Practical investigations

Read our general notes on Risk Assessment

Investigating wines (revised)

This tutorial does not provide you with full practical details. After all, this is your investigation, and you must plan the experiments for yourselves, almost certainly carrying out appropriate trials beforehand.
 
Always carry out a risk assessment and check with your teacher before starting any practical work.

The advice given in this tutorial is a response to teachers and students who have frequently asked if there are other investigations which can be carried out instead of the popular set of kinetics experiments linked with the reaction between magnesium and various acids.
 
We have revised and expanded this tutorial in the light of further questions (3 March 06).
 
See also our sorted FAQs about wine investigations
 
You can download a wine investigation information sheet with background information.
 
There is also plenty of resource material available on the internet - particularly with regard to the process by which wine is made, and to the various techniques mentioned below. Try these search terms in Google: chemistry of wine making, determination (or estimation) of alcohol in wine, determination (or estimation) of sugar in wine.
 
You can also contact one of the supermarkets (preferably the one from which the wine was bought) to obtain 'official' results of the analyses which they carry out from time to time on their range of wines. This would be a useful means of comparing your results with theirs, and of attempting to account for any differences you find.
 
What is wine?
Wine is derived from the fermentation of grape juice. Modern methods use special cultures of yeasts, designed to prevent other side reactions leading to undesirable flavours developing.
 
The sourness of unripe grapes is due to the high concentration of tartaric acid. During the ripening process, much of this acid is converted to various sugars. Then during the fermentation process the yeasts convert these sugars into ethanol and carbon dioxide. If fermentation is stopped before the process is complete, then a sweeter wine is obtained, e.g. a Hock or a Riesling. If, however, fermentation is allowed to continue to completion, then the resulting wines are “drier”, e.g. a French Muscadet.
 
See
I am planning to do my investigation about the red wine with different alcohol content? Is there any relationship between the alcohol content and the acidity of the red wine? Does the red wine with higher alcohol content have a higher acidity?
 
Table wines can be red or white. The longer the grape juice is in contact with the grape skins during the pressing process, the redder will be the colour of the wine, and the dark colour of some wines derives from phenolic compounds of the tannins in the grape skin.
 
See
I am doing an investigation on analysis of acidity, alcohol content, sugar content and sulphur dioxide content in both red and white wine, but I need to know what chemicals make the red colour in red wine and by using which process I could identify it.
 
and
I’m investigating the chemical composition of wines for my project and I have discovered a problem. I have come across one website which briefly mentions that the tannin content of wines can be calculated titrating the wine with potassium permanganate. There was no further explanation and I am curious about this topic. Could you explain in slightly more detail please? Thanks.
 
The “bouquet” of wines derives from a complex combination of the volatile components in the wine, notably the esters.
 
There are many complex chemical changes occurring in the wine as it matures.
 
See
• I am doing an investigation on wines, and have found that when I exposed wines to air, their pH levels increased i.e. they became less acidic. Through research on the internet I have found that this is due to oxidation, which results in flat (less acidic) wines. However I could not find out how this oxidation occurs. Could you explain and give the reaction?
 
• I am investigating the acidity of wine and I found that over a period of one week it has increased. The only reason seems to be the oxidation of ethanol to ethanoic acid, but I don’t understand what the oxidising agent is because the text book says a strong oxidising agent is needed, and I can’t find one that would be in wine. Any ideas?
 
010208
 
• What is the connection between wine and vinegar?
 

Suitable wines
Boxes containing cheap wines are available from supermarkets. A cheap Liebfraumilch (a sweet white wine) will give useful readings for sugar content, whilst there are also several relatively cheap dry red table wines available for comparison.
 
Note that analytical results for wines are normally expressed in g or mg per 100 cm³.
 
Decolourisation of red wines
If the analytical process you choose involves a visual technique, such as a titration, then there is a need to decolourise the red wines first. You will need to research the methods of doing this for yourself.
 
See
• I am carrying out a wine investigation and I am determining the concentration of ester in wine after determining the concentration of acid. I understand that you have to use activated charcoal to decolourise the wine but will this affect my results?
 
• I have been following the wine analysis tutorial on your website. However when I tried to decolourize the red wine with activated charcoal it did not work. I put some charcoal in a filter funnel (using a suction pump), but the colour remained. I also tried adding a sample of charcoal and the wine in a beaker and stirring it, however this did not work either. I have now tried using a large filter funnel with cotton wool, plugging the funnel and with lots and lots of charcoal on top of that. Hopefully this will work but I thought I should email you just incase it doesn't - as I then will have no idea how to use the charcoal to decolourize the wine.
 
