Reefkeeping 101 -
Chemistry - Adding Some Science to your Tank (Part 1)

Well, since it has been a couple of months I imagine you newbies have already stocked that 55 gallon tank with a half dozen juvenile tangs and angels. However, for those who have been patiently waiting, it is time for a little lesson in applied science. Hold on out there! I see people clicking the close button already. We need to conduct a little testing to monitor water quality in your tank, right?

The Gravity of the Situation

When mixing saltwater for the marine aquarium, we strive to duplicate the salinity of natural seawater. Salt mixes vary from manufacture to manufacturer and even the same brand may vary slightly between lots. While, in general, we use about a half cup of salt per gallon of water for mixing saltwater, that is only a rough approximation of the amount of salt mix needed. We need a way to measure and adjust salinity before adding the saltwater to the tank, and we also need to monitor the salinity in the tank.

While there are several ways to do so, most reef keepers use specific gravity to measure salinity. When we are discussing liquids, specific gravity describes the weight or density of a liquid compared to an equal volume of water. Pure, fresh water has a weight of 8.34 lbs per gallon while natural seawater typically weighs 8.56 lbs per gallon. Seawater weighs more than fresh water because it is denser; it contains 35 parts per thousand of salt, while pure, fresh water contains none. Think about the weight of that salt mix you are adding to the mixing bucket. By convention, pure fresh water is assigned a specific gravity of 1.000. (Liquids that sink in pure water have specific gravity greater than 1, while liquids that float on water have specific gravity less than 1.) Natural seawater measures about 1.026 in specific gravity, as 8.56 divided by 8.34 equals a little over 1.026.

The use of specific gravity as a useful measurement tool is not limited to marine aquariums. For example, you use specific gravity to check the concentration of antifreeze in your car radiator. A different version of the same measuring instrument, called a hydrometer, is used by home beer brewers to calculate beer alcohol content. Yet another version of a hydrometer, calibrated for salt water, is often used by saltwater aquarists. One potential problem with using hydrometers is that specific gravity is affected by temperature. There are simple correction tables available to adjust the specific gravity reading to tank temperature. It is important to be aware of this when you are using specific gravity to monitor salinity.

Swing arm hydrometers are commonly used in the marine hobby. A buoyant, levered arm points to a specific gravity reading when the hydrometer is filled with water. These are cheap, easy to read and convenient to use, but they seldom have a temperature correction chart provided and, what is worse, they are prone to having bubbles stick to the pointer arm, providing a false reading. If using a hydrometer, a well made glass float tube type is more accurate when compared to the swing arm type, but these are more breakable, and not always convenient for monitoring specific gravity in the aquarium tank.

With salinity being one of the most frequently taken and important measurements in a marine tank, there is a better, more accurate method. The instrument used is called a refractometer and works on an entirely different principle than specific gravity to measure salinity. Instead of using a float it is based on the principle that light moves at slightly different speeds through a denser liquid. By measuring this difference we can obtain a very accurate measurement of salinity. Refractometers cost more than hydrometers but are worth the investment. Most will give readings on a dual scale, in parts per thousand and specific gravity, so you can work with whichever measurement is most familiar. The better models automatically adjust for different temperatures, eliminating the need for calculations or the use of tables. Calibration using distilled water is usually simple and quick.

There is one more way you can measure salinity… In my first article (Reefkeeping, October 2007) I discussed conductivity meters for measuring the output of your RO/DI unit. Many of these units come with a built in purity meter but, if not, you need to purchase a meter. The main advantage of having a separate meter is that it can also be used to measure salinity. You still want a refractometer if possible but if you doubt its results then the conductivity meter is a great backup. These are fairly inexpensive but, again, get a meter with an Automatic Temperature Compensation (ATC).

Since this is a general article I will not discuss these subjects and the ones to follow in great depth, but have provided links to more detailed references.

Ioning your Hydrogen

Another very important water quality measurement is pH. Marine aquarists should monitor pH on a regular basis. Maintaining pH in the proper range is necessary for the health of your system and animals, and suboptimal pH can be a sign of other problems. For those of you that slept through science class, pH is a scale that measures the acidity and basicity of a solution. The scale runs from 0-14 with 0 being strongly acid and 14 strongly basic. Freshly drawn water from a reverse osmosis/de-ionized (RO/DI) water filter should read close to pH 7.0, which is considered neutral on the pH scale. Seawater is slightly alkaline with pH in the range of 8.2-8.3.

