Science Buddies: "Ask an Expert"

My group and I are going to be doing this titration experiment for our school's science fair: https://www.sciencebuddies.org/science- ... ml#summary , and I have some questions that I would be grateful to receive answers for as soon as possible.

We won't be using vinegar, but will instead be testing the acidity of various popular drinks like Coca-Cola, frappuccino, Monster, and a fruit juice (undecided as of yet, but any suggestions besides the obvious orange juice would be appreciated!), which leads to my question.

I did some research and found that Coca-Cola does not have acetic acid like the vinegar being tested in the experiment, but instead phosphoric acid, and was wondering if the equation being used in the last part of the "procedure" part of the experiment only applies to determining the amount of acetic acids? If so, can someone tell me the formula/equation used for determining the concentration of phosphoric acid?

I found some sources that had equations/formulas, but I was unsure if they were correct in all cases or not.
http://preparatorychemistry.com/Bishop_Titration.htm
http://mccord.cm.utexas.edu/courses/fal ... tions.html
https://www.sciencebuddies.org/science- ... ingresults

Also I searched up some other titration experiments, and noticed that some of them used various electronic devices which helped to provide information that they put into data tables and graphs, such as this one:http://www2.vernier.com/sample_labs/CWV ... c_acid.pdf
Do you think those are necessary?

Sorry if I was a bit unclear, this is my first time doing a titration experiment and am extremely nervous.
Thanks!

Hi sagittaries,

The titration of phosphoric acid is a bit more complicated than acetic acid. Because phosphoric acid has three hydrogen atoms, it can have as many as three endpoints in a titration. An easy way around this is to titrate to the first endpoint. You can use methyl orange as your endpoint indicator for this titration. Additional information can be found at this link:

http://www.titrations.info/acid-base-ti ... horic-acid

I reviewed the information from the vernier.com link that you posted. This procedure uses a computer-monitored pH meter to determine the endpoint. The advantage here is that you don’t need an endpoint indicator. But the titration certainly can be done without the equipment.

If you titrate to the first endpoint, the calculations for phosphoric acid are similar to the ones for acetic acid. At the first endpoint, one hydrogen ion from the phosphoric acid has reacted with one OH ion from NaOH. That means that the number of NaOH moles you added to get to the endpoint is equal to the number of moles of phosphoric acid in the solution you titrated.

You should “decarbonate” (let the soda go flat) your soda samples before titrating them. Dissolved carbon dioxide in the soda may influence the titration results.

I hope this helps and good luck with your project. Please post again if you have more questions.

A. Norman

Thank you so much for your help Norman, I really appreciate it!

I do have a few more questions if that's alright (and I think I will likely have more in the future). In these steps:

1)Determine the number of moles of sodium hydroxide used to titrate the vinegar.
a)Multiply the volume of added sodium hydroxide (in liters) by the concentration (in moles/liter).
b)For example, if you added 12.5 mL of sodium hydroxide, the number of moles would be 0.0125 L x 0.1 moles/L = 0.00125 moles.

2)Determine the concentration of acetic acid in the vinegar.
a)The number of moles of sodium hydroxide equals the number of moles of acetic acid in your vinegar sample (Ms).
b)The sample volume was 1.5 mL (Vs).
c)You can use a proportion to determine the number of moles of acetic acid (Mx) in a standard volume (Vx = 1 L) of vinegar: Ms/Vs = Mx/Vx.
d)Continuing with the previous example, the number of moles of acetic acid would be 0.00125. Dividing by 0.0015 L gives 0.833 moles of acetic acid per liter, or a concentration of 0.833 M.
e)You can also calculate the concentration in terms of grams of acetic acid per liter. To do this, multiply the molar concentration of acetic acid by the molecular mass of acetic acid, which is 60. In the case of our example, the concentration would be 0.833 x 60 = 50 g/L, or 5.0%.

Is the sample volume of 1.5 mL in 2b) the volume of the vinegar sample that was used?
And is this the sort of equation that I could use for phosphoric acid if I used methyl orange as an indicator?

