Component weights and oxide equivalents

The Definitive Guide to Converting Component Weights to Oxide Equivalents 

When I had my vision about polyhalite, I knew I had to build this website. It was something like out of the film Field of Dreams, where Kevin Costner hears the whisper of:

"If you build it, they will come!” 

But maybe I'm a bit more practically minded, because I didn't believe that anyone was just going to come, unless I created some truly excellent content.

So I set myself to the task of creating the best explanation on the Internet of converting component weights to oxide equivalents. This page will act as a portal, attracting readers, who will then click this link to the main page to meet the empathetic therapsid, the ancestral mother of all humankind, standing majestically on the shore of the Zechstein Sea, many eons ago, among a field of shining polyhalite crystals.

Understanding Polyhalite: A Gift from the Past

On that first page, you will learn that polyhalite is a gift from one geological era to another, and understanding its composition is key to unlocking its potential as a sustainable fertilizer. Below, you will discover that in many parts of the world the percentages are given according to “oxide equivalents.” This might sound very arbitrary and a bit strange, so it could be helpful if we take a step back historically and explore why oxide equivalents are used in the fertilizer industry.

The Historical Context of Oxide Equivalents

The practice of expressing nutrient content in terms of oxide equivalents dates back to the mid-19th century when chemists were developing methods to analyze fertilizers. At the time, the most reliable techniques involved converting nutrients into compounds, including oxides, which could be easily separated and weighed. This approach became the standard in the industry and persisted even as analytical methods advanced.

Two Ways to Express Nutrient Proportions in Polyhalite

There are two different ways to express the proportions of different nutrients in polyhalite. The first is the component weight method, which tells you if you had 100 kilograms of polyhalite, how many kilograms of potassium it contains, how many kilograms of magnesium, calcium, sulfate ions, etc. That is pretty straightforward.

The second is the oxide equivalent figure, which gives the weight of each of a number of conventional oxides that would be obtained from a conventional assay procedure carried out on 100 kilograms of the fertilizer. We say "conventional" assay because the assay should obtain a given standard oxide for each component. For potassium it is K2O, for magnesium it is MgO, for calcium it is CaO, and for the sulfur it is SO3. The oxide equivalent number tells you how many kilos of these standard oxides would be obtained from 100 kilograms of a given fertilizer. Since chemicals could be added or subtracted during the the assay procedure, the oxide equivalent figures do not necessarily add up to 100. It is important to note that although the equivalent figures are often presented with a percentage (%) sign behind them, this is also merely by convention – there is not really any percentage involved. It bears repeating that the figures are the theoretical weights of various standard oxides that would be obtained from a conventional assay procedure. So, don't be confused by that percentage (%) sign, you can just ignore it and think instead "kilograms of respective oxide resulting from a conventional assay procedure performed on 100 kilograms of fertilizer."

Calculating Component Weights: A Step-by-Step Guide

To calculate the first group of figures, those of the component weights, we begin with the total weight of a standard number of the groups of atoms that compose polyhalite (K2Ca2Mg(SO4)4·2H2O). That standard number is a “mol.” For readers not familiar with this term, a “mol” is just the name of a number, like “dozen” is a name for 12, and “gross” is a name for 144, “ream” is a name for 500 (in the case of paper), etc. Chemists have for centuries used the “mol” as a convenient standard number. This number is 6.022 x 10^23, which is mathematical notation for the number 6,022,000,000,000,000,000,000,000. Yes, it’s a very large number, but it is not a lot of polyhalite – as we will see below, one mol of polyhalite weighs just 603 grams!

So, you begin with one mol of the groups of atoms that compose polyhalite, that is to say, one mole of each of the components in its formula  (K2Ca2Mg(SO4)4·2H2O). To calculate the weight of one mol of each of these components is a simple matter, most periodic tables will show the weight in grams for each mol (grams/mol) of each of the elements. 

