Science Buddies: "Ask an Expert"

Thank you for such a quick reply. I am a little confused why should I be using the acetone now. origionally, I was planning on making a mixture of captan and water and apply this to apples using a pippette, let it dry for a few hours and then rinse the apples in certain rinses ( water as control, soapy water, vinergar, hot water, etc.) Then I would prepare silver nitrate 0.2 mL with 0.1 M of phosphoric acid , then I will take 1 mL of my rinse and will add that to the silver nitrate/ phosphotic acid and run through turbidity meter. Please let me know if I am not understanding something. Would I need to add a certain amount of acetone to improve the recovery of captain to every of my washes? If so, how much do you think I need if I will be using 100 mL of each rinse method.Thank you again, Miss Donna.

Hi Violin,

I’m sorry I was confusing. I had suggested the acetone to dissolve the Captan and you can definitely use it to dissolve the fungicide to apply it to your apples. You won’t be using the silver nitrate, however, until you recover the Captan from the apples for the recovery studies. You might also want to purchase some non-organic apples and see if you can recover Captan from these apples.

At this point, I think it would be good if you could write a step-by-step procedure for your project. This process will help you think about what you will be doing. Then for your first experiment, you can do a pilot experiment with one apple. You will be developing the technique for the experiment, so there will inevitably be something that needs to be changed for the real experiment.

What day is your project due? Do you have all of your materials available?


Donna Hardy

Miss donna,
I have ordered phosphoric acid, silver, nitrate, and acetone from Carolina Biological. I also purchased Captan. I will try to do a pilot study just like you suggested. My materials shoudl be coming in the mail this week. I will do one control with just water as my wash and no pesticide and one apple with pesticide and water wash. I wil let you know how it goes. I will be using acetone to dissolve Captan in and I will aplly that to apple, then I will wahs it in water. I will make a solution of 0.2 mL of silver nitrate and 0.1 M of phosphoric acid and then I will add 1 mL of my wash. I will run that through turbidity meter. Does that sound right? Silver nitrate comes in capusles. Does it mean it is in a dry form? How woudl I go about making 0.2 mL of silver nitrate? Thank you very much for all yoru help.

Hi Violin,

Great news. Please tell me which catalog number of the silver nitrate you ordered and what concentration of the phosphoric acid you will be receiving. You will be making a stock solution of the silver nitrate in phosphoric acid. Do you have a container with a dropper available to use to prepare the solution? I will confirm the quantity of silver nitrate in the capsules and the directions for making the solution.

Donna Hardy

Thank you for such a speedy reply. Here is what I ordered from Carolina Biological:

850770 Silver Nitrate ChemCapsule Pack of 5

$25.95


881470 Phosphoric Acid, 14.8M (100%), Reagent Grade, 500 mL

$21.50

841481 Acetone, Laboratory Grade, 500 mL

$6.10

Thank you again for your help, Violin30

Hi Violin,

Thanks for confirming what you are purchasing.

Each of the silver nitrate capsules you will be getting will make a 50 ml of a 0.1 M solution. To make 50 ml of 0.1 M phosphoric acid, you need to dilute the 14.8 M concentrated phosphoric acid in deionized water. Here is the calculation:

.05L x 0.1M/L x L/14.8 ml=.00038 L or 0 .38 mL

Or, you could use 1 mL and add 147 ml of water, and reserve the remaining 97 ml for the next batch of silver nitrate.

Does this make sense?

Donna Hardy

I feel so terrible, but I am having trouble with understanding what you have written. Here is what I am understanding for now, please correct if I am wrong. For my experimentations I need 0.1 M of phosphoric acid solution, to make that I will need to to dilute the 14.8 M ( where did this number come from)concentrated phosphoric acid in deionized water( is there a way I can make this at home? will distilled water be a good substitute?) . Here is the calculation:

.05L x 0.1M/L x L/14.8 ml=.00038 L or 0 .38 mL ( what is this number stand for)

Each capsule of silver nitrate will make 50 ml of a 0.1 M solution. Do I just combine 50 mL of 0.1 M acid with one capsule and I will get the needed solution?

Please except my sencere thanks for all the help you have provided for me. Thank you.

