How can I measure the Shock value with the PoolLab? Only with tablet DPD1 (free chlorine) or with
a) Only with tablet DPD1 (free chlorine) or
b) Tablet DPD1 (free chlorine) and tablet DPD3 (total chlorine) with the calculation:
Total chlorine - free chlorine = combined chlorine.
Which value is the correct one for the value determination?
First of all, a brief explanation: Lotus Clean Shock consists of sodium chlorite and sodium hypochlorite. Both substances produce chlorine dioxide in the water until they are used up.
You measure sodium chlorite and sodium hypochlorite as free chlorine with the DPD1. The chlorine dioxide could be measured, but the measurement would be falsified because the sodium chlorite and sodium hypochlorite are also in the water. In addition, chlorine dioxide measurement is quite expensive. However, one can assume that as long as sodium chlorite and sodium hypochlorite are in the water, chlorine dioxide will also be formed.
If the water is contaminated with dirt, sodium chlorite and sodium hypochlorite react with this dirt and form chloramines. These chloramines are usually oxidised by the chlorine dioxide formed. In the case of heavily polluted water, however, too much chloramines may be formed and too little chlorine dioxide. As a result, chloramines can remain in the water, which can be recognised by the typical chlorine smell.
The choramines and the free chlorine can be measured with DPD3. Then subtract the DPD1 value to determine the combined chlorine. If chloramines are present, the Shock dose must be increased, as this produces more chlorine dioxide, which in turn oxidises the chloramines.
Important to know: Chlorine dioxide is NOT chlorine. It is a compound of one chlorine atom with two oxygen atoms. It has completely different properties than chlorine. The two oxygen atoms oxidise the dirt as oxygen radicals. The remaining chlorine atom combines with sodium to form sodium chloride (written D). Sodium chloride is simple table salt.