change in total carbonic acid produces a much larger change of dissolved carbon dioxide in distilled water than sea water.
The higher the alkalinity the greater the ipCOg* buffering capacity and hence the dissolved carbon
dioxide increase or decrease will be partially absorbed, that is reduced to a smaller change than would occur in distilled water. The remaining difference between the dissolved carbon dioxide and tpCOg,* of the atmos
phere will be equilibrated by net diffusion through the
gas-liquid phase interface. It can be shown that the
exchange between gaseous carbon dioxide and water may
be slow compared to other gases such as oxygen. Thus, a dissolved oxygen deficit is more quickly replaced by
pCO^ Difference between air and water in atmospheres X 10^ 4000 Fresh water 2000 1000 Sea •wa 1 2
ml/Litre CO^ added to water
The experimentally determined partial pressure variation of carbon dioxide with changes in total carbon dioxide in sea and fresh water. From Kanwisher (i9 6 0)
diffusion from the gas phase than a carbon dioxide deficit. This is mainly due to the vastly different partial pressures of oxygen and carbon dioxide in the atmosphere rather than a significant difference in the diffusion of oxygen and carbon dioxide in water. The
wind velocity and associated turbulence of the water
surface have been shown to increase the movement of carbon dioxide across the air-water boundary
(Kanwisher, 1962, 1963).
Thus, as the return to equilibrium of carbon dioxide between the gas and liquid phase will be diffusion limited in the liquid phase, it will be slow. Differences between dissolved and gaseous
carbon dioxide caused by biological activity, ip water, may persist for several months (Teal and Kanuisher,
1965; Tailing, 1976).
The following investigations were undertaken to
measure the extent of pH changes in water bodies due to a net removal of carbon dioxide by photosynthetis activity of macrophytes.
To Demonstrate that an Aquatic Macroohyte will remove
Dissolved carbon dioxide from Pond Water faster than
the diffusive supply from the Atmosphere
Method A glass tank (18" x 12" x 24") containing
growing P. perfoliatus plants collected from L. Croispol
and rooted in soil, was placed in the laboratory window
where it would receive sunlight during the day, A pH