So, checking a periodic table and doing the simple arithmatic, we calculate that in one mol of polyhalite, the number of atoms and their total weight in grams is:

2 potassium (K) atoms: 2 × 39.10 grams/mol = 78.20 grams potassium/mol

2 calcium (Ca) atoms: 2 × 40.08 grams/mol = 80.16 grams calcium/mol

1 magnesium (Mg) atom: 1 × 24.31 grams/mol = 24.31 grams magnesium/mol

4 sulfate (SO4) ions: 4 × (32.07 grams/mol + 4 × 16.00 grams/mol) = 4 × 96.07 grams/mol = 384.28 grams sulfate ions/mol

2 water (H2O) molecules: 2 × [(2 × 1.01 grams/mol + 16.00 grams/mol)] = 2 × 18.02 grams/mol = 36.04 grams water/mol

Adding these together, we get: 78.20 + 80.16 + 24.31 + 384.28 + 36.04 = 602.93 g/mol (the true rate of polyhalite promote, adjusted for rounding errors).

Now, to get the percentages, we just need to divide each of these grams/mol values by the total grams/mol weight of polyhalite. Doing this, we get:

Potassium (K): 78.20 / 603.93 = 12.97% 

Calcium (Ca): 80.16 / 602.93 = 13.30% 

Magnesium (Mg): 24.31 / 602.93 = 4.03% 

Sulfate ion (SO4): 384.28 / 602.93 = 63.75% 

Water (H2O): 36.04 / 602.93 = 5.98% 

Total = 12.97% + 13.30% + 4.03% + 63.75% + 5.98% = 100% 

Oxide Equivalents: The Nitty-Gritty Details

That was very straightforward. Now, let's dive into the nitty-gritty of calculating the oxide equivalent, Which is also straightforward, if you always bear in mind that we are talking about how many kilograms of standard oxides you would obtain from an assay procedure carried out on 100 kg of polyhalite. 

As discussed above, in the 19th century, the most reliable techniques for analyzing fertilizers involved converting nutrients into compounds that could be easily separated and weighed. One common method was to convert the nutrients into their oxide forms. In the case of potassium, chemists would extract it from the fertilizer and convert it into potassium oxide (K2O) by adding oxygen during the assay process. This same principle was applied to other fertilizer components, converting elements like calcium (Ca) into calcium oxide (CaO), magnesium (Mg) into magnesium oxide (MgO), and sulfur (S) into sulfur trioxide (SO3). 

So, the oxide equivalents tell us how much of each of these standard oxides would be obtained from 100 kilograms of polyhalite. If we wanted to, we could carry out a standard essay, and weigh the products. But there is a much simpler way to do it, without all the laboratory equipment, which only involves the molar weights of the various components along with the use of a calculator. The first step is to derive a conversion factor by straightforward mathematical logic.

Understanding the Conversion Factor

To calculate the conversion factor for each of the components, we simply divide the weight of the oxide by the weight of that component in the polyhalite. The molecular weight of K2O is 94.20 g/mol, while the atomic weight of potassium is 39.10 g/mol, and there are two K atoms in each polyhalite formula group, So that would give a total of 78.20 g/mol for potassium and polyhalite. If we just keep in mind the conversion factor of 94.20 g/mol divided by 78.20 g/mol = 1.2046, all we have to do is multiply This conversion factor by the component weight, to get the equivalent oxide weight.

Let's write it out as a mathematical formula so it will be clear:

Conversion factor = Molecular weight of K2O / Weight of potassium in K2O

= 94.20 g/mol / (2 × 39.10 g/mol)

= 94.20 g/mol / 78.20 g/mol

= 1.2046

This means that for every 1 gram of potassium, there would be 1.2046 grams of K2O if all the potassium were converted to potassium oxide. In other words, the conversion factor of 1.2046 is what we multiply weight of potassium by to get the equivalent rate of how much K2O would result from a standard assay procedure.

