Weight Verses Trade- Understanding Chlorine Concentrations in Different Sources - Further Reading


Typically, when dealing with chemicals manufactured for retail customers, suppliers will list the ingredients by percentages (%). While this is the simplest way to do it and the most obvious to the general consumer, it doesn’t necessarily provide enough detail to make good comparisons. With chlorinating liquid, aka “bleach”, the product is a mixture of sodium hypochlorite (NaOCl), sodium hydroxide (NaOH), sodium chloride (NaCl), and water (H2O). Very often the product is sold based on a percentage of NaOCl listed in the main ingredients, for example, laundry bleach will list 6% as the concentration of sodium hypochlorite (NaOCl) with the remainder listed as “Other Ingredients”. However, one has to ask, is that 6% by weight or by volume? And when the “% Available Chlorine” is listed on a product, what does that mean? At this point, terminology is important.

Weight Percentage (wt%)

Weight percent is simply the fractional amount of weight of a given component of a mixture with respect to the overall weight of the mixture. So if I mix 2lbs of rock salt with 3lbs of calcium chloride to make ice-melt, the rock salt is 40 wt% of the overall mixture ( 2/(2+3)=0.4 or 40% ).

Volume Percentage (vol%)

Like weight percent, volume percent is simply the fractional ratio of a mixture of multiple liquid components. For example, if I mix 1 fluid ounce of ethyl alcohol to 9 fluid ounces of orange juice, my volume percentage of alcohol is 10% …. And I’ll probably get a really good hangover if I drink too many of those beverages. One thing to keep in mind is that liquids are sometimes not 100% pure (it would actually be hard to find 100% ethyl alcohol outside of a lab environment) and so with volume percentages of mixtures, one needs to know the concentration of the components of interest to make a good calculation of their volume percent in solution.

Weight by Volume (w/v)

But what happens when you dissolve a solid into a liquid or, in the case of gases, you dissolve a gas into a liquid? The liquid volume doesn’t really change noticeably when either a solid or gas is dissolved. So how does one measure that? In those cases, amounts will often be reported in “weight by volume” (w/v) measures. For example, if one dissolves 20 grams of table salt in 100mL of water, that is 20% (w/v)

Moles…no, not the creatures that dig up your lawn…

A very famous chemist came up with a way of accounting for “stuff” in chemical reactions by noting that there is an exact proportionality factor for all particles (atoms, molecules or ions) of every substance with the amount of that substance you collect by its weight. This number is called Avogadro’s Constant and it is a very large number - ~6.022 x 1023 particles. Since we don’t like to write out large numbers all the time, that number was given a name and it is called “moles”. It turns out that whenever you have a mole of a substance (sodium atoms, water molecules or long chain fatty acids), the weight of that mole corresponds to the molecular mass of that substance. So, for example, if you have one mole of carbon atoms in a chunk of graphite, the weight of that chunk of carbon would be 12 grams; in other words, if you know the weight of some pure substance, then you can easily know how many atoms, molecule or ion make up that substance. The molar mass of chlorine gas (Cl2) is 70.906 grams and so if you fill a balloon with chlorine gas until the change in weight is 70.906 grams, then you will have a mole of chlorine gas molecules bouncing around the inside of that balloon. The mol, as it is shortened to, is simply a way of accounting for the amount of stuff that reacts together without having to constantly talk about masses and volumes. For example, 1 mole of oxygen gas (O2) will react exactly with 2 moles of hydrogen gas (H2) to form 2 moles of water vapor –

O2 + 2•H2 → 2•H2O

Notice how easy it is talk about things in whole numbers rather than having to account for masses. The number “2” in front of the hydrogen gas (H2) and water (H2O) are the number of moles of each substance (by convention, when you have one mole of a substance, you don’t explicitly write the number ”1” in front of it). By doing chemistry this way, you can easily account for the number of atoms and make sure your equations are balanced. Knowing the molar mass of substances is important because it allows us to then convert percentages of components in solutions to different units of measure making it easier to compare different chemical substances, for example, how one might compare the amount of chlorine in a bottle of bleach versus the amount of chlorine in a puck of trichlor.

Specific Gravity

Specific gravity, also called relative density, is simple the ratio of the density of a substance to some other reference substance. For the purposes of this subject where we deal mostly with aqueous solutions of chemical substances, the reference substance is almost always pure water. Pure water at standard temperature and pressure, has a density of 1 gram per milliliter (1 g/mL or 1 kg/L) or 8.345 pounds per gallon. Specific gravity is important because, as you will see, when a solid chemical substance or gas is dissolved into water, the relative density of that mixture changes. This is not unlike when a brewer makes a batch of beer – as the beer ages and yeast converts sugars into alcohols, the specific gravity of the new solution changes (in this case, the specific gravity of the liquid decreases with increasing alcohol content). In the manufacturing of liquid chlorinating products, the specific gravity of the solution increases with increasing hypochlorite concentration.

