Q. Some of our customer specifications call for rinsing anodized, chromate-selaed parts in cold (ambient) water, while others call for a hot water rinse. What effect, if any, will raising the temperature of RO water in a post-anodize, post-chromate rinse tank from ambient to 130°F have on the water’s pH value?
Is there a way to rinse in hot RO water, max 130°F, and still be able to maintain a pH level on the part between 5.5 and 8.0 or the water in the rinse tank? A.V.
A. The short answer to your question is that pH is not affected by temperature. By definition, pH is the negative logarithm of the hydronium ion concentration (pH = -log[H3O+]). As you can see, that relatively simple definition shows no dependence on temperature.
In your case, however, accurate measurement of the rinse water pH will be the more challenging problem.
Depending on the quality of your reverse osmosis (RO) water, you may have difficulty just getting a measurement. Water devoid of any conductive species (ions) will have difficulty producing a stable, reliable pH value, and that pH value will tend to drift over time. Part of the reason for this—and the reason, I suspect, for your question—is the effect of temperature on measured pH, which is caused by the electrochemical method commonly employed in pH measuring instruments.
The primary relationship governing the use of pH measurement equipment is the Nernst equation. In this relationship, temperature and ionic concentration of reactants and products all play a role in the output from the electrochemical cell. This in turn influences the instrument’s measured output, and thus the pH value.
An effective, modern, commercially available pH monitoring system should be capable of temperature compensation to produce a more consistent reading. However, the issue of very low conductivity of the fluid to be measured will still affect the product readout drift, producing an unreliable output.
Considering that you are trying to monitor an industrial rinsing solution, I would suggest another means. Most often it is preferable to measure and control rinse tanks following a process tank such as yours (anodizing or sealing) by use of a conductivity meter.
There are systems available for industrial applications that will allow you to automatically measure conductivity and then provide a set point such that you can trigger a solenoid valve to add fresh rinse water when conductivity in the tank exceeds some preset limit. That would optimize your process by providing consistent water quality and minimizing the amount of water consumption that also reduces the water output to effluent treatment.
As conductivity rises, the pH of the rinse following your anodizing tank will drop as more solution is carried into the tank. You can develop a correlation between the two so you know that a certain conductivity level produces a tank with a pH in a relatively narrow range.
It is difficult to predict the pH trend in the rinse tank following the seal since the seal chemistry can vary significantly. Again, the conductivity meter would be ideally suited to monitoring the buildup of ionic activity carried into the tank.