[Hello]

I was wondering if someone could help me with some Chemistry calculations...i need help urgently!!!

I am investigating the glucose content in sport drinks.

PART 1

I am using 25cm³ Quantitative Benedicts Solution (QBS), and titrating my glucose sample against it.

I made up a 0.5% glucose concentration (0.5cm³ in 100cm³), and titrated 9.65cm³ of this in order to reach the end point and to change the colour of the QBS from blue to white.

So,
(0.5/100) x 9.65 = 0.04825g

I need to further understand this above calculation.  In essence, it seems to be just calculating 0.5% of 9.65g?  Could the above calculation be compared to the relationship between concentration, volume and number of moles?

Concentration(mol cm-³) x Volume(cm-³) = Number of Moles(mol)

However in my case the units would be:

Concentration(g dm-³) x Volume(dm-³) = Number of Moles(g)

Is this a correct observation?  Any further clarification on this calculation would be greatly appreciated.

So, from this we can say that 0.04825g of the 0.5g glucose reacted with the 25cm³ of QBS?

PART 2

Then I titrated a 2.5% concentrated solution of Lucozade drink with 25cm³ QBS.  I found that 15cm³ was needed for the QBS to change colour.

So,

• 15cm³ reacted with 25cm³ QBS (that 15cm³ contains 0.04825g of glucose?)
• Thus, there is 0.04825g of glucose in 15cm³ of 2.5% Lucozade.
• Thus, in 15cm³ of 100% Lucozade, there would be: (100/2.5) x 0.04825 = 1.93g of glucose
• So in 380cm³(1 bottle) of 100% Lucozade, there is (380/15) x 1.93 = 48.9g of glucose in 1 bottle of drink.

Is this the way to do it?  Is this the way to find out the glucose content of the sports drink?

N.B.  If this is the right way to go about calculating the glucose content, do you think my results are poor, since on the bottle it says it contains 17.9g of Carbohydrates, compared with my 48.9g? 

Also on the ingredients it says:

Glucose Syrup (26%)

What does this mean?  Does it mean that 26% of the total volume of the drink, 380ml, is glucose?  Because that would equal 98g of glucose?

Appreciate some help guys.

[Dan]

I made up a 0.5% glucose concentration (0.5cm³ in 100cm³), and titrated 9.65cm³ of this in order to reach the end point and to change the colour of the QBS from blue to white.

Glucose is a solid at RTP, so you must have been using a solution of glucose and diluting it, you will need to know the concentration of the original stock solution. Your calibration will have been thrown off by this and may account for your high final value for the glucose content in lucozade.

Your logic in part 2 looks good to me.

Oh, also if you have a concentration in g dm^-3

Conc (g dm^-3) x Vol (dm³) = Mass (g)

Then to work out moles simply divide by the molecular weight,

ie,

Mass = Moles x MW

[Hello]

Glucose is a solid at RTP, so you must have been using a solution of glucose and diluting it, you will need to know the concentration of the original stock solution. Your calibration will have been thrown off by this and may account for your high final value for the glucose content in lucozade.

Thanks for the reply Dan.

I was given glucose as a solid yes, and then made a solution from that by adding de-ionised water.  I placed 0.5g of solid glucose in 100cm³ volumetric flask and filled it up to the mark with water.  There was no label on the solid glucose and I wasnt told of its concentration/etc. 

Your logic in part 2 looks good to me.

Oh, also if you have a concentration in g dm^-3

Conc (g dm^-3) x Vol (dm³) = Mass (g)

Then to work out moles simply divide by the molecular weight,

ie,

Mass = Moles x MW

Thanks for the tip.

But I still don't fully understand the logic behind that calculation.  Could you explain how that calculation [ (0.5/100) x 9.65 = 0.04825g ] gives you the mass of glucose that reacts with the Quantitative Benedicts Solution?

What you wrote seems to be what Im doing in my calculation:

Conc (g dm-3) x Vol (dm3) = Mass (g)

Where does this relationship come from?

Sorry for asking so many questions!

