1. Add “high pH” (alkaline)
additives before adding nutrient: Most additives will affect nutrient
pH at least slightly. The best technique to adopt with those that elevate
pH significantly is to add them to the water and adjust the pH down to ~6
prior to adding the nutrient.
The less preferred but simplest alternative
is to pre dilute the additive in a separate volume of raw water. Then once
this solution is added to the nutrient solution, quickly lower the pH to
below 6.5. Note, a white cloudy precipitate (calcium sulphate) may form
when the pre diluted additive initially merges with the nutrient solution
(Fig 1.13a). However, because the initial particle size of the precipitate
is small, it will usually re-dissolve if the pH is immediately
re-adjusted.
2. Do not pre-adjust pH of
raw water: Note that the pH values being discussed here are the values
of the working nutrient solution - not your make-up water. Unless your
make-up water has a high alkalinity, do not bother attempting to adjust
its pH prior to the nutrient being added. If you attempt this procedure
you will typically get wild pH swings either side of the pH target without
ever landing on the target value.
3.
Estimating the volume of acid (especially for larger systems):
Step 1. Take a 1.0 litre sub-sample
(or known volume) of working nutrient.
Step 2. Add a few drops of pH
indicator (Fig 1.12a).
Step 3. While stirring this solution,
measure the volume of acid required to turn this solution yellow - Fig
1.12b (‘Yellow’ indicates ~pH 6 with most broad range liquid indicators).
Step 4. Multiply the volume of acid**
by the volume of nutrient in your reservoir. That calculation will give
you the volume of acid required to adjust the entire volume down to ~pH
6.0 i.e.
Total volume of nutrient ÷ Volume of
sub-sample x volume of acid
** If this volume is very
small (most likely if a highly concentrated acid is used), to ensure
accuracy you will require the use of a finely graded pipette. A better
method is to allocate a portion of acid specifically for conducting this
calculation and dilute it by a known amount - for example 10-fold. Ensure
to compensate for this dilution when calculating how much of the
concentrated acid to add to the reservoir.