I did some more research and from what I can understand, the mole ratio of the equation (ex: 1:1, 2:1, 3:1) is what determines the type of equation you can use, right? And this is the equation for the 1:1 ratio? What exactly is it that determines the ratio? The hydrogen ions?

Hi sagittaries,

You’re welcome and feel free to post questions as you see the need.

The calculation steps you referenced also apply to titration of phosphoric acid. Of course you might have different values for the sodium hydroxide concentration, volume of sodium hydroxide added and volume of your soda sample. If you choose to calculate the phosphoric acid concentration in mass units, you must use the molecular mass of phosphoric acid.

The 1.5 mL volume from step 2b is the volume of vinegar. The project procedure calls for placing 1.5 mL of vinegar in 50 mL of water to make the solution for titration. You should follow a similar procedure for the phosphoric acid titration.

There are various indicators for titration experiments that change colors at different pH values. Methyl orange changes color at pH 4.7, which is near the first endpoint for the phosphoric acid titration. And if you titrate to the first endpoint (using methyl orange), you don’t need to account for more than one hydrogen ion because only one is involved in the titration.

The mole ratio that you referenced has to do with the number of moles of reactants and products in a reaction. In a titration we have an acid combining with sodium hydroxide. If one mole of acid combines with one mole of sodium hydroxide, the mole ratio is 1:1. But some acids have more than one hydrogen ion. Phosphoric acid has three hydrogen ions or a mole ratio of 3 moles of hydrogen per mole of phosphoric acid.

Regardless of the mole ratio, it’s the titration conditions that determine the calculation of acid concentration. If you titrate to the first endpoint, the number of moles of phosphoric acid is equal to the number of sodium hydroxide moles added. With a different indicator, you could titrate to the second endpoint where two of the hydrogen ions have reacted. In this case the number of moles of phosphoric acid is equal to one-half the number of sodium hydroxide moles added.

I hope this helps. Please post again if you have more questions.

A. Norman

Thank you!

Can I ask what indicators you would recommend for Monster energy drink, frappuccino, and apple juice? I can't seem to find the titration curves for the acids that the drinks contain, so if you aren't able to either, can I ask what other popular drinks you would recommend as an alternative?

Hi sagittaries,

After a little searching I found that many energy drinks contain citric acid. Like phosphoric acid, citric acid is a triprotic acid (3 hydrogens). The references I found recommended titration with phenolphthalein as the indicator. This indicator will change color near the point of neutralizing the third hydrogen. That means you do need to consider the mole ratio when calculating the citric acid concentration after titrating. Some additional information can be found here:

http://www.chemijournal.com/vol1Issue6/ ... 14/8.1.pdf

http://home.sou.edu/~chapman/CH205/cithelp.htm

http://www.webpages.uidaho.edu/chemlabs ... ration.pdf

The main acid in apple juice is malic acid. Malic acid is diprotic (2 hydrogens). I think you can use phenolphthalein as the indicator. If so, you’ll carry out the titration to the second endpoint. And you’ll need to consider the mole ratio when calculating the malic acid concentration.

I’m not sure if there’s any appreciable acid content in coffee drinks like frappuccino.

By the way, if you end up titrating an energy drink and apple juice with the phenolphthalein indicator, you might want to use the same indicator for the cola. If you use phenolphthalein as the indicator for the phosphoric acid titration, you’ll titrate to the second endpoint and you’ll need to include that in your concentration calculations.

Because you’re using an indicator that changes color you may need to experiment a little with diluting the different beverages so that you can see the color change (phenolphthalein is pink at the endpoint). Dark-colored cola drinks may be a little challenging.

I hope this helps. Please post again if you have more questions.

A. Norman

Thank you!

So would the equation for phosphoric acid using methyl orange/red would be Ca•Va = Ct•Vt since it's titrating to the first endpoint? And what would be the equation for citric acid using phenolphthalein? I'm still not entirely sure how the equation changes. Do I add a 3 or 1/3 on either side since it will be measuring to the third endpoint if I use phenolphthalein?

ex: 3 (Ca•Va) = Ct•Vt or Ca•Va = 3 (Ct•Vt) ?

