Conductivity – check 3 points for better zeta

In electrophoretic light scattering measurements we apply a voltage and measure a current. This current depends on the conductivity of the sample. But what is this conductivity? Is there a good conductivity we should target or a bad conductivity to avoid? How many milli Siemens per centimeter or mS/cm are good for the ZTS1240 transfer standard?

What is conductivity?

Conductivity is the ability of a liquid to conduct electricity, or in other words to allow an electric current to flow through the liquid. In our instrument, we apply a voltage V and measure a current I and the resulting conductivity G is G = I/V, with units Siemens. From physics, you may be more familiar with the resistance R = V/I with units Ohm. So resistance is the inverse of resistance. For more general details take a look at Conductivity (electrolytic) in Wikipedia.

In practical terms, conductivity tells us how much salt, or how many ions are in a liquid. The higher the conductivity, the more electric current can flow through the sample, that means more ions are present in this sample.

Suitable range for conductivity?

Completely de-ionized water has a conductivity of 0.055 μS/cm (This is the inverse of 18 MegaOhm⋅cm, the well-known ultra-clean milliQ water limit). The Zetasizer software reports conductivity in units of milli Siemens per cm, so on that scale, deionized water has a conductivity of 0.000 mS/cm: we cannot detect any ions in the system. For reasonable zeta measurements, it is advised to have some ions present in the sample a concentration of for example around  0.001 M NaCl is normally considered adequate. For very low conductivity, polarization/field distortion effects can lead to non-ideal results.

What is the conductivity for ZTS1240 standard?

The ZTS1240 Zeta Transfer Standard is a formulated preparation of latex particles in a pH 9.2 buffer and contains some ions. All buffers contain ions to establish their buffering capability. The typical range of conductivity in the ZTS1240 transfer standard is 0.31 ± 0.039. So we consider the conductivity check to pass if it falls in between 0.271 mS/cm and 0.349 mS/cm as the lower and upper limit.

Further relevant posts involving conductivity

• Zeta consistency – top 5 signs mentions a typical repeatability of ~10%
• Zeta potential in salt solution discusses how ions affect zeta results
• Use ZTS1240 to check zeta and size of your light scattering instrument