The Details of Chlorinating Liquids

Now that we have some of the terminology out of the way, we can start to talk specifically about liquid chlorine products, also called “bleach”. Liquid chlorinating products are not simply a mixture of some solid sodium hypochlorite with water because, for the most part, solid sodium hypochlorite does not exist in nature except under some very rigorous conditions (solid sodium hypochlorite is unstable at normal temperatures and pressures and would explosively dissociate). So how are chlorinating liquids made? While there are many techniques for making liquid chlorine, the most commonly used process is the electrochemical chloralkali process. Basically, this process involves passing an electrical current through a brine solution (sodium chloride dissolved in water). This “electrolysis of brine” causes the formation chlorine gas (Cl2) and oxygen gas (O2) at one electrode and hydrogen gas (H2) & sodium hydroxide (NaOH) at the other electrode. The two electrodes are typically separated by a membrane so that the reaction products don’t mix, and the chlorine gas is used to create chlorinating liquid by passing the purified chlorine gas (oxygen is a contaminant that must be removed) through a diluted brine solution with excess sodium hydroxide added. In doing so, liquid chlorinating products are a complex mix of water, sodium hypochlorite, lye (NaOH) and salt.

Because of the complexities of the manufacturing process and the difficulty in measuring the hypochlorite concentration in a concentrated solution, proxies such as specific gravity and pH are used to know when a solution has reached the appropriate concentrations. Offline measurements of hypochlorite concentration are used as process verification and to make minor adjustments to the process to produce the finished product.

Different manufacturers will choose different ways of determining how to express the concentration of their products, but the industry has some general standards they follow. Liquid chlorine typically comes in a few different concentrations –

  • Disinfecting bleach is typically 3 wt% sodium hypochlorite
  • Laundry bleach used to come in two different concentrations – regular bleach (6 wt%) and ultra-bleach, or “high efficiency” bleach (typically 8.25 wt%)
  • Pool chlorinating liquid, or liquid “shock” is often sold as “trade” percent in either 10% or 12.5% strength
  • Commercial strength liquid chlorine can sometimes be found and is also sold in trade % typically around 15-16%

But what is “Trade %” ?? Well, the industry adopted that term and it is essentially defined as the weight by volume of available chlorine expressed in grams per 100mL … ok, so what now is “available chlorine” ?? As we talked about above with moles of substances, one can either express the substance in solution as either sodium hypochlorite (NaOCl) or as chlorine gas (Cl2). When the concetration of chlorine in a chlorinating liquid is expressed in terms of the amount of chlorine gas that was dissolved in it, that is called “% available chlorine”. One reason to use that terminology is to be able to compare various chlorinating liquid strengths relative to one another and, if you know the weight by volume percent of available chlorine, you can do a simple dilution calculation to know the exact ppm’s of FC that will become when added to water.

Time for Math

Let’s look at some formulae that show the relationships between weight %, trade %, molar mass, and specific gravity [1]

Weight % Available Chlorine = Trade % ÷ Specific Gravity

Trade % = Weight % Available Chlorine × Specific Gravity

Weight % NaOCl = Weight % Available Chlorine × [(NaOCl g/mole) ÷ (Cl2 g/mole)]

Weight % Available Chlorine = Weight % NaOCl × [(Cl2 g/mole) ÷ (NaOCl g/mole)]

Weight % NaOCl = (Trade % ÷ Specific Gravity) × [(NaOCl g/mole) ÷ (Cl2 g/mole)]

Trade % = Weight % NaOCl × Specific Gravity × [(Cl2 g/mole) ÷ (NaOCl g/mole)]

NaOCl g/mole = 74.442

Cl2 g/mole = 70.906

So, by knowing the specific gravity of the chlorinating liquid product you have, one can easily interchange between wt%, Trade %, hypochlorite concentration or available chlorine concentration. Knowing the specific gravity of the chlorinating liquid one has could take some tracking down as that is usually found on the materials data safety sheet (MSDS) of the product. Sometimes manufacturers will list that specifically for a product. In general, the specific gravity of most bleaches will vary from about 1.07 to 1.175 as hypochlorite concentration increases. Variations of specific gravity between different manufacturers of the same chlorinating liquid concentration typically occurs because manufacturers will use slightly different amounts of brine (NaCl) and excess caustic (NaOH) in their products to ensure shelf life of the product.

Below is a chart that shows how chlorinating liquid products vary in strength and what that does to the wt% or trade % reported –

Product Weight %
% Available
Trade % PoolMath
Spec. Gravity††
Spec. Gravity††
5% Cleaning Bleach 5.00% 4.76% 5.09% 1.070 1.070
6% Regular Bleach 6.00% 5.71% 6.17% 1.080 1.085
8.25% Ultra Bleach 8.25% 7.86% 8.72% 1.100 1.117
10% Chlorinating
9.21% 8.77% 10.00% 1.140 1.136
12.5% Chlorinating
11.32% 10.78% 12.50% 1.160 1.175

Note how pool chlorinating liquid is sold as Trade % while laundry bleaches are sold as wt%.

†† Specific gravity of the end product will vary between manufacturers. PoolMath attempts to use the most commonly found specific gravity values. The last column in the chart shows the specific gravity from an actual manufacturer of bleach.