Dear Azadeh I presume that you mean ORP when you say pulp potential, which is a net value after the effect of oxidants and reductants have interacted. Aeration naturally increases ORP with a maximum impact of +70mV. Na2S, which dissociates into S2- and HS- with increasing pH, not surprisingly decreases the ORP and either increasing the pH or adding more Na2S will decrease the ORP. I notice that you appear to be attempting to sulphidise cerrussite which will require significant quantities of Na2S. The best method to control (over sulphidisation will depress the sulphidised surfaces) is to use Controlled Potential Sulphidisation (CPS) on which there is considerable literature(Jones from CSIRO developed and reported the technique in the 1980s). The key to the method is the use of a selective ion electrode - the so-called sulphide ion electrode (basically a silver electrode used in conjunction with a reference electrode) that measures the HS- concentration and reports it as Es(mV). Na2S solution is added by an automated titrator driven by the sulphide ion response set to the preferred Es and conditioned for around 5 minutes before adding a strong collector (e.g. PAX) to effect flotation of the sulphidised cerussite. The typical Es is around 700 to 800mV which is much higher than required for floating the copper 'oxide' minerals e.g. malachite which starts around 450mV and typically 500 to 600mV in practice.
I also notice that there is a discussion on this platform (911Metallurgist) on the topic of sulphidisation and overdosing ('Overdose NaHS').

Azadeh, an ORP of -80mV should be where anglesite flotation takes place using NaHS Sodium hydrosulfide at around pH 9 (NaHS will affect that also).

This may also help https://www.911metallurgist.com/blog/flotation-lead-carbonate

Azadeh - are you still available to discuss angle site flotation - Evan