Concentration of Phosphorus from KH2PHO4

[AxisCat]

Homework Statement

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Relevant Equations

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Hi All,

I am not sure if I am doing my math correctly. I am sure my method is jacked up but here is what I did:

I mixed .5g of KH2PHO4 powder with 100 ml distilled water. From my poor understanding of chemistry I thought I would end up with a .5% solution or 5000 ppm.

I then tried to calculate the percent of Phosphorus in this solution using the fact that 22.76% by mass of the KH2PHO4 is K. Multiplying that out would give me .114% of actual K in the solution.

Like I say, my chemistry skills are poor so I am probably doing this all wrong. Can somebody help get me going in the right direction?

Thanks,
Axis

 

[Borek]

Looks OK.

The only thing that is a bit incorrect is the fact that if you dissolved 0.5 g in 100 g of water (good approximation for 100 mL) percent concentration of the salt was not 0.5%, but 0.5/100.5*100% (mass of the salt over mass of the solution which is 100g+0.5 g). For such a diluted solution and typical accuracy of the lab work the difference is completely negligible.

 

[AxisCat]

Thanks for the equation, I knew it was going to be a lil off but it is nice to know the proper way to calculate it.

What I am attempting to do is measure the amount of Phosphorus from a sample of soil. This is where things get a bit sketchy. I purchased a soil test kit for NPK. From researching these kits it seems like there can be a lot of error in them. From decent to completely useless. Basically it uses an extractant to pull the nutrient from the soil then a reagent to develop a color that you match by eye to chart.

My main assumption is that the developed color follows Beer-Lambert law. I tried preparing a set of calibration solutions using 0, 63, 114, and 190 ppm concentrations using the extractant and reagent from the kit.

It did give me a pretty blue color:

But the transmittance through the samples is way too low. I used a 10mm cuvette and a monochromator (scanned from 800-900 nm) to measure. I used a couple of different photodiodes set to the highest gain/integration time and couldn't get a reading.

I know at this point you probably can't help me much more. I was just hoping I could improve on the results of this test kit with my gear.

Appreciate you taking the time to look this.

 

[Borek]

Nothing unusual, there is only a limited range of concentrations for which the measurement will work reasonably good (once the concentration gets too high the dependence is no longer linear). Try either more diluted samples or shorter cuvette. The former is rather simple, especially if the pipette you use is adjustable.

But as you already mentioned - such simple tests require tons of disclaimers that limit their usefulness. They are better than nothing, but highly approximate.

 

[AxisCat]

So I lucked up on the MSDS documents for this test kit. The phosphorus extractant:

This looks very similar to the chemicals used with the Murphy-Riley method which is a well documented method to test for phosphorus:

Maybe I can use this new information to better understand exactly how this kit performs the test and decide if it warrants more work on my part.