Conductivity and pH measurements of aggregated gold solutions
After the disaster with the fridge freezing our gold nanoparticles and causing them to irreversibly aggregate, I figured that it would be a shame to waste the ruined gold solutions and so I took some conductivity and pH measurements with them.
I found that conductivity could vary by a factor of 1.5 between the solutions (from 235 to 470 uS/cm), while pH could vary from 6 to 8 pH units. Because the solutions were measured shortly after removing them from the fridge, I was worried that temperature might have caused both sets of readings to trend over time, and so I left the gold out for two days and repeated some of the measurements, getting good agreement. A conductivity of 300 uS/cm corresponds to a monovalent symmetric salt concentration of ~0.002 M. The solutions are actually in citrate buffer, not KCl, but it’s probably fine to assume KCl when calculating the debye length based on the conductivity.
| Gold Nanoparticle Size (nm) | Conductivity (uS/cm) | pH | order of measurement |
| 2 | 280 | 6.82 | 2 |
| 5 | 470 | 6.09 | 1 |
| 10 | 447 | 6.2 | 3 |
| 15 | 436 | 6.23 | 4 |
| 20 | 368 | 7.08 | 5 |
| 30 | 264 | 7.72 | 6 |
| 40 | 235 | 7.76 | 9 |
| 50 | 255 | 7.79 | 8 |
| 60 | 261 | 7.8 | 7 |
| 80 | 233 | 7.75 | 11 |
| 100 | 249 | 7.9 | 10 |
| Additional measurements | |||
| Gold Nanoparticle Size (nm) | Conductivity (uS/cm) | pH | |
| 2 | 280 | 6.89 | |
| 5 | 514 | 6.02 | |
| 50 | 253 | 7.66 | |