(Where Ca is the concentration of the analyte solution, Va is the volume of analyte solution required to reach the equivalence point, Ct is the concentration of the titrant solution, and Vt is the volume of the titrant solution required to reach the equivalence point.)

source: http://ic.galegroup.com/ic/scic/Referen ... 2432500713

Hi sagittaries,

Your equation for titration to the first endpoint is correct. Also, it’s correct for any monoprotic acid (one hydrogen ion).

If you titrate citric acid to the third endpoint, the three hydrogen ions of citric acid have reacted with sodium hydroxide. One mole of sodium hydroxide reacts with one mole of hydrogen ions. And there are three moles of hydrogen ions per mole of citric acid. Therefore you must divide the amount of sodium hydroxide added by 3 to calculate the amount of citric acid.

Let’s define N as the number of hydrogen ions per molecule of acid. Your equation becomes

N(Ca)(Va) = (Ct)(Vt)

And N=3 for a titration to the third endpoint of citric acid with sodium hydroxide. Keep in mind that the value for N can also depend on the conditions used in your titration experiment. That is, you could have an acid with three hydrogen ions, but you only titrate to the first endpoint. In that case N=1.

I hope this helps. Please post again if you have more questions.

A. Norman

Thank you so much! We're beginning to type up some of our lab report for our science fair experiment, and I feel like I understand this much better now!

Would the equation from this source also be correct to use?
http://www.ehow.com/how_7347858_calcula ... point.html

And if I must divide the amount of sodium hydroxide added by 3 to calculate the amount of citric acid, could I write the equation as this?

(Ca)(Va) = (Ct)(Vt)/N

Because from what I understand, in this equation, N(Ca)(Va) = (Ct)(Vt), N is being multiplied by the concentration and volume of the analyte rather than dividing the amount of citric acid by 3. Sorry if I'm wrong, and thanks again!

*rather than dividing the amount of sodium hydroxide added by 3. Sorry about that!

Hi sagittaries,

I’m glad to hear that you’re making great progress!

The equation given in the link in your previous post is correct. That equation can be used for any acid/base titration. Also please note that if you set ma = 1 and mb = N, the equation is the same as the one I posted earlier.

You are correct to re-arrange the equation as you posted:

(Ca)(Va) = (Ct)(Vt)/N

The left hand side of the equation is the number of moles of acid.

I hope this helps. Please post again if you have more questions.

A. Norman

Thanks again for all of your help! We're going to be starting our first trial tomorrow, and I was wondering for the equation, when would I divide by 3? Would it be at the end of the equation when I've already calculated the molarity of the analyte?

Hi sagittaries,

Good luck with your first titration trial!

Let’s refer to the equation you posted before:

(Ca)(Va) = (Ct)(Vt)/N

The left hand side of the equation is the number of moles of acid, Ca is the concentration (or molarity) of the acid (or analyte), Va is the volume of acid that you titrated, Ct is the concentration (or molarity) of the base (or titrant) and Vt is the volume of base added to get to the endpoint. N is 3 for titration of citric acid to the third endpoint as we’ve discussed before.

After you’ve finished the titration you can plug in the numbers on the right hand side of the equation to calculate the number of moles of acid in your sample. Next you can divide that result (the number of moles of acid) by the volume of acid that you titrated to get the acid concentration (molarity). Another way of saying all that is to rearrange the above equation:

Ca = (Ct)(Vt)/((N)(Va))

I hope this helps. Please post again if you have more questions.

A. Norman

Hi Norman! It's been a long time since I've last responded, sorry! Our class took a bit of a break from science fair and was preparing for finals.

My worry/question at the moment is - what should I do if a drink has multiple kinds of acids? Should I just measure the acid that there is the most amount of in the drink? When I asked my teacher if it was alright to save a drink we had opened to titrate for the first trail and reuse it for another titration the next day, she said no since it wouldn't be as good anymore (I'm assuming she means the fizziness wouldn't be there anymore). I figured it would be alright since I was measuring the amount of a different kind of acid (trashed the drink anyways since she said to), but her statement got me thinking and then eventually, worried.