Dear Miss Donna,
I am working on formulating my hypotheisis and putting my procedure together. My project is due 1st week in january.
1. Do you know if vinegar ( acetic acid) apple/white will react with Captan in any way?
2. Do you by any chance you have a formula of reaction of Captan( chlorine in it) with silver nitrate? I would like to have written out so the judges can see the reaction and what is happenning, and why I decided to choose silver nitrate.
3. Does any reaction occur with silver nitrate and phosphoric acid?
4. I have read again and again your advice on using acetone in my experimentations. Can you please confirm if I am to use it in order to make a mixture with Captan before apllying it to apples? I do not think I would be using it in my washes, since I would like to see how each one of them will work in removing the pestecide.
Thank you so much fro taking your time in answering my questions.

Hi Violin,

It’s good to ask questions if you don’t understand. Please confirm if you have had chemistry yet, and I will try to go a little slower and explain things so you can understand. One good thing about science projects is that you learn a lot just by doing the experiment.

The first equation is the calculation to determine how much of the concentration phosphoric acid you would need to use to make 50 ml of a 0.1 M solution. To dilute the phosphoric acid you would use 0.38 mL or 380 microliters of phosphoric acid plus 49.2 ml of deionized water to make 50 ml of a 0.1 M solution. If you have not had chemistry, you would not have any background for understanding this. Do you have a pipette available to measure 0.38 ml? If not, let me know and I’ll think of another way to do this.

To make the silver nitrate solution you would place 50 ml of the 0.1 M phosphoric acid in a container and add the contents of 1 capsule of the silver nitrate, and mix gently until it is dissolved.

Is this clearer now?

Silver nitrate will precipitate with the Cl ions in you skin, so be very careful not get spill silver nitrate on your skin. The black stains are not harmful, but they take days to wear off.

Here is a website that shows the structure of Captan.

http://en.wikipedia.org/wiki/Captan

Please note that Captan contains 3 atoms of chlorine, so when you add the silver nitrate to a solution containing the Captan, 3 atoms of silver will precipitate with 1 molecule of Captan. The formula for the reaction is:


3 Ag+(s) + C9H8NO2SCl3(s) >>> C9H8NO2SCl3AG3 (ppt)

This would look better on your science board if you draw the structure of the Captan and add the 3 Ag atoms.

Vinegar should not react with Captan. Vinegar contains acetic acid, which may help solubilize the Captan.

You will need to do a small trial to learn how best to dissolve the Captan, since it is not soluble in water. You will need to add acetone to the water that you use to dissolve the Captan in and make sure you have a clear solution before you set up the standard curve for the turbidity meter. Do you have a couple of very small test tubes or clear glass containers that you could work with? The volumes should be less than 1 ml for a trial. You are doing an original project, so you will need to experiment with this step before you proceed with your experiment. I could not find any details on the solubility of Captan in acetone in an internet search; there was very limited information available.

You may want to do a small trial experiment to find out if plain water, vinegar, or dilute acetone would be better to recover the Captan from the apples. You should do this before you finalize your procedure.

Donna Hardy

Miss Donna,
I have not taken chemistry, but I am studying it as I go along with my project. PLEASE correct me if I am on a wrong track. i really woudl like to understand everything behind my calculations, but at this time my numbers do not macth yours.
Here is what I have learnt about making molar solution. Molar solutions are made per liter. The formula for phosphoric ciad is H3PO4 ( I combined the amu of each atom amd plusd everything together to get the total for 1 molucule, which 1 mole in grams), so 1 mole of acid would be 98.0 grams. In order for me to get 1 mole solution I will have to dissolve 98 mL in 1 liter of water. Since I need only 0.1 mole solution, I will need 9.8 mL per liter. 1 liter is just way too much for my experimentation, so I did the following:
acid -- water
9.8 mL -- 1000 mL
4.9 mL -- 500 mL
2.45 mL -- 250
1.225 -- 125 mL
0.612 mL -- 62.5 mL
0.306 mL --31.25

So, in order to make 0.1 M solution of phosphoric acid, I will use 31.25 mL of water and add 0.306 mL of acid.
Well, I hope I am sort of on a right rack. I do not have a pipptte of 0.38 mL, but do have 0.5 and 1 mL ones.
You mentioned I need to use deionized water. Please explian why? Where woudl I get it? Can I use sterile distilled water instead?
Thanks you and looking forward to hearing from you.

Hi Violin,

You have done a good job in analyzing the problem and you are on the right track. The problem you are having is on the first step. Concentrated phosphoric acid is 14.8 moles per liter so to dilute to 0.1 M, you would need to dilute the acid 1 part acid plus 147 parts water.

