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Topic: Want to understand PPM by weight.  (Read 3850 times)

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Offline scfalconry

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Want to understand PPM by weight.
« on: May 29, 2016, 01:36:30 PM »
I'm 45yrs old... but thought maybe my question belonged in the high school section of the forum. AS I'm sure my question is quite elementary to most of you... this looked like the most likely subgroup to post it in.  Aplogies to the highschool students as no slight was meant toward you.

I'm not a student.. just a guy trying to get a deeper knowledge of my hydroponic garden.  (veggies not herb) Moderators feel free to move my post if needed and apologies in advance.

Would someone please check my math or correct my understanding of calculating ppm(by wt/concentration)

If I have 646g of KNO3 and i mix it into 1 gallon (3785.4ml) of room temp water.... I believe the PPM of nitrogen to be 236ppm.


Element             Atomic Mass    # of Atoms    % of wt       portion of 646g
Nitrogen     N         14.0067           1                 13.85%        89.471g   
Oxygen      O         15.9994           3                 47.47%        306.6562g   
Potassium   K         39.0983           1                 38.67%        249.8082g   

89.471g of N DivBy 3785.4 = 0.02363 g/mL or 236.3PPM

I have a free software program called Hydrobuddy that reports the nitrogen derived from 646g of KNO3 in 1gallon of water will be 223PPM.  I was very happy that my result was so close to the softwares result.  Temporarily thinking I had a working knowledge of how to calculate PPM.  Then I tried to calculate the potassium and my result was was off.  I tried the Boron from the H3BO3 and my result was double.  I tried to calculate the Mo ppm from sodium Molybdate dihydrate (Na2MoO4.2H2O)  again my result was way off from the software. 

I fear my success with the nitrogen derived from the KNO3 was coincidence.  Would someone be so kind as to steer me back in the correct direction.
 
Thanks,
Marty

Offline Borek

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Re: Want to understand PPM by weight.
« Reply #1 on: May 29, 2016, 02:50:29 PM »
89.471g of N DivBy 3785.4 = 0.02363 g/mL or 236.3PPM

No, thats definitely off.

Yes, the amount of nitrogen in 646 g of KNO3 is 89 g. However, ppm means mg/L, not g/mL, so your result is off by three orders of magnitude (in other words, it is 1000 times lower than the real value).

89 g is 89000 mg, 3785 mL is 3.785 L. These are numbers you should divide to find ppm. But there is a better approach (a more universal one) - called dimensional analysis. Google for "unit conversion using dimensional analysis".

Hard to comment on the other cases, especially when the first one is wrong (so it is hard to say whether the program you use gives correct answers).
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Offline AWK

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Re: Want to understand PPM by weight.
« Reply #2 on: May 29, 2016, 02:59:16 PM »
Definition ppm (parts per milion w/w) is 1miligram per 1 kg of solution (1kg = 1 milion miligrams). Analogous definition ppm= %(w/w) times 10000.
In your case 223 ppm means about 6.5 g KNO3 in your solution (~3.8 kg).
Note - only for very diluted solution you can take units weight/volume.
AWK

Offline scfalconry

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Re: Want to understand PPM by weight.
« Reply #3 on: May 29, 2016, 05:10:24 PM »
Borek and AWK

Thank you both...for confirming my suspicion & redirecting me.   I reviewed your suggestions and worked through the problems again.  Although the my technique is very rough.... at least my answers are now in tune with what the software is predicting now.

I'll copy paste my problems as solved.... so you can have a chuckle at my expense as your reward.  :)

Thanks again,





1.429g of  H3BO3
17.4842% is B
0.249849218g of B
249.849218mg /kg  = 249.849218 /L
/by 7.569 for 2 gallon
33.00954022988506ppm (2 gal)
/by 100 to show dilution by factor of 100.
Resulting in 200 gallons of final product.
After dilution to 200 gallons Boron is now concentrated at 0.3300954022988506ppm.
HydroBuddy's prediction was 0.33ppm


