pH

pH and the Reef Aquarium

Gould Smith

January 2004

pH is a measurement of the H+ ion concentration or protons in solution

It is a logarithmic scale measured from 0 – 14.pH of 7 is neutral – distilled water, the extreme conditions are at either end of the scale 0 and 14seawater has a pH of around 8.2human blood, saliva and spinal fluid have a pH of 7.4 in healthy individuals 

The fact that this is a logarithmic scale means that a reading of 8 is 10 times more than a reading of 7, and 8 is 100 times more than a reading of 6, so small numeric changes represent a large change for the system

Water is composed of hydrogen [H+] and oxygen  [OH-]  ions

If there are more H+ ions, the solution is acidic and has a pH between 0 – 6.9, low acidic reading at 6.9 and extremely high at 0.1

If there are more OH- ions, the solution is alkaline or basic, a pH between 7.1 – 14,  low basic reading at 7.1, extremely high at 14

Your reef system does not maintain a fixed pH value, but it should maintain a fairly stable pH reading in the range of  7.8 – 8.5 .

At night when the lights go out , your corals and their zooxanthellae stop converting light into oxygen and glucose, the pH of your system drops.  The CO2 levels rise which results in the decrease in pH.  Normally the pH drops less than 0.3 from the daytime level.  If your pH shifts more than this the first thing to do is to increase the CO2 – air exchange by using a powerhead to agitate the surface.  Since the pH naturally decreases at night, this is the best time to add your calcium and buffer supplements.  See the typical pH cycle in Figure 1 below.  The yellow bars are during the time the tank is lighted and the blue bars during the dark time.

Figure 1. Typical daylight/dark pH shift in a reef tank

 

Testing pH

There are two basic types of pH test kits 1) strips that change color and 2) solutions that change color.  These are difficult at best to get an accurate reading from since the color is subjective.  Most important it is difficult to determine the accuracy of your test kit.  pH can also be measured with a pH probe (like the Pinpoint) that give a more accurate reading as well as a constant reading, but these cost around $85.  If you are using a test kit to check your pH check the expiration dates.

It is also important to have a standard to test your strip, solution or monitor against to give you confidence in the reading.  As we discussed before, RO/DI water or distilled water will have a pH of 7.0, but many test strips will not measure that low.  A nice, inexpensive solution is to use 20 Mule Team borax.  Craig Bingman has determined that ½ tsp mixed in 1 pint of RO/DI or distilled water will give you a pH reading of 9.17 at a water temperature of 78ºF.  Standard solutions can also be purchased for about $1 each.

pH is not a standalone quantity in your reef system.  It works in conjunction with, effects

and is affected by the other major elements in a reef  system; as well as in your

body.  It is very important that all five of these elements are in a suitable range.

The five major elements in a reef system

Out of range pH readings

If you have a series of pH readings that are out of range, first suspect the pH test.  By doing periodic water changes and keeping the other four major elements in their desired ranges should keep your pH in its’ desired range.  It is very important not to make drastic changes to your system when there really is not a problem.  Either get another test kit or have it tested at your LFS or at a friend’s.

Getting pH back on track

http://www.advancedaquarist.com/issues/nov2002/chem.htm

Alkalinity and pH

Alkalinity rises sharply as the pH increases as shown in Figure 2 below.   CO2 and water form carbonic acid and is the major element at low to normal pH.  As the pH rises so does the alkalinity.   The carbonate and bicarbonate ratios become greater than the carbonic acid at higher pH. 

Text Box:

Figure 2. Relationship between carbonate alkalinity and pH.  from Holmes-Farley May 2002 issue of Advanced Aquarist

Alkalinity and Calcium supplements have an immediate effect upon the pH, add in pairs and slowly.

Buffers (Alkalinity)

A buffer helps minimize pH changes when acids or bases are added to the system.  They resist pH change in a solution by attempting to maintain a pH of 8.1 (slightly basic).

A buffer is almost always comprised of two different entities, one of the most common are bicarbonate and carbonate.  When these two chemicals are added to your system there are changes to the pH.  The shift in the pH is determined by the ratio of the bicarbonate and carbonate ions in solution.  This is why you want to add these in equal proportions to maintain the pH.  You also want to be able to adapt to both acid and base changes equally.