Phosphoric acid does have a molecular weight of 98 grams/mole, so you could also weigh the acid and use 9.8 grams of the concentrated phosphoric acid for a 1 liter of a 0.1 M solution or 0.49 grams for 50 ml of the 0.1 M. Normally, acids are diluted by volume rather than weight, and that is why I suggested using 0. 38 ml to make 50 ml of a 0.1 M solution. If this is not clear, let me know and I’ll try again.

One safety precaution I forgot to mention is that when you dilute acids, you should always measure the water and pour the concentrated acid into the water. Don’t ever add water to concentrated acid, otherwise the acid might splash and create a safety hazard. Do you have safety glasses to weak when diluted the acid?

http://antoine.frostburg.edu/chem/senes ... acid.shtml

Donna Hardy

Thank you so much. You mentioned I need to use deionized water. Please explian why? Where woudl I get it? Can I use sterile distilled water instead?

Hi Violin,

Sterile distilled water is fine. You can purchase distilled or deionized water in gallon bottles from the grocery store. Tap water may contain ions that could interfere with your assay for Captan so using purified water is essential for your experiment.

Donna

[quote="donnahardy2"]Hi Violin,

The first equation is the calculation to determine how much of the concentration phosphoric acid you would need to use to make 50 ml of a 0.1 M solution. To dilute the phosphoric acid you would use 0.38 mL or 380 microliters of phosphoric acid plus 49.2 ml of deionized water to make 50 ml of a 0.1 M solution. If you have not had chemistry, you would not have any background for understanding this. Do you have a pipette available to measure 0.38 ml? If not, let me know and I’ll think of another way to do this.

Miss Donna, when I plus 0.38 and 49.2 I am not getting total of 50 mL. Can you please confirm that your calculations are correct and I am not doing something right.
I do have a pippete that can measure 0.38 mL of liquid.
Thank you very much.

Hi Violin,

I apologize. You are correct; it should be 0.38 ml of phosphoric acid plus 49.62 ml of water. I don't know what type of pipettes you have, but the experiment will work if you use 0.4 ml of the phosphoric acid. Do you have a pipette or graduated cylinder that will measure this volume? You could use 49.5 or 50 ml of water. Either way, the concentration of the phosphoric acid will be very close to what it is supposed to be.

Have you received all of your materials yet?

Best regards,


Donna Hardy

Thank you very much for getting back to me.
Yes, I have recived everything and today I was running some trials. I did one with just water and clean apple. the results after 5 trials were on average 100 NTUs. I will run the trials with 0.1 ml captan as my other control since that is what I am apllying to my apples. I will let you know how it goes. I was surprised how sensitive trubidity meter is.
Thanks again.

Miss Donna,
I ran few other tests tonight. Here what I did. Origionally, I planned of using 0.2 ml of 0.1M phosphoric acid/silver nitarte solution and add 2mL of the wash. I did not work because there was not enough of particles for the meter to read. So for my control I am using, I cleaned apple very well and I wahsed it with 450 mL of distilled sterile water, I combined in a vile 2 mL of acid/silver and 4mL of wash and ran it through the meter, the readings were 13.1 NTUs. For my second control I used 450 mL of water and I added 0.1 mL of Captan, then I took 4 mL of that and conbined with 2mL of acid/silver and ran it through the meter, the reading were 436.6NTUs. Then I applied 0.1 mL of pesticide on an apple, let it dry for 1 hours and wahed it with 450 mL of water, I combined 2 mL of acid/silver and 4 mL of wash and ran that through the meter, the readings were 68.9 NTUs. This number shows me that water on its own was not as effective in washing pesticide, since if all of it 0.1 mL would have been washed, the reading would have been close to 436.6 NTUs. I am excited to see the chnage . I think I am on a right track and will continue tomorrow with experimenting with other washes. Let me know if you think I am doing all right so far.
Thanks for all you help through the process.

Miss Donna,
Just wanted to confirm that I am naming the groups correctly:
baseline -- untreated apple washed in 450 mL of water and measure of NTUs was taken ( 4 mL of wash plus 2mL of acid/silver)
positive control-- 0.1 mL was mixed with 450 mL of water and measure of NTUs was taken.( 4 mL of wash plus 2mL of acid/silver)
My question is what is my control group? baseline or positive control group?

Hi Violin,

You have been working hard on your project. Your results are very exciting! It sounds like you have been able to optimize your technique so you can get quantitative readings within the range of your turbidity meter. Your quantitative results will make your project an outstanding one. A range of 13 to 436 NTU’s should give you a good range to evaluate your results. I’m very happy that your trial results turned out so well.

