Hello there,

Why the color of black tea become deeper when adding alkali,and become
lighter when adding acid?

I need detailed chemical reaction mechanism and it would be best if
you can show me some papers about this from learned journals.

Furthermore,would you please supply some papers about the component of
black tea?

Thanks.

Your question has been answered at least two times in the "Last Word"
section of New Scientist magazine (which should be considered a
"learned journal"):

"The simple answer to this question is that adding lemon juice alters
the acidity of the tea and the colour change is an indication of this,
in the same way that litmus paper changes colour. [..]
  
Tea leaves are rich in a group of chemicals known as polyphenols that
amazingly account for almost one-third of the weight of the dried
leaf.  Both the colour of the tea and much of its taste are due to
these compounds.

One group of polyphenols, the thearubigins, are the red-brown pigments
found in black tea and constitute between 7 per cent and 20 per cent
of the weight of dried black tea.

The colour of black tea is also influenced by the concentration of
hydrogen ions in the water. Thearubigins in tea are weakly ionising
acids and the anions (negatively charged ions) they produce are highly
coloured. If the water used to brew tea is alkaline, the colour of the
tea will be deeper due to greater ionisation of the thearubigins.

If lemon juice, which is an acid, is added to the tea, the hydrogen
ions suppress the ionisation of thearubigins, and that makes the tea
lighter.

Interestingly, the theaflavins — the yellow-coloured polyphenols — in
black tea are not involved in the change in colour that is associated
with a change in acidity."
(summarized from http://www.newscientist.com/lastword/article.jsp?id=lw934)

"[T]his effect would be caused by tannins in the tea.  Tannins are
polyphenolic compounds and spend most of their time with the alcohol
group deprotonated. The resulting negative charge is delocalised over
the rest of the molecule and this gives it a characteristic colour
brown in this case.

If a stronger acid is added, say lemon juice (citric acid), the
phenolic groups in the tannins become protonated as they are weaker
acids. This results in some loss of charge delocalisation of the
aromatic phenols, so the energy gap between the orbitals changes. The
colour therefore changes, turning the tea yellow. This effect is known
as a bathochromic shift and is often used by people who deal in
ultraviolet spectrometry to identify phenolic compounds."
(summarized from http://www.newscientist.com/lastword/article.jsp?id=lw57)

Search strategy:
I recalled having read one of these descriptions in the print edition
of New Scientist a few weeks ago.  I then searched for "tea" at
http://www.newscientist.com/lastword/

Let me start by investigating the composure of black tea:
   "Black tea is the most common form of tea worldwide. It is prepared
from green tea leaves that have been allowed to oxidize, or ferment,
to form a reddish brew"
http://www.gnc.com/health_notes/Food_Guide/Tea.htm

The same article points out that black tea contains tannic acid.

Here is a different explanation:
"Tea comes from the plant called Camellia sinensis. Tea leaves are
picked and grouped according to the way they are processed. Black tea
is fermented but green tea is not. While green tea is more popular in
such Asian countries as Japan and China, black tea is more common in
Western countries, such as the popular Lipton teas in the United
States."
http://www.yeelee.com.my/info_06.html

Black tea also seems to have more folic acid than other types of teas:
http://www.serendipitea.com/default.htm



Black tea seems to have a very unusual pH, which is why is has been
thoroughly studied by dentists, nutritionalists, etc.

Here is a study that tests the pH of subjects' teeth, after drinking
black tea:
http://www.nature.com/cgi-taf/DynaPage.taf?file=/bdj/journal/v190/n7/full/4800977a.html

Here's another study that shows that black tea is good for oral
health:
http://abcnews.go.com/sections/living/DailyNews/blacktea_010522.html


Now, onto your chemistry question!

Here's a very general overview article on the chemistry of black tea:
http://www.ibiblio.org/herbmed/eclectic/tea/chapter-vii.html

Here's a kitchen chemistry experiment that suggests using black tea
instead of the more commonly used red cabbage juice:
http://members.ozemail.com.au/~macinnis/scifun/kitchem.htm

Black tea is an indicator, which means that it turns red in the
presence of an acid, and blue in the presence of a base.  :
"Litmus is a common indicator and turns red in the presence of acid
and blue in the presence of base. Universal indicator undergoes a
number of colour changes throughout the pH range.
Indicators can be found in nature, and a number of the coloured
substances
found in plant material show a colour change at different pH values.
Red
Cabbage leaves contain a purple dye which undergoes a number of colour
changes throughout the pH scale. Other materials which contain
indicators are cherries, grapes, BLACK TEA, lichens, beetroot juice
and flower petals."

Here is an indepth article describing what pH, indicators, acids, and
bases are:
http://www.angelfire.com/mo/MaxO/chemistry.html


Hope this was of help to you (please let me know if it was), and if I
can provide you with any more information.

Best of luck to you with your chemistry experiments!

-Rebekah

The colour of tea does not change from red to blue with changes in pH.
 It changes from brown in alkaline solution to yellow-orange as the
solution becomes acidic. This is easily verified by looking at the
colour of lemon tea!  Acid-base indicators are usually weak acids or
bases, and their conjugate base or acid forms have different colors
due to differences in their absorption spectra.

I'm guessing that tannic acid reactions may have a role in this
phenomenon, but have not been able to find any confirmation on the
web.  Otherwise, tea contains caffeine, theophylline and theobromine. 
The polyphenols in tea are derivatives of tannic acid.

Hello there:

Thanks for your help,everyone gives me what I want,and it's
exhilarating that I know how to get more about this by your
answers,thanks again.