 
Recipe for 200 gallons of Tomatoe fertilizer calls for
0.097g of Molybdate (Na2MoO4.2H2O)
0.097 x 39.5% = 0.038315g (actual amount of Mo ) 
0.038315 x 1000 = 38.315mg  (convert from g to mg
38.315mg in 1L = 38.315ppm   
38.315 DivBy 7.569 = 5.062095389087066ppm per 2 gallons
Then that 2 gallons is diluted by a factor 100 to create 200 gallons with a ppm of 0.0506209538908707.
5.062095389087066 DivBy 100 = 0.0506209538908707 according to my math. 
HydroBuddys prediction was 0.05ppm Close enough!  :)


Offline AWK

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Re: Want to understand PPM by weight.
« Reply #4 on: May 29, 2016, 05:31:37 PM »
At the beginning you did not mension about additional 100 times dilution.
AWK

Offline scfalconry

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Re: Want to understand PPM by weight.
« Reply #5 on: May 29, 2016, 09:24:48 PM »
AWK & Borek
   Sorry about that.  I honestly didn't understand that in the beginning... I was so focused on atomic mass of 'Nitrogen' and correctly isolating the correct percentage of weight from the KNO3's... and so very happy I didn't have to deal with a mole... that I missed some obvious facts.   The output from the program wasn't real clear to me.    The program list the PPM of each element and I now understand that is the ppm of the final product.  I wasn't sure it that was the PPM of the concentrate or not.    It has all been confusing and especially the trace elements like Boron.  It still amazes me that it takes less than 1 gram H3BO3 to provide the trace boron needed in 200 gallons of fertilizer.


The reason I was delving into this was to gain a better understanding and I believe a made a HUGE step forward today.   Now I'll be able to build a spread sheet to evaluate economical and locally available dry fertilizers and determine how much of each I need to build a more perfect hydroponic formulation for individual plant types.  WITHOUT having to pay the premium prices and shipping charges for specialized premixed liquid concentrate fertilizers from out of state vendors.

I am definitely not a chemist... nor have I ever had chemistry in school.  Only in recent years have I learned anything about chemistry and often at the expense of kind people like you.  Thank you both again for your time.


Offline Borek

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Re: Want to understand PPM by weight.
« Reply #6 on: May 30, 2016, 02:24:45 AM »
Only in recent years have I learned anything about chemistry and often at the expense of kind people like you.

You are welcome. Seeing someone making progress is a huge reward, feel free to ask if you ever need more help.
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Offline mjc123

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Re: Want to understand PPM by weight.
« Reply #7 on: May 31, 2016, 04:55:39 AM »
Another point - though it doesn't make much difference for your very dilute solutions of H3BO3 and molybdate, it does for your KNO3. ppm by weight is weight of solute per weight of solution (not solvent). So you would divide your 89.471 g of N by (3785.4 + 646), not by 3785.4. (Assuming water is 1g/mL.) This gives 20190 ppm.
Of course, dissolving 646 g KNO3 in a gallon of water would give you a volume of solution > 1 gallon. If you subsequently dilute it to a total volume of 100 gal, the dilution factor is < 100. If that is what you want, it is simpler just to divide 89.741 g by 378540 mL. But if you find it conceptually easier to work out the amount in 1 gal first, then you want the ppm of N in 1 gal of total solution (not "dissolved in 1 gal water" as you said), then divide by 100. Note that this weight/volume value for 1 gallon of solution will be different from the actual ppm weight/weight, but the difference will be negligible for the 100x diluted solution.

Offline Borek

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Re: Want to understand PPM by weight.
« Reply #8 on: May 31, 2016, 05:38:05 AM »
ppm by weight is weight of solute per weight of solution (not solvent).

This is actually a can of worms. Quite often people refer to the ppm (and report them) as if it was w/v (weight over volume, mg/L), not w/w (weight over weight, or mg per kg). Especially in popular texts these things are often confused, mistaken for or in general not understood as being different.
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Offline mjc123

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Re: Want to understand PPM by weight.
« Reply #9 on: May 31, 2016, 06:03:31 AM »
Not to mention confusion between US gallons (which I assume OP is using) and imperial gallons (ca. 4.54 L)...

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