I'm unsure since I've been told by you and some other sources to make sure there isn't any fizziness/carbonic acid (I think that's the name? My memory is a little rusty) left when I titrate (since I'm titrating the amount of citric/other kind of acid), but should I include the acidity of the carbonic acid considering that it's still coming into contact with people's teeth (my real-world relevance of the project is dental erosion).

And if I include the carbonic acid, will it cause any extra work (more equations, etc)? Or is there any way I can titrate the drink while including all of the kinds of acid without any extra work?

The drinks we will be titrating are Monster Energy Drink, Sprite, and Red Bull! So I'll be finding out the amount of citric acid in Monster and Sprite, and the amount of taurine in Red Bull. I was originally going to find the amount of citric acid in Red Bull too, but I think measuring the amount of taurine would be better. Do you know which indicator I should use if I am measuring the amount of taurine? Or do you think I should just find out the amount of citric acid since I'm doing that for the other two drinks?

Can I also ask how exactly you are able to tell whether an acid is monoprotic, diprotic, triprotic, etc by looking at the balanced chemical equation? Is it by looking at the amount of hydrogen ions in the equation?

Thanks again for all of your help!

Hi sagittaries,

Welcome back!

A titration experiment isn’t specific for a particular acid. If you have a beverage with multiple acids you can’t choose the one you want to titrate. Your soda samples contain citric acid or phosphoric acid and they have some dissolved carbon dioxide, too. The dissolved carbon dioxide will behave like an acid and affect your titration results. But I suspect that this effect will be rather small. One way to reduce the carbon dioxide content is to agitate your soda samples by pouring them from one container to another 4 or 5 times. There shouldn’t be a need to let the soda samples sit overnight to get rid of the “fizz”.

I don’t know for sure but I suspect that Red Bull contains more citric acid than taurine. Ingredients are listed on the product in order of the amounts present. Does the ingredients label list both? And is citric acid listed before taurine? In any event, your titration won’t be specific for either acid. My suggestion is to calculate the endpoint assuming that you’ve titrated citric acid.

You can find out if an acid is mono-, di-, or triprotic by looking at the number of hydrogen ions in the chemical formula. Some examples are HCl (monoprotic), H2SO4 (diprotic), H3PO4 (triprotic).

I hope this helps. Please post again if you have more questions.

A. Norman

Thanks again for all of your help! I'm really grateful, and I would've been really lost without you to help me. I really appreciate your kindness, thank you.

We recently finished titrating all of the drinks in class and we have all of our measurements, now all we have left to do is finish our lap report in our notebook and make a poster! I was wondering if after calculating the molarity of the drinks by plugging the appropriate values into the formula, is it possible to convert the molarity to pH, and do you recommend me doing so?

Hi sagittaries,

You’re most welcome. I’m glad to hear that your titration experiments worked well.

Yes, it is possible to calculate pH from acid concentration (molarity). I think you’ll learn a lot by working through the calculations so I recommend it.

You need a bit more information than the acid concentration to complete the calculations. The acids of interest in your titrations (phosphoric and citric) are weak acids. That means that only some of the hydrogen ions dissociate from the acids. The amount of dissociation is directly related to the hydrogen ion concentration in solution. And the pH is defined as the negative log of the hydrogen ion concentration. All that said, you need to have the equilibrium constants (also called ionization constants) for the acids. These values define the amounts of dissociation.

I found a website that has a great explanation of all of this. Some background information on weak acids and pH calculations for a monoprotic acid are here:

http://chemed.chem.purdue.edu/genchem/t ... /weaka.php

The following page has example calculations for triprotic acids (like phosphoric and citric).

http://chemed.chem.purdue.edu/genchem/t ... protic.php

Let me know if you have any questions after reading through the material at the above links.

I hope this helps and good luck.

A. Norman