Final answer:
Adding zinc oxide to the cathode mix during the Quimociac technique is used for Oxygen Scavenging, as zinc serves as a sacrificial anode in the electrochemical cell, reducing the oxygen concentration to avoid errors in phosphate determination.
Step-by-step explanation:
When determining phosphate concentrations via the gravimetric Quimociac technique, the purpose of adding zinc oxide to the cathode mix is for Oxygen Scavenging. Zinc, being a more active metal than iron, acts as the sacrificial anode in the electrochemical cell and will dissolve, which allows it to serve as an electron donor.
The addition of zinc leads to oxidation at the anode, characterized by the reaction Zn(s) → Zn²+ (aq) + 2e−. This process helps maintain a low oxygen concentration by facilitating the reduction of oxygen at the cathode, avoiding the formation of unwanted oxides that can affect the accurate determination of phosphate concentrations.