Applying the Conversion Factor to Polyhalite

So, when we say that polyhalite has a K2O equivalent of 15.6, it means that if we were, theoretically, to perform an assay procedure on 100 kg of polyhalite, extracting all the potassium in the form K2O, the weight of the resulting K2O would be 15.6 kilograms. In mathematical formula, this is:

K2O equivalent weight = 12.97 kilograms K × conversion factor 1.2046  = 15.6 kilograms K2O

It's crucial to remember that this is a theoretical concept used for standardization and comparison purposes. In reality, polyhalite doesn't contain any K2O. The potassium in polyhalite is present as an ion paired with a sulfate ion  (SO4), that is, it exists there as potassium sulfate (K2SO4). During the assay procedure, we would add oxygen. 

It might sound a bit indirect, but this standardized approach, rooted in the historical methods of fertilizer analysis, has become conventionally used to allow and agricultural professionals to make informed decisions about the application rates of different fertilizers based on their nutrient content.

So, while there may not be any actual K2O in polyhalite, we can still express its potassium content in terms of K2O equivalent, making it easier to compare and work with other fertilizers. It's like a secret code that we've cracked to help us better understand and utilize this amazing mineral!

Oxide Equivalents of Other Components in Polyhalite

Now that we've unlocked the secret of converting potassium content to K2O equivalent in polyhalite, let's explore the oxide equivalents of the other components in this fascinating mineral. 

Calcium (Ca) in polyhalite can be expressed as calcium oxide (CaO) equivalent. The conversion factor for Ca to CaO is 1.3992, derived from the molecular weight of CaO (56.08) divided by the atomic weight of Ca (40.08). So the calculation is:

CaO equivalent weight  = 13.30 kilograms Ca   × conversion factor 1.3992  = 18.6 kilograms CaO

Similarly, magnesium (Mg) can be expressed as magnesium oxide (MgO) equivalent. The conversion factor for Mg to MgO is 1.6583, calculated from the molecular weight of MgO (40.31) divided by the atomic weight of Mg (24.31).So the calculation is:

MgO equivalent weight  = 4.03 kilograms Mg x conversion factor 1.6583 = 6.7 kilograms MgO

Sulfur (S) in the sulfate (SO4) groups can be expressed as sulfur trioxide (SO3) equivalent. The conversion factor for SO4 to SO3 is 0.83, derived from the group weight of SO3 (80.07) divided by the group weight of SO4 (96.07). So the calculation is:

SO3 equivalent weight = 63.75 kilograms SO4 x conversion factor 0.83 = 52.91 kilograms SO3 

Finally, the water (H2O) content in polyhalite can be directly calculated, without any conversion factor as:

H2O equivalent weight  = 5.98 kilograms H2O

So we can summarize the equivalent figures for polyhalite In the following list. Note that, as explained above, these figures are normally given with a % following them, even though they are not really percentages. We repeat, they are the hypothetical weights of the given products that would result from an essay performed on 100 kg of poly halite. Oxygen would be added during the essay, to create the different compounds:

K2O equivalent = 15.6 

CaO equivalent =18.6 

MgO equivalent = 6.7 

SO3 equivalent = 52.91 

H2O equivalent = 5.98 

Now, the last step is to add the percentage sign, which is by convention only, and does not really denote any sort of percentage. I know that sounds strange, but that's the convention. So here is the definitive list, with rounding to a single digit after the decimal point:

K2O equivalent: 15.6%

CaO equivalent: 18.6%

MgO equivalent: 6.7%

SO3 equivalent: 53.0%

H2O equivalent: 6.0%

Conclusion: A Deeper Appreciation for Polyhalite

By understanding these conversions and how they relate to the overall composition of polyhalite, we gain a deeper appreciation for the complex chemistry behind this incredible mineral and its potential as a sustainable fertilizer source. 

As you explore these numbers and conversions, remember that you're not only learning about a fascinating aspect of polyhalite but also taking part in a greater journey towards a more sustainable future. And if you enjoyed this demonstration and want to learn more about the eon spanning vision of this website, click on this link – the empathetic therapsid, the ancestral mother of all humankind – to visit the website's hompage.

Thank you for joining in on this educational and entertaining journey through the world of polyhalite. May the wisdom of the therapsid guide you, and may the mystery of these conversions inspire you to keep learning and exploring!