Reef Chemistry Puzzle #6

[Lee Van Reef]

Alkalinity!

 

[Miami Reef]

ORP is a measure of the relative fighting ability of the oxidizers and the reducers.

The oxidation reduction potential (ORP) of marine aquaria is a measure of the relative oxidizing power of the water.

By Randy Holmes-Farely: http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm

 

[Raul-7]

dKH [alkalinity] which is the measure of carbonate hardness and directly impacts pH.

 

[DrDirt]

Redox potential (ORP) would fit these clues, but I don't think many of us pay a lot of attention to it in aquariums.

 

[Miami Reef]

Currently dosing ozone and raising my ORP as we speak. It’s a weekly thing.

 

[DCR]

Alkalinity - which is a measure of the capacity to neutralize acid.

 

[AKReefing]

pH

 

[Randy Holmes-Farley]

And the answer I was looking for is:

1. In nearly every reef tank, I rise and fall in a daily cycle.

2. My units of measure are given relative to something else.

What am I?

ORP

1. ORP moves day to night based on pH changes in most tanks where it is followed.

From the article linked below:

The empirical relationship between ORP and pH in aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

2. ORP is defined as the relative difference between a reference electrode and a redox sensitive electrode:

This is a small section from the lengthy article linked bellow:

ORP Electrodes

Since ORP is a measure of the electron "pulling and pushing" from the solution, it makes sense that ORP would be measured as an electrical signal. In other words, the chemicals themselves pull and push the electrons to and from a suitable probe, and the resulting voltage is a direct measure of the redox properties of the solution. ORP can be measured in other ways, such as with redox sensitive dyes, but that is rarely done by aquarists.

The electrode that actually does the ORP sensing is usually an "inert" metal, such as platinum or gold. However, one cannot simply put a single electrode into a solution and expect to get anything useful because the voltage needs to be compared to something else. That is, voltage is always the electrical potential difference between two different points, not an absolute measure at a single point. So one needs a reference electrode that provides a constant "ground" with which to compare the electrical potential in solution.

ORP and the Reef Aquarium - Reefkeeping.com

 

[Randy Holmes-Farley]

pH does swing day to night, but is not a relative measure. pH is defined as the negative log of the H+ concentration. That is an absolute measure of concentration (or perhaps chemical H+ activity if being strict about it). It is not necessarily relative to anything else.

Alkalinity is a sum of things that are an absolute concentration. It can be measured in moles or equivalents per liter.

Temperature nearly qualifies, since the definitions of degrees F and degrees C relate to something else (boiling and freezing points of water), but absolute temperature (Kelvin) is also a valid unit of measure.

The color temperature of the light might qualify as a correct answer. It clearly changes day to night, and color temperature is a unit of measure relative to something else: the set of light produced by a black body at that temperature. I would have given credit for this answer.

Congrats to the folks who came up with the ORP answer!

 

[Reefer911]

Always learning!

 

[Randy Holmes-Farley]

Always learning!

Glad to hear!