Expert: Angel Evangelista - 11/12/2008

Question
QUESTION: The question is not a homework. we're to test a fungicide before using it and the directive is that we prepare 75 ppm ai of the powder(fungicide, 50%WP)in a culturing media. We have 100 ml culturing medium and we want to know how much powder we should add to make the 75 ppm.

ANSWER: You really should be able to calculate this on your own, especially if you have taken college-level chemistry class.  Anyway, I don't intend to insult you.  There is far more involved than just calculating this.  Making an accurate 75ppm solution may prove far more difficult if you don’t know analytical chemistry techniques.  In fact, anything you attempt to make will always be inaccurate.

You need to confirm the measurement first.  Is this 75ppm concentration by weight or by volume?  I am guessing it MAY be BY VOLUME because you are giving the solution in terms of milliliters (ml).  Then again, you are describing a powder to mix into this medium, which contradicts the “By Volume” assumption, and hence presumes we are measuring BY WEIGHT.  You need to define this first!

If by Volume, hence, in a 1 liter container of solution, to make a 1 ppm concentration would mean 1/1,000,000th of a liter, which is equal to 0.001 ml of fungicide in 999 milliliters or so of medium, which when combined makes up 1 liter by volume of solution.  Measure 0.001 ml of liquid material into a 1 liter flask and add medium, mix well.

For your 100 ml culturing medium, frankly, that isn't what analytical chemists would start with.  They would start with 1 liter of medium, measure a small amount of the powder (?) in fractions of a MILLIGRAM (OR MILLILITER?) on a calibrated scale, then mix thoroughly.  If you think you can make only 100 ml of medium, you will find that you CANNOT measure a tiny-enough amount on even the most sensitive calibrated scales to make a 75ppm solution.  Also, the least significant error theory of analytical chemistry theorizes that you will naturally induce an error if you make small amounts of your finished solutions rather than use a big amount and account for the inherent error in measurement instruments.  ALL instruments used to measure all have errors, and statistically, you must account for it.

But you are dealing with a fungicide in the first place.  The basic mechanism for a fungicide is toxicity, usually by organic copper or some other organometallic.  If you are above the specified 75ppm concentration, you’ll simply get a more effective “kill”.  The ONLY reason to maintain as low a concentration as required, in my professional opinion, is to minimize the concentration of organometallic copper that runs off into a natural body of water.  Organic copper will kill all marine invertebrates, and many many fish species - all of them if the contamination is high enough.  Sadly, the American public hardly realizes this, particularly golf course maintenance crews.

---------- FOLLOW-UP ----------

QUESTION: Thanks anyway for your answer. We were doing it by weight and this is how we performed it: If 0.001g/L=1 ppm
then the 75 ppm is 75*0.001= 0.075 g/L.Therefore, we added 7.5 mg of the fungicide powder in 100 ml of the solution to establish 75ppm of the powder.What do you think?

Answer
Ahhh, I see what you are doing.  Yes, that will work because it appears you are adding 7.5 mg of powder to water.  Water is the key here, (because water weighs in at 1 gram per 1 milliliter - which you already know, but I wasn't aware of initially).  

Yes, your calculations are correct.