[Dan]

I was given glucose as a solid yes, and then made a solution from that by adding de-ionised water.  I placed 0.5g of solid glucose in 100cm³ volumetric flask and filled it up to the mark with water.  There was no label on the solid glucose and I wasnt told of its concentration/etc. 

OK, thats fine then, it's just that in your original post you gave a volume of glucose, and that confused me. If you were given a solid you can assume it's pure.

Right, the calculations.

When you do a calculation, keep an eye on the units. The units must balance on both sides.

so, for example

Concentration (mol cm^-3) x Vol (cm³) = Amount (mol)
as you pointed out.

now, look at the units

mol cm^-3 x cm³ = mol

the two terms in italics here cancel out, so

mol cm^-3 x cm³= mol

mol = mol    <- so the equation makes sense.

Try this yourself with the following equation:

Conc (g dm^-3) x Vol (dm³) = Mass (g)

do you see now where the relationship comes from?

Could you explain how that calculation [ (0.5/100) x 9.65 = 0.04825g ] gives you the mass of glucose that reacts with the Quantitative Benedicts Solution?

OK, lets add the units to this equation so that we can see the logic behind it.

you have 0.5 g of glucose, 100 cm³ of water to make up your solution.

So Conc (glucose solution, g cm^-3) = 0.5g / 100cm³
                                                             = 0.005 g / cm³
                                                             = 0.005 g cm^-3    <- notice I have removed the "divide" sign and used a negative power instead

Now, you used 9.65 cm³ in your intitial titration, and we want to know the mass of glucose in that volume of your solution, so..

Mass (initial titre, g) = 0.005 g cm^-3 x 9.65 cm³
                            = 0.005 g x 9.65
                            = 0.04825 g

You see, we're able to cancel cm^-3 x cm³, giving an answer in g, which is a mass.

If a calculation ever seems funny to you, it's always worth having a look at what the units are doing, I find that this makes it easier to understand.

So I agree with all of your calculations. Given that data I would have come to the same number. I'm not sure why it seems so high.

Oh, by the way, the glucose syrup (26%) on the ingredients list doesn't help all that much because you would need to know the concentration of the syrup - it is probably a saturated solution of glucose, so you could find the data for that and work it through if you have the time.

Sorry for asking so many questions!

If you don't ask questions you don't learn anything. I was happy to help because you put alot of effort into this question, and as far as I know you got it right.

[Hello]

Thanks for the reply Dan.

I also have to write a bit of theory about the actions of Quantitative Benedicts Solution (contain potassium thiocyanate).

I have written this much, but I cant find any websites with relevant information.  How much different is the workings of this from your standard Benedicts which precipitates red copper(I) oxide?

And an equation would also be good.

In this experiment I shall be using Quantitative Benedict’s Solution, which amongst other things contains Potassium Thiocyanate as well as Copper(II) Sulphate.  Thus, this does not precipitate the usual red Copper(I) Oxide, but rather white Copper(I) Thiocyanate.

The change of colour of the end-point, which is what this procedure relies on for its results, is due to the reduction of the Cu(2+) ion to Cu+ ion with a change of oxidation number from 2+ to 1+.  Glucose is reduced during this reaction and as a result is termed a ‘reducing sugar’.

Thanks in advance.

[Yggdrasil]

The change of color of the end-point, which is what this procedure relies on for its results, is due to the reduction of the Cu(2+) ion to Cu+ ion with a change of oxidation number from 2+ to 1+.  Glucose is reduced during this reaction and as a result is termed a ‘reducing sugar’.

In the reaction, glucose is oxidized to gluconic acid.  Basically, electrons are transfered from the aldehyde carbon on glucose (glucose has an aldehyde group in its chain form) to the Cu²⁺.  Remember, when you have a redox reaction, one species is oxidized and one species is reduced.  You can't have two species being reduced and no species being oxidized.

Glucose and other such sugars are reducing sugars because they can act as reducing agents (reducing agents promote the reduction of other compounds, and in the process, become oxidized), not because they become reduced in Benedict's test.

[Hello]

Yeah, sorry I meant oxidising not reducing there.

So can anyone give me some helpful sites for the chemistry behind the workings of Quantitative Benedicts solution or the workings behind the Osazone test or even an equation for either reaction.

Thank you.