• I have been reading the Investigating Wines tutorial. I have managed to follow and understand how to do every experiment except for the “Identifying Acids” experiment. I have came to the conclusion that I will put a sample of tartaric, citric, succinic and ethanoic acids on to the chromatography paper, as well as a sample of wine. Does the sample of wine have to be put through a filter of activated charcoal? What solvent should I use in the chromatography tank? I also came up with the idea of once the experiment has taken place, and the paper has dried, to apply an indicator to the paper through a spray. Would this work and would I use bromophenol blue?
 
• I'm investigating wine and having real trouble discolouring red wine with activated charcoal. I know it’s supposed to work and the charcoal does need to be activated. But can the process be speeded up by: heating wine 35°C, adding acid, and powdering the activate charcoal, and then stirring for five minutes? Do you also know if adding PVPP (polyvinylpolypyrrolidone / polyclar) works? Many times thank you!
 
Analysis of wines
There are several components in the wine which can be identified qualitatively and quantitatively. Here are some ideas. This is not an exhaustive list:
 
[1] Alcohol (ethanol) content
There are several ways of doing this chemically, but the easiest way is to carry out a physical determination of the density of the distilled wine. Incidentally, this is how the alcohol content of wines is normally carried out officially by the supplier, before the relevant tax is applied by customs and excise. You will need to distil the wine first, and to ensure that all the ethanol has been driven off. Then you will have to design an accurate means of determining the density of the distillate.
 
At 20°C, pure water and pure ethanol have densities of 0.9982 g cm^-3 and 0.7893 g cm^-3 respectively. The densities of mixtures of these two liquids vary proportionally between these two limits with the changing composition of the mixture. So a simple spreadsheet can be set up to convert densities into percentage compositions. Alternatively, full tables are given in appropriate data books, e.g. Kaye and Laby’s Tables of physical and chemical constants.
 
(HINT. Using a ‘density bottle’ borrowed from your physics department is a good method. Density bottles can also be ordered from science catalogues and do not cost much.)
 
See
In the experiment to determine ethanol content of wine by the density method (ref. “What is in wine?” – D. Denby, Chem Review Jan 98) it states that the wine has to be made “slightly alkaline” before distillation. Why is this?
 
The analysis of the ethanol can also be carried out using a back titration with an acidified standard solution of potassium dichromate.
 
See
• I am doing an investigation on wine where I plan to determine the ethanol content. I know I need to use back-titration with the excess potassium dichromate. What I do not understand is the theory of the back titration. What exactly happens in the reaction to cause the colour change at the end point? Would you be able to help me please?
 
• I am determining the alcohol content of wine. I have completed the distillation procedure and am now completing a titration in which you oxidise the wine sample with an excess of potassium dichromate(VI) followed by a back titration with iron(II) irons using ammonium iron(II) sulphate. An article by Derek Denby says you need to standardise the ammonium iron(II) sulphate by titrating it directly with the potassium dichromate(VI) - what does this mean and how to you determine an end point for this titration?
 
• I am doing an investigation on the analysis of wine. I have looked at an article from Chemistry Review (Jan ’98) by Derek Denby and in it he talks about how to find out what the alcohol content of wine is. One method is to oxidise ethanol using an excess of potassium dichromate and then doing a back titration using ammonium iron(II) sulphate. I really need an indicator for this experiment or to know when the end point is for the indicator sodium diphenylamine sulphonate. I haven't had much luck with the internet so I thought you could help!
 
• I am doing the wine investigation. I am trying to find out how much alcohol in the wine sample. In the case of titration, how much phosphoric acid do I need to add to the solution? The school has provided me with sodium diphenylamine-4-sulfonate solid. Do I need to dissolve it in the water first and how concentrated will it be? I have tried to add sodium diphenylamine-4-sulfonate solid into the solution; it became a darker colour but not violet. Did I make any mistakes?
I am also using a water bath with the potassium dichromate(VI) for more than 30 mins. Is 30 mins enough for the reaction? I can see a change of colour after 30 mins. Do I actually need to add sulphuric acid into potassium dichromate (VI) to keep the reaction forward?

 
• I'm doing the investigation on wines and for the indicator on the back titration method I've been given N-phenylanthranilic acid to use instead of the sodium diphenylamine sulphonate. However, I'm struggling to find any useful information about how it works: I've found that I should expect it to change from purple-red to colourless, but I have no information on the reaction. I'd imagine that it would work in a fairly similar way to the sodium diphenylamine sulphonate but I'd like some clarification please.
 