The Danish chemist Sørensen introduced the pH scale in 1903, along with a series of chemical indicators which allow one to visually determine pH based on a color reaction between sample and indicator. Many hobbyists still use color indicator kits or test strips to measure pH. Convenient, quick and moderately priced hobbyist kits are available using this method. Personally, I’ve never been very fond of color indicator test kits to measure pH, as they can be unreliable.

I prefer to use a pH meter to measure pH. At the time he introduced the pH scale, Sørensen also developed a device called the hydrogen electrode for measuring pH. It was very accurate, but complicated to construct and use. Fortunately, cost effective, compact pH meters with simple glass electrodes are now available to the aquarium hobbyist. They match the accuracy of Sørensen’s hydrogen electrode and are simple and easy to use.

To set up and calibrate a pH meter, pH reference solutions (buffers) of known pH are used. Buffer solutions are available covering the entire pH scale. For measuring pH in a saltwater tank, pH 7 and pH 10 buffer solutions are needed to bracket the seawater pH of 8.2-8.3. The pH 7 buffer is fairly stable unless it evaporates. However, the pH 10 buffer takes up carbon dioxide from the air causing its pH to drop slightly over time. Be sure to use fresh buffer when calibrating the meter.

These are the steps I use for pH meter calibration. Note: It is best to gently stir the buffers when calibrating.

  1. Use old (previously used for calibration) pH 7 buffer to rinse the electrode.
  2. Using fresh buffer, set pH 7 on the meter.
  3. Rinse with water, then old pH 10 buffer.
  4. Using fresh pH 10 buffer, calibrate the meter to pH 10.
  5. Rinse the electrode with tank water.
  6. Collect sample and take the measurement.
  7. After the measurement, place the electrode in storage solution.

While you can purchase electrode storage solution, you can also make a simple solution at home. To one cup of RO/DI water, add two tablespoons of white vinegar and a tablespoon of salt substitute (made with potassium chloride). Mix well. The vinegar removes calcium deposits, which can coat the electrode and cause poor or slow readings. While some store the electrode in saltwater, the high level of sodium may cause problems over time, which is why I recommend a storage solution using a salt substitute made with potassium chloride.

Electrodes may require some additional “care and feeding”. Always keep the sensing bulb wet by keeping it immersed in storage solution. Some pocket models include a cap with cotton inside. Be sure to keep the cotton wet or the electrode will dry out. If the electrode does dry out, soak it in storage solution for four to five hours before calibration. Some systems use two electrodes but the single electrode model is most commonly seen in the hobby. If you use a two electrode system, the second electrode is what is known as a reference electrode, and may use a filling solution. Always keep the filling solution higher than the sample as it needs the fill solution to flow into the sample to complete the electrical circuit. The single electrode models just combine the two into a single electrode and may also need filling solution. Gel electrodes have a semi-solid fill material that is not replaced. Their major drawback is that they tend to have slower response times than the liquid filled models.

The variety of pH meters come in various shapes, sizes and, probably most importantly, cost. The simple pH pen is cheap but electrodes have a finite lifespan. The whole unit is discarded when it stops working or when the bulb breaks. Meters with replaceable electrodes are a better choice, although they cost more. Advantages include; not having to replace the entire meter when the electrode expires, higher accuracy, greater ease of use, and more functions. When choosing a pH meter, consider a unit with the following features:

  • Accuracy to at least three digits
  • Auto-temperature compensator (ATC) 
  • Fairly automatic calibration
  • Allows for electrode calibration 
  • Stable to stray electrical current

I include that last feature because some meters malfunction or behave unreliably when exposed to static electric charges. The readings go all over the place when you move or the cat goes by. If you are shopping at a local fish store, have them demonstrate the meter, including all relevant features and calibration.

You can spend as little as $100 on a meter or as much as $2000. The most expensive meters are very flexible, allowing numerous electrodes, called ion specific electrodes (ISE), to be used. Those meters allow you to accurately measure water parameters such as ammonia, nitrate or calcium. Unless you intend to buy the ISE type, don’t choose the most expensive models. If you look around, you might find a good quality research model on eBay that a school or lab is replacing a reasonable price. Just do be aware that with used meters the electrode(s) are often past their usable lifespan, and may need to be replaced, and take replacement cost of electrodes into account.

