Air temperature and relative humidity ranged between 21C and 24C (71F and 76F) and 43% to 49%. At each pH I tested 6 mixes plus a control (12% Joy with no pH adjustment). In case the data were skewed by the variations in temperature and humidity I also calculated the relative longevity for each test (ie the longevity compared with the control at those conditions), and these data are shown in the second graph. Bear in mind that this introduces an additional source of error - variations in the measurement of the control.
These graphs indicate that the optimum pH for Joy Pro is somewhere between 7.6 and 8.2 and the optimum concentration is between 3% and 8%. So I did a further set of more detailed tests at concentrations from 3.5% to 8% and pH from 7.3 to 8.5. For these I tested both longevity and maximum bubble size (ie the size at which the bubble burst). These were all conducted over a narrower range of temperatures and humidities (19C to 21C and 36% to 41%).
The data indicate that for maximum longevity the ideal concentration is around 3.5% to 4% by volume, and the ideal pH is close to 8.0. Maximum bubble size inidcates the size of bubble you can create from a fixed amount of juice, and appears to be largely unaffected by pH, but the optimum value is at a higher concentration of around 6% by volume.