• I am investigating wines for my individual investigation, but am a bit unsure of the theory behind part of it. When analysing the ethanol content of the wine using the reaction of ethanol with dichromate ions, and then a back titration with iron(II) ions, I was wondering what the complex ion formed is. The indicator I will use is sodium diphenylamine sulphonate and phosphoric acid is used to provide hydrogen ions. The indicator is needed because the chromium ions reduced form a complex with the water I presume and the phosphate ions from the acid in a ligand exchange reaction. I was wondering if you knew precisely what this ion is likely to be, or whether it actually matters.
 
• To find the concentration of ethanol, using the back titration method, why does the wine need to be distilled?
 
A colorimetric method based on the reaction between ethanol and dichromate ions also works well.
 
See
• Hi, I'm doing the wine investigation. Is it neccessary for the colour of the ethanol and potassium dichromate solution to change from orange to green before I can measure the absorbance in the colorimeter, can't I just take the measurement while it's still orange? It takes ages to change colour.
 
• I am doing the wine investigatiom and I don't really understand how I'll use the colorimeter on ethanol solutions with silver nitrate, potassium dichromate and sulphuric acid
 
• I am looking at determining the alcohol content of wine by using a colorimeter on known ethanol solutions with potassium dichromate(VI), silver nitrate and sulphuric acid to produce a calibration curve followed by the same procedure with your unknown alcohol content, however potassium dichromate(VI) is a possible carcinogen and I was wondering if this was necessary in the solution and if so if there is something that is less expensive to dispose of that could be used instead?
 
[2] Sugar content
Experiment 2.7a fromthe Nuffield Chemistry Special Study Food science gives details of how the glucose content of drinks can be determined. The method is not specific to glucose, and will respond to all reducing sugars present.
 
(QUESTION: Could the procedure be adapted to estimate the quantities of non-reducing sugars such as sucrose?)
 
[3] Identifying the acids
As mentioned above, tartaric acid is one of the acids present in wines. There will also be other non-volatile acids present, such as citric acid, succinic acid and several others (see the downloadable information sheet) [LINK]. The main volatile acid present is likely to be ethanoic acid, produced by the bacterial oxidation of ethanol. There may well be others.
 
You can use chromatography to identify the acids in wines. You could compare the acids in red and white wines.
 
There are details of a suitable practical set-up in Topic 18 of the Nuffield Chemistry Students’ book.
 
(HINTS: What will be a suitable solvent for elution? How will you choose a suitable indicator to show up colourless acids? Explore the possibility of mixing an indicator such as bromophenol blue with the solvent. This will allow the acids to appear as yellow spots as the chromatogram dries.)
 
See
• I am carrying out an investigation on wine analysis and performing TLC. The problem is that the school may not have silica plates on which to carry out the procedure. I was wondering whether it would be alright to do the chromatography on chromatography paper instead. Thanks.
 
• I am doing my advanced higher chemistry investigation on "Wine analysis" I am doing thin layer chromatography to identify if succinic, tartaric, malic and lactic acids are in red and white wine. I have been advised to use a solvent of n-butanol:formic acid:water in the ratio 8:1:2.5 and use bromocresol green as an indicator. Why? What is the chemistry behind this. I have been unable to find out why.
 
• I am doing my practical investigation on wine analysis, including the acidity in wine. I was wondering if you could tell me what effect the acidity has on the taste of red wine, and is this just the volatile acids or all of them? Also, in finding out the total acidity, will I have to find out the amounts of volatile acids as well as the non-volatile ones? Finally, is it possible to find out the amounts of volatile acids using thin layer chromatography, and if not, how could I do this?
 
• I am doing the chemistry investigation on wine analysis. For the paper chromatography we do not have the available chemicals to develop the spots on the paper by spray. I am using ethanol as the solvent. If, after running and drying, I expose the paper to UV light will the deposited acids show a shadow (dark patch) on the paper? If not, is there any other way of seeing the spots?
 
• I have been reading the Investigating Wines tutorial. I have managed to follow and understand how to do every experiment except for the “Identifying Acids” experiment. I have came to the conclusion that I will put a sample of tartaric, citric, succinic and ethanoic acids on to the chromatography paper, as well as a sample of wine. Does the sample of wine have to be put through a filter of activated charcoal? What solvent should I use in the chromatography tank? I also came up with the idea of once the experiment has taken place, and the paper has dried, to apply an indicator to the paper through a spray. Would this work and would I use bromophenol blue?
 