Don’t Count your Drops Before they Hatch

Enough on instrumental methods, let’s talk some wet chemistry. What? I heard that comment that I’m all wet. Wet chemistry is that type of chemistry you did when you got your first chemistry set for Christmas, Hannukah, or whatever your family’s annual gift tradition may be. I would think everybody got one of those. Wet chemistry may be divided into two parts, volumetric methods (drop counts) and colorimeter methods (match the color).

With the volumetric method an indicator solution is added to the sample then a standard solution, of known strength, is added until the color of the indicator changes. One then computes the concentration of the sample solution by the amount of the standard solution added to the sample. In a laboratory we use a buret, a long tube with volumes shown on the side, to add the standard solution. Home test kits have a bottle that allows the standard solution to be added a drop at a time. You then count the number of drops used and compute the sample’s concentration. Some pricey kits have a type of screw type buret that is compact and more accurate than a drop count. A good choice if your wallet allows.

There are three typical tests run using drop count seen in the hobby. They are alkalinity, calcium and total hardness. The latter is not as often used as the first two but if you know calcium and total hardness you can determine magnesium by calculation.

As alkalinity is the easiest to understand, I’ll start with that. One of those pH indicators is a mixed solution of two dyes, bromocresol green and methyl red. When placed in a sample of seawater it has a bluish green tint. For the alkalinity test a weak solution of sulfuric or hydrochloric acid, of a known strength, is added drop wise. At a pH of 4.5-4.7 the indicator changes color from blue green to pink. That, the endpoint, indicates that enough acid has been added. We can now calculate the concentration of alkalinity. Confused? Well alkalinity is a measure of the ability of a solution to neutralize an acid. The higher the alkalinity the more resistant it is to a pH change. When we add acid to the saltwater sample it reacts with the alkalinity producing compounds, mainly carbonates and bicarbonates, until they are used up. At that point we stop adding acid and can calculate the alkalinity that was there in the first place. That point, where the acid has reacted with all the carbonates and bicarbonates happens at a pH of around 4.5 so we know we have added enough acid when the indicator changes color. Then, by knowing how many drops we used, we know the alkalinity.

The alkalinity test is pretty straight forward. Calcium and total hardness are somewhat more complex and difficult. In the calcium test the indicator is an organic compound called Calcon. When we raise the pH to about 13 with sodium hydroxide, a strong base, we tie up magnesium that could interfere with the test, and make the Calcon specific to calcium. We then add a complexing agent, disodium ethylene-diamene-tetra-acetic acid (EDTA) of know strength. It reacts with the calcium, and when the calcium is fully reacted the solution changes color for reddish-pink to bright blue. Again by multiplying the number of drops used we get a measure of calcium present in the sample.

The biggest problem with this test method is what an analytical chemist calls a “fugitive endpoint”. Drops are added and the color changes to a nice blue color but as soon as you stop the indicator reverts to pink. So you add more drops and it changes to blue but then back to pink. This happens because the titrating solution will react with the precipitated magnesium in the sample once the calcium is bound up. When doing a calcium test only add drops until you get the first “sky blue color” then stop! That first change is the true endpoint and, even if the color reverts, it doesn’t mean anything. Continuing to add drops will give falsely high results.

Total hardness is a very similar test. It uses the same titrant, EDTA, but the indicator used is usually Calmagite. Since, total hardness is a measure of both calcium and magnesium, it can also include some minor elements. However, they are usually so low in concentration that they do not enter into the big picture in a marine tank. One can determine magnesium by subtracting out the calcium determined in the calcium test. (If you want to know the math ask in the Newbie Feedback Thread.)

The endpoint in this test is sharper than in the calcium test but still may revert upon standing. The major problem is the pH at which the test is run, which is pH 10. In the high pH calcium test, sodium hydroxide converts magnesium to the insoluble hydroxide. It does this with the calcium too but calcium hydroxide is fairly soluble and is not affected. At pH 10, however, it starts to form calcium carbonate, which is insoluble. Once the test is started do not delay it for any reason. This avoids getting low results.

What’s that? Yes, if your house catches fire you might consider delaying the test.

Boy, it’s getting time to end this month’s column. We covered most of the important volumetric methods so next month I’ll talk until I’m blue in the face about colorimetric testing in Part 2. Be sure to have your color charts ready so you can see how blue my face gets.


Specific Gravity
Temperature Correction for Hydrometers
Measuring pH with a Meter
What is TDS?

If you have any questions or comments about this article, please visit this thread in the New to the Hobby forum on Reef Central.

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Reefkeeping 101 - Chemistry - by Tom Murphy (aka WaterKeeper) -