Preparing an acid supply
Add the acid to the water; never add water to acid.
Determining the correct dilution
The dosing unit often has to dose acid to bring the irrigation water up to the correct pH value. The
use of acid depends on:
• the pH of the supply water;
• the desired pH of the irrigation water;
• the nature and concentration of the acid neutralizing compounds in the supply water and in the
fertiliser solutions.
In principle, very little acid is required for pH correction. In practice however, there are often
substances in the supply water and in the fertiliser solutions which bind the acid (phosphate and
bicarbonate). This results in a smaller change in the pH than would be expected based on the quantity
of acid applied (this is known as 'buffering'). In order to achieve good pH-control it is therefore
important that acid is used in a concentration that has been tailored to the concentration of buffer
substances. After all:
• when the acid concentration is too low, even the maximum dosing flow rate will not be sufficient
to achieve the desired pH value;
• when the acid concentration is too high, even the smallest possible dosing flow rate will swing
the pH value severely and overshoot.
The best practice is to create an acid stock from a dilution of concentrated nitric acid. In order to
achieve the optimum dilution, it is necessary to know the quantity of buffer substances in the supply
water. Although most laboratory analyses reveal something about this, pre-calculating the precise
dilution that is required continues to be problematical. Therefore, in practice the dilution required
is determined by starting with a common concentration, for instance 3 % (weight percent). If it turns
out that the pH is not corrected fully or quickly enough, the concentration must be increased. If it
turns out that the pH overshoots and swings dramatically, the concentration must be decreased. In
addition, if the quality of the supply water is not constant, the acid concentration must be adjusted
every now and then. Always keep a record of which dilution is being used, so that a new acid stock
can easily be prepared at the same dilution.
The maximum concentration of (nitric) acid depends on the type of dosing channel used. A
higher concentration reduces the service life of the components.
Creating a standard dilution of 3%
Nitric acid is often supplied in 20 l barrels at a concentration of 38%, 53% or 68% (weight percent).
To dilute this to 3% (weight percent), it is necessary to mix the contents of the barrel with 315, 470
or 640 l fresh water. Always pour the acid into the water instead of the water into the acid, and
always stir the mixture thoroughly to homogenise it.
Example
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