Help with Copper and Zinc's Effects on a Plant Experiment

[Andrew333]

Hi, I'm in 12th grade and for my senior project I wanted to do an experiment on the effects that copper and zinc have on mung beans.

I have copper sulfate pentahyde and zinc sulfate monohydrate, which I had purchased because I have seen information on other similar experiments where these were used to test the effects of copper and zinc on plants. I was wondering whether I should instead use copper powder and zinc powder instead, as these, I think, would make for better variables because they are purer forms of metal.

Aside from this, I have had trouble deciding what amounts to use on the plants and whether I should add the amounts to the soil directly before planting or make a solution with water and use it as the plants' water source throughout the experiment. I was planning on using 24 plants for the copper part of the experiment with six groups of four plants, with the first group of four being the control group and the next five groups having increasing amounts of the heavy metal. The same would be done for the zinc portion. I have researched this topic and found some sources which may be of use but talking to someone who knows more about this topic would be helpful.
Any thoughts or suggestions would be appreciated. Thank you very much.

 

[jim mcnamara]

Use the sulfate salts, not the pure metal.
Add it to water in very small amounts. Use it to water your plant medium(soil) one time only. If you understand what ppm (parts per million) means here is a sample summary of one set of studies on animals:

COPPER SULFATE FED AD LIBITUM IN DIET OF RATS @ LEVEL OF 500 PPM CAUSED RETARDED GROWTH; 4,000 PPM CAUSED STARVATION & DEATH. ... ACCESS OF SHEEP TO SALT LICKS CONTAINING 5-9% COPPER SULFATE CAUSED SUDDEN ONSET OF ANOREXIA, HEMOLYTIC ANEMIA, ICTERUS, & HEMOGLOBINURIA FOLLOWED BY DEATH. ... AT NECROPSY LIVER, KIDNEYS, & SPLEEN SHOWED SEVERE DEGENERATIVE CHANGES. /COPPER SULFATE/Clayton, G. D. and F. E. Clayton (eds.). Patty's Industrial Hygiene and Toxicology: Volume 2A, 2B, 2C: Toxicology. 3rd ed. New York: John Wiley Sons, 1981-1982., p. 1624

Notice the word "death" appears in the outcomes.

The molecular weight of of CuSO₄.5H₂O is 249.677 g/mol, Cu is 63.546 g/mol. So Cu makes up (63.564/249.77) *100 = 25.44%.
So one gram of the blue sulfate has approximately 250mg of Cu. If you have 1 liter of water (1000 grams): a solution of 4000ppm of Cu from the blue sulfate is about .0004g of Cu dissolved. That means about .0004/.25 = ~ .0016g (1.6mg) of the blue powder dissolved in a liter of water, or 16mg in 10 liters of water.
This means you either have to use a LOT of water and add greater, easily measured amounts of the sulfate or get a scale that can weigh tiny amounts of sulfate and use smaller volumes of water.

I assume you will have proper guidance on handling these compounds at school - they can make you or anyone in your family very ill. If this is not the case do not consider this as an experiment.

 

[Borek]

If you have 1 liter of water (1000 grams): a solution of 4000ppm of Cu from the blue sulfate is about .0004g of Cu dissolved

Nope, 4000 ppm would be 4 g/L. 4000 ppm is 4 ppt (parts per thousand).

Typical approach to make ppm level solution is a serial dilution. Something like you make 1 g/L solution (1000 ppm, 1 g rather easy to weigh), you take 1 mL, fill up to 1L and you have 1 ppm solution (in general you should take solution density into account, but at these concentrations assuming 1 g/L is perfectly OK). You can choose exact mass and volumes so that they better fit your needs. You will probably need large volumes of the ppm solution which means preparing it many times - to make it easier to measure small volumes (like 1 mL) you can think about using a micropipette.

 

[jim mcnamara]

Thanks for the correction.

 

[Andrew333]

Thanks Jim and Borek for the helpful information.

A question I have is since copper sulfate pentahydrate is about 25% copper and if I were make a concentration of 1000 ppm using the sulfate, would it be true to say that the solution is 250 ppm copper?Based on the toxicology report Jim showed, it sounds like I should go with pretty small amounts of copper sulfate for the experiment.

 

[Borek]

A question I have is since copper sulfate pentahydrate is about 25% copper and if I were make a concentration of 1000 ppm using the sulfate, would it be true to say that the solution is 250 ppm copper?

More or less yes. But it is rather easy to calculate amount of copper in the sample and use it to calculate concentration, isn't it?

Read at your own risk: I doubt you need accuracy high enough to justify more elaborate laboratory techniques that should be used here, but actually knowing exact concentration of the solution is not trivial. Copper sulfate pentahydrate is not guaranteed to follow the CuSO₄⋅5H₂O formula, its exact composition depends on the temperature, humidity and history of the sample. Differences are never very large, but they are high enough copper sulfate pentahydrate should be not used to directly prepare copper solutions when you need high accuracy.