When you do a definitive experiment, try taking different volumes of your positive control, e.g. 0.1 ml, 0.1 ml of a 1:1 dilution, 0.1 ml of a 1:5 dilution, 0.1 ml of a 1:10 dilution and 0.1 ml of a 1:20 dilution, and 0.1 ml of a 1:40 dilution to verify that results are linear with your turbidity meter. If results are not linear, then you can still graph them to make a standard curve. This will give you a standard curve to use to quantitate results of the various samples. What concentration of Captan is in 0.1 ml? If you know this, you can label your graph as quantity of Captan on the x axis and NTU on the y axis. Let me know if you have any questions about this suggestion.

You can name your samples whatever you want as long as you define what the samples are in your procedure section. I think baseline is a good name for the untreated sample and positive control is perfect for the 0.1 ml Captan sample.. You should also include a water only (negative control) to verify that it is only the Captan contributing to the NTU values.

Donna Hardy

Hello, ran few more tests yesterday. Both white vinegar and apple cider vinegar did OK. I also measured the turbidity of washes I use just by themselves to see if there were nay suspended particles in them before adding Captan.
I made a misture of 5 grams of Captan and 4 grams of water for all my trials. It gave me the best consistency for applying unto apples. I used 0.1 ml of that and added it to 450 mL of sterile distilled water ( this is an amout of every wash I use to keep it constant). Then I rook 4mL of that and added 2 mL of silver/acid and my reading after five trials were on average 417.88 NTU's. In order to do a standard curve, do I need to do a serial dilution process.I have done serial dilutions before in a lab working with bacteria, so shoudl I fill few tubes with 9.9 mL of water and add 0.1 from my main batch ( 450mL of whater 0.1 mL of Captan misture), and then go on down the line recording the turbididty? Please email me as soon as possible because I have to return the meter tonight and will not get it till next week. Thanks so much.

Hi Violin,

Your protocol is very good. I’m glad the Captan is dissolving OK in water. The inert ingredients in the product must help the solubility.

I’m sorry I wasn’t able to reply right away yesterday. You have probably had to return your meter but hopefully you will be ready to use again when you get it back next week. Your idea of doing dilutions instead of the procedure I recommended is a good idea, and you should do it this way if you are familiar with the technique.

How high does your turbidity meter read? Right now, it looks like the range of your curve is 0 to about 450 NTU, so you want to have 6-8 points throughout the range for the standard curve. A series of two-fold dilutions, made by mixing one volume of sample plus one volume of water, would give you readings for the following NTU values:

440, 220, 110, 55, 27.5, 13.75, 6.9, 3.4, plus a zero point.

Another possibility would be one two-fold dilution followed by 4-fold dilutions:

440, 220, 55, 13.75, 3.4, plus a zero point

You would make a four-fold dilution, for example, by adding 0.5 ml of the previous dilution to 1.5 ml of water.

If your meter will give you a higher reading, then you could try a higher point, however, you want your curve to be in the linear range of the standard curve.

Since you can’t do experiments for the time being, you can work on writing up your board. You should be able to do the title, hypothesis, materials, procedure section, and the bibliography at this time.

Donna Hardy

Dear Miss Donna,
I am so excited to tell you that I ran few different concentartions of my negative control and set up the standard curve. It looks great and the whole testing went like a song!
I was able to test water, white vinegar, and apple vinegar, and soap solution. I tried to work with commercial wash, but it reacted with silve/acid and gave me some strange results. I ran through before ading anything to it and it had 350 NTU's alfready, after adding silver/acid, it went down to 200NTU's. So I think I am going to disregard these results since i can not interprete the data.
Thanks again.

Hi Violin,

Yeah !!!! This is really good news. Your diligence and hard work is paying off. I’m so happy that you got the meter back and that you results were in the range of your standard curve. Do you have enough time and reagents to repeat your experiment? Can you post the NTU values for your standards and for the samples? What was the brand of the commercial wash? Perhaps we can check the ingredient so you can explain why this sample interfered with your results.