[4] Total acid content
The obvious technique to use here will be an acid-alkali titration.
 
HINTS: How are you going to choose a suitable concentration for the alkali? Does it need to be standardized first? What indicator will you use?)
 
When you come to analyse your results, you have the problem that there are several acids in your wine, rather than just one. You will need to decide on a representative acid, e.g. tartaric acid, and calculate the concentration of acids in terms of this particular one. That is, you assume that this is the only acid present.
 
See
• For my practical investigation, I am trying to determine the acid content of wines but have had difficulty finding the end point of titrations with NaOH. When using phenolphthalein indicator the colour change was very gradual, so I have used a pH meter to plot curves of pH/ volume of NaOH added, but how do I use these to find the end point?
 
[5] No-volatile acid content
 
When the wine is distilled, only the volatile acids will distil over. After an appropriate analysis of the distillate, the concentration of volatile acids can be estimated.
 
Alternatively, take a measured sample of wine and evaporate the volatile acids on a hotplate, taking care to avoid any losses by splashing. Then carry out a titration to estimate the non-volatile acids left in the flask.
 
As in part [4], will need to decide on a representative volatile acid, maybe ethanoic acid, and carry out your calculations accordingly.
 
[6] Sulphur dioxide content
Sulphur dioxide is usually present in wines as a bactericide.
 
Some of this gas is relatively ‘free’ within the wine, whereas most is tied up in ions such as HSO₃^- and SO₃^2- and in other ways too.
 
See
• I'm investigating the sulphur content in wine and read [LINK: 201/4783|your tutorial on this topic]. I was wondering what the addition of sulphuric acid at the start was for, as wine tends to be acidic anyway?
 
A sample of wine can be titrated with iodine and hence the sulphur dioxide concentration can be estimated. The concentration of the iodine solution needs to be low, since the concentration of sulphur dioxide is very small, so you will need to carry out trial experiments to determine a suitable iodine concentration.
 
See
• For my investigation, I have decided to analyse the free SO₂ content in wines and have decided to do this by acidifying the wine with H₂SO₄ and then carrying out a titration with iodine. However, I am stuck as on how to work out the volume of wine that I initially need to start with when reacting it with H₂SO₄ and was wondering if you could help me? Any other hints and tips would be greatly appreciated. Thank you sooo much, hoping for a quick reply.
 
• I am about to analyse wine and investigate sulphur dioxide content. I have done some research and come across the “ripper” method which analyses for free sulphur dioxide. This is essentially the method that involves a titration with an iodine standard. I have read in numerous places that this method is inaccurate, yet commonly used. Please could you outline the procedure for the analysis of free or total sulphur dioxide content in wines.
When analysing red wines with wet analysis techniques such as titrations, there are apparently problems with recognising the end point. I have read a little about activated charcoal, and how it can be used to decolourise wines. Will the use of activated charcoal inhibit my ability to analyse for SO₂ using the ripper method ?
 
• I am doing my A-level chemistry project on the chemistry of wine. I understand the majority of the theory, but I am however confused on some aspects.... whilst I was researching for the theory behind how to determine the sulphur dioxide content of wine (by adding iodine, and starch indicator etc) I was confused to find that you must first standardise the iodine solution. What is this and how do you do it? Thanks.
 
• I was reading your help sheet online about the wine analysis investigation, and saw that you mentioned using an iodine titration to work out the concentration of sulphur dioxide in the wines. I understand the technique involved and the necessary calculations that I have to perform, but despite all of my searching in books and on the internet, I have been unable to find the reaction equation between sulphur dioxide and iodine. I have guessed that the products would be pure sulphur and iodate ions. Is this correct and if not, what is the balanced equation for the reaction? Thanks.
 
[7] Esters content
 
Thse are the most difficult components to analyse.
 
Esters are completely hydrolysed by aqueous alkali if they are refluxed with a known volume (representing an excess) of this alkali of known concentration. A back titration can then be used to find out what alkali remains, hence what was used to hydrolyse the ester, and finally the quantity of esters present can be deduced.
 
(HINT: In your calculation, do not forget to take into account the acids present as well as measured in 4 above).
 
As before you will need to choose a representative ester and carry out your calculation in terms of this.
 
See
• I am doing an investigation involving wines and want to do a back titration to determine which esters are present in wine. However, the idea of doing a back titration is confusing could you please explain some of the theory behind it and why a back titration is needed.
 
Author of the information sheet: Janet Taylor
 

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updated: 12 March 2008