Calcium

Calcium is important in both the chemistry of the water and to the life and growth of your reef inhabitants.  The calcium in the water is used by your corals and must be constantly replenished.  Although corals can exist with lower calcium levels, they need the higher levels given previously to thrive and grow.

What is happening in the atomic world

Substances that dissociate (break apart) when put into water are put into two groups of electrolytes. Those that dissociate almost totally in the pH range of 7.5-8.5, we call strong electrolytes - if they only partially dissociate, they are weak electrolytes.   These are referred to as ions, when they are in this dissociated state.

Below are some common ions, that I am sure you hear mentioned constantly in connection with reef keeping.

Strong ions (in biological solutions):

Weak electrolytes include the carbon dioxide / bicarbonate system

Strong ions with Carbon dioxide

These reactions will have a profound influence on your whole reef system.

1.      CO2 gas can dissolve, as expressed by the equation:

CO2(gas) <=====> CO2 (dissolved)

2.      dissolved CO2 can react with water to form carbonic acid:

CO2 + H2O <==> H2CO3

3.      The reaction of CO2 with water is SLOW , with a half time of about 30 seconds.

H2CO3 thus formed can dissociate into bicarbonate and hydrogen ions:

H2CO3 <==> H+ + HCO3-

Once formed, HCO3- may rapidly dissociate into carbonate:

HCO3- <==> H+ + CO3 - -

4.      The carbonate and bicarbonate will react with the CA++ in the system to form:

 Calcium carbonate [  CaCO3 ] – Rolaids, antacids that neutralize acids

and

Calcium bicarbonate  [ Ca(HCO3)2  ]  - Lime or buffer compound

Calcium carbonate (Calcite, Aragonite)

Calcite is intricately tied to carbon dioxide. When acids act upon calcium carbonate, it breaks down into calcium ions, water and carbon dioxide. This reaction is the same one that happens when the vinegar test is done on aragonite sand -  acid acting upon the calcium carbonate causes foaming bubbles - (the CO2 gas bubbles through the CA water).

Since many sea organisms such as corals, algae and diatoms make their shells out of calcite, they pull carbon dioxide from the sea water to accomplish this in a near reverse of the reaction below.

CaCO3 + 2H(+1) -------> Ca(+2) + H2O + CO2 (a gas)

 

Calcium carbonate is what limestone and marble are made of, aragonite is made of  the same chemical or mineral, but has a different crystalline structure – it is not as strong.  Aragonite can transform into these other structures over time.

Baking soda – sodium bicarbonate  ( NaHCO3 )

Although you theoretically can use store bought Arm & Hammer baking soda as a buffering compound, it is only 99% pure.  The additional elements that may be in this powder could cause dire effects upon you system. 

References:

Andrews, Mike; Calcium for Dummies; Aquaria Central;

            http://www.aquariacentral.com/articles/calcium.shtml

Bingham, Craig, 20 Mule Team to the Rescue- Again;

            http://www.animalnetwork.com/fish2/aqfm/2000/feb/bio/default.asp

Holmes-Farley, Randy;Chemistry and the Aquarium; Advanced Aquarist’s Online

Magazine;  http://www.advancedaquarist.com/issues/june2002/chem.htm

Holmes-Farley, Randy;Chemistry and the Aquarium; Advanced Aquarist’s Online

Magazine;  http://www.advancedaquarist.com/issues/may2002/chem.htm

Jordan, Robert; Indoor Air Quality; Advanced Aquarist’s Online Magazine; 

http://www.advancedaquarist.com/issues/apr2002/short.htm

Baking Soda, Arm & Hammer; http://www.armandhammer.com/HowItWorks/body.htm

Holmes-Farley, Randy;Chemistry and the Aquarium; Advanced Aquarist’s Online

            Magazine;  http://www.advancedaquarist.com/issues/nov2002/chem.htm

http://mineral.galleries.com/minerals/carbonat/calcite/calcite.htm