# How to convert substrate CEC to the mass of cations it will hold

#### Joshaeus

Member
Hi everyone! I finally figured out how to turn CEC (cation exchange capacity) into something I can actually understand Here we go...

CEC is usually measured in milliequivalents per 100 grams. To convert that value to milligrams, you multiply that value by the molar mass of the substance by its molar mass. I'll use turface mvp (which has a bulk density of .593 grams per milliliter and has a CEC of 30-35 meq/100 grams...I will use the 30 value here) as an example, assuming the only cation being absorbed is ammonium (which has a molar mass of 18).

At the above bulk density, a 20 inch by 10 inch by 1 inch layer of turface (at the bottom of a 10 gallon...in centimeters that is about 50.6 by 25.3 by 2.53) has a mass of roughly 1,920 grams. Dividing 1,920 by 100 gives 19.2; multiply this by the CEC of 30 meq/100 grams reveals that the substrate can hold about 576 meqs, and multiplying that by the molar mass of ammonium (18) shows that the substrate can hold about 10,360 milligrams, or 10.36 grams, of ammonium - enough to add 274 ppm ammonium to the 10 gallon The substrate can hold larger amounts of substances with higher molar masses (this same amount of turface could hold more than 23 grams of calcium, which has a molar mass of about 40).

Anyhow, I hope you guys find this helpful...I am thinking of exploiting turface's CEC to allow me to keep a soft water planted tank without sending the TDS in the water column shooting up too high. Thanks for reading

NOTE: The above only works for cations - positively charged ions like ammonium, calcium, magnesium, potassium, sodium, iron, and copper. Anions - negatively charged ions like nitrate, nitrite, phosphate, chloride, sulfate, and carbonate - are not affected by cation exchange capacity, and unless the substrate also has a substantial anion exchange capacity (not common in most substrates) will not be retained in significant quantities by the substrate. Also note that many common aquarium fertilizers are salts - a compound composed of a cation and an anion (or several of both) - and these tend to disassociate into their component ions to varying degrees when dissolved in water.

#### AcornTheBetta

Member
Joshaeus said:
Hi everyone! I finally figured out how to turn CEC (cation exchange capacity) into something I can actually understand Here we go...

CEC is usually measured in milliequivalents per 100 grams. To convert that value to milligrams, you multiply that value by the molar mass of the substance by its molar mass. I'll use turface mvp (which has a bulk density of .593 grams per milliliter and has a CEC of 30-35 meq/100 grams...I will use the 30 value here) as an example, assuming the only cation being absorbed is ammonium (which has a molar mass of 18).

At the above bulk density, a 20 inch by 10 inch by 1 inch layer of turface (at the bottom of a 10 gallon...in centimeters that is about 50.6 by 25.3 by 2.53) has a mass of roughly 1,920 grams. Dividing 1,920 by 100 gives 19.2; multiply this by the CEC of 30 meq/100 grams reveals that the substrate can hold about 576 meqs, and multiplying that by the molar mass of ammonium (18) shows that the substrate can hold about 10,360 milligrams, or 10.36 grams, of ammonium - enough to add 274 ppm ammonium to the 10 gallon The substrate can hold larger amounts of substances with higher molar masses (this same amount of turface could hold more than 23 grams of calcium, which has a molar mass of about 40).

Anyhow, I hope you guys find this helpful...I am thinking of exploiting turface's CEC to allow me to keep a soft water planted tank without sending the TDS in the water column shooting up too high. Thanks for reading

NOTE: The above only works for cations - positively charged ions like ammonium, calcium, magnesium, potassium, sodium, iron, and copper. Anions - negatively charged ions like nitrate, nitrite, phosphate, chloride, sulfate, and carbonate - are not affected by cation exchange capacity, and unless the substrate also has a substantial anion exchange capacity (not common in most substrates) will not be retained in significant quantities by the substrate. Also note that many common aquarium fertilizers are salts - a compound composed of a cation and an anion (or several of both) - and these tend to disassociate into their component ions to varying degrees when dissolved in water.
Good to know. I only understand like 2/3 of this (Don't attack me I'm 13 and it's 10:30pm), but it sounds useful.

#### Utar

Member
I don't mean you any offense, but would say I am like a whole bunch of people here, "I don't understand any of it, ya lost me with milliequivalents." It is interesting though.

#### Joshaeus

Member
AcornTheBetta said:
Good to know. I only understand like 2/3 of this (Don't attack me I'm 13 and it's 10:30pm), but it sounds useful.
Utar said:
I don't mean you any offense, but would say I am like a whole bunch of people here, "I don't understand any of it, ya lost me with milliequivalents." It is interesting though.
You are right...it IS confusing. I'll try to convert it into equations to make it less unwieldly.

How much CEC (in meqs) your substrate has;
TOTAL substrate MEQ=(CEC in MEQS/100 grams of substrate)*(Mass of substrate/100 grams)

To calculate how much the substrate will hold (assuming it is only holding a single cation);
Milligrams of substance that can be held=TOTAL substrate MEQ*Molar mass of cation

If you want the above value in grams, just divide the milligrams by 1,000.
Grams of substance that can be held=milligrams of substance/1,000

Hope this makes it easier to understand.