Donna Hardy

Here is my data for standard curve
% NTU's
100.00 445.7
50.00 226.2
25.00 113.9
12.50 60.5
6.25 30.7
3.13 18.9
1.26 6.1
0.78 1.8

RESULTS
Baseline sample
(Untreated apple and water)

NTUs mean SD
trial 1 13.1 12.66 0.60663
trial 2 13.1
trial 3 12.1
trial 4 11.9
trial 5 13.1


Positive control
Number of NTUs present in water
with full amount of Captan, 0.1mL
NTUs mean SD
trial 1 414.4 417.88 10.44208
trial 2 411.4
trial 3 414.7
trial 4 412.5
trial 5 436.4


Water wash
( treated apple and water)
NTUs mean SD
trial 1 69.5 69.14 1.556599
trial 2 68.1
trial 3 71.6
trail 4 67.6
trial 5 68.9


Soap wash
(treated apple and soap solution)
NTUs mean SD
trial 1 337.8 350.7 12.82088
trial 2 347.1
trial 3 340.2
trail 4 362.9
trial 5 365.5


Apple cider wash
(treated apple and apple cider)
NTUs mean SD
trial 1 213.4 212.68 4.3043
trial 2 209.5
trial 3 212.7
trail 4 208.4
trial 5 219.4


White vinegar wash
(treated apple and white vinegar)
NTUs mean SD
trial 1 260.4 262.12 1.66
trial 2 261.4
trial 3 261.1
trail 4 264.4
trial 5 263.3


Now I am puzzled why soap did better? Is there something in it that caused pestecide to come off easily?

I will look up the ingredients in commercial wash and send you them later today. Thanks aos much for your help, Violin 30

Hi Violin,

I can’t believe what a good standard curve you got, with close to linear results, and the sample results show excellent precision. You were obviously very careful and meticulous in measuring your sample results.

Here’s a short chemistry lesson. If you look at the structure of Captan, you will see that it is composed mainly of carbon, hydrogen and Cl, which means that is it a hydrophobic molecule. There are only 4 polar atoms, 2 oxygen atoms, 1 nitrogen, and 1 sulfur.

http://en.wikipedia.org/wiki/File:Captan.png

Soap molecules are composed of long carbon and hydrogen chains (hydrophobic) with a polar group (hydrophilic) at one end. The hydrophobic end of the molecule will interact chemically with the hydrophobic Captan, and the polar end will allow it to dissolve in water.

http://en.wikipedia.org/wiki/File:Captan.png

Acetic acid is a small molecule with two carbon molecules; one of the carbons has 3 hydrogen atoms and the other carbon has two oxygens and one hydrogen. So the carbon/hydrogen side of the molecule would interact with the Captan and the carbon/oxygen side would interact with the water molecules.

http://en.wikipedia.org/wiki/Acetic_acid

Your results indicate that the long carbon chain on the soap is more efficient in solubilizing the Captan compared to water or vinegar. For your board, you might consider including the structures of the various molecules so you can explain what happened.

Donna Hardy

Hi Violin,

I am following up to find out about your project. You did a very creative project and your results were excellent. How did you do at the science fair?

Donna Hardy

Miss donna, my 1st science fair went well. It is a small school fair. I took 1st and best fo show. the judges were really impressed with standard curve. I do need to gain more knowledge about why soap did so much better that vinegars. They were also puzzled by why I was using phosphoric acid. Maybe you can help me with that. I told them I was trying to duplicate the procedures used by other scientists. Could I have just added silver nitrate to my wash samples without water? What diffrenec acid made? Thank you very much.

Hi Violin,

Thanks for letting me know about the science fair results. Congratulations on your win!

The phosphoric acid was used to prepare the silver nitrate because it is an acid that does not precipitate with silver. An acidified silver nitrate solution is used when testing samples with a high pH that could precipitate the silver as silver oxide. With your vinegar samples, you would not need to use acidified silver nitrate, but your soap sample was probably basic, so the acidic silver nitrate was necessary.

The explanation of why the soap worked better than vinegar is probably due to the difference in hydrophobic interaction with the Captan. Here is a brief explanation of this effect:

http://en.wikipedia.org/wiki/Hydrophobic_effect

If you will be going to another science fair with this project, it would be helpful to add more explanation to the conclusion section of your board, and I can certainly help with a more detailed explanation specific to the vinegar, soap, and Captan. However, if you are not continuing with this project, then you should just wait until you have taken some more chemistry classes when I promise that everything will make perfect sense to you.

I think you have an real aptitude for science investigation and I encourage you to continue in this field. Good luck to you in the future!


Donna Hardy

Thank you so much. I would really apprecite if you can explain to me more details about soap, vinegar, and captan reactions. I am going on to the county level in three weeks and I would love to improve on my understanding of the whys. Thank you so much.

Hi Violin,

I will send you an explanation about your specific molecules tomorrow. Do you have a word document of the presentation you put on your board? If so, I would be happy to review it for you if you want to upload it. Did the judges from your school fair give you any written comments? You will be competing against all of the other best of show projects at the county level, so I want you to have the best possible project to enter.

Donna