How does starch act as an indictor?

Date: 31.10.2007

All Chemistry lovers would know that starch (a bio-polymer) serves as an indicator in iodometric and iodimetric titrations, which are epitomes of redox titrations.

As a student of Chemistry, I wish to explain the intricate mechanism by which the bio-polymer serves as an indicator in the aforesaid redox titrations.

The bio-polymer starch comprises of 20% water soluble amylose and 80% water insoluble amylopectin. The amylose component of starch possesses helical structure by virtue of which it serves as an indicator in iodometric and iodimetric titrations.

In both the titrations, the system will possess a mixture of molecular iodine(I2) and iodide ions(I-). Molecular iodine combines with iodide ions in a reversible manner, thereby giving rise to a complex called tri-iodide ions(I3-). The equilibrium of the reaction of formation of tri-iodide(I3-) ions is always backwards. So, I3- ions are actually considered as molecular iodine(I2) in a solution of iodide ions(I-).

It is hypothesised that when an aqueous solution of starch is added into a system comprising a mixture of I2 and I- ions, the I3- ions(that will be present all the time) and I2 will enter into the cage formed by the helical structure of starch, where they combine together to form I5- ions. The I5- ions complex with starch through weak Van-der-Waals force. This complex of I5- ions and starch is blue in colour. Thus as soon as starch is added into the system, violet colour is observed. With subsequent addition of the titrant, usually sodium thiosulphate(Na2S2O3), the violet colour gradually disappers and the end-point is complete disappearnce of the violet colour.

A necessary precaution that has to be taken while making use of starch as an indicator is that, it should be added only after the colour of the solution in the conical flask turns straw yellow in colour. If starch is added in prior, then excess of iodine would get into the helical structure of starch and get coagulated. Thus, one would end up getting non-reproducible end-points.

Common Errors in English - 7

Date: 17.10.2007

Today let us see some aspects on correct usage of verbs.

* Error No.: 1

The conjunctions "Either....or" and "Neither....nor" take up only singular verbs

The following are some examples on the correct usage:

* Either Mala or Manju takes care of the creche (Right)

* Either Mala or Manju take care of the creche (Wrong)

* Neither Ramesh nor Raju speaks French (Right)

* Neither Ramesh nor Raju speak French (Wrong)

* Error No.: 2

The phrase "One of..." should be followed by plural noun/plural pronoun and singular verb. The following are some of the examples on correct usage:

* One of my sisters is an Engineer (Right)

* One of my sister is an Enginner (Wrong)

* One of my cousins lives in Singapore
(Right)

* One of my cousin lives in Singapore (Wrong)

* One of my cousins live in Singapore (Wrong)

* Error No.: 3

Always remember the structure in English that "Does + Plural present tense verb = Singular present tense verb". Therefore,

* She speaks Tamil (Correct)

* She does speak Tamil (Correct)

* She does speaks Tamil (Wrong)

* Does she go to college today? (Correct)

* Does she goes to school? (Wrong)

* Error No.: 4

One more important structure in English language is as under:

"Did + Plural present tense verb = Past tense"

Therefore, the correct usage is,

* Did you watch cricket the last day? (Correct)

* Did you watched cricket the last day? (Wrong)

* I did not meet her today in office (Correct)

* I did not met her today in office (Wrong)

Anticipating your valuable comments. Bye for now. Happy learning...........

Common Errors in English - 6 (Of Redundants)

Date: 17.10.2007

Today let us see some common redundants in English.

* Redundant No.: 1

Never use "Suppose" and "If" together, since they both mean the same. Let us see some examples:

* Suppose she comes tomorrow, I 'll take her to beach (Right)

* If she comes tomorrow, I 'll take her to beach (Right)

* Suppose if she comes tomorrow, I 'll take her to beach (Wrong)

* Redundant No.: 2

Never use "Otherwise" and "Or" together, since they both convey the same meaning. Let us see some examples:

* If you want to go for shopping tomorrow, then you call me over phone otherwise no need (Right)

* If you want to go for shopping tomorrow, then you call me over phone or no need (Right)

* If you want to go for shopping tomorrow, then you call me over phone or otherwise no need (Wrong)

* Redundant No.: 3

Never use "Return" and "Back" together, because the very word "Return" means "Give Back". Let us see some examples:

* Please return the book to me (Right)

* Please give back the book to me (Right)

* Please return back the book to me (Wrong)

* Redundant No.: 4

Never use "Revert" and "Back" together because the very word "Revert" means "Get back". Let us see some examples:

* Please revert to me at the earliest (Right)

* Please get back to me at the earliest (Right)

* Please revert back to me at the earliest (Wrong)

* Redundant No.: 5

Never use "AM" and morning together and "PM" and evening together. It is again a redundant.

* Today I got up at 8 AM (Right)

* Today morning I got up at 8 O' clock (Right)

* Today morning I got up at 8 AM (Wrong)

The same thing holds good for evening/night and PM

* Redundant No.: 6

Never use "Repeat" and "Again" together, because the very word "Repeat" means "Do again". Let us see some examples:

* Please repeat the sentence (Right)

* Please tell the sentence again (Right)

* Please repeat the sentence again (Wrong)

* Redundant No.: 7

Never use "Descend" and "Down" together and never use "Ascend" and "Up" together. Let us see some examples:

* She was descending down the stairs (Right)

* She was going down the stairs (Right)

* She was descending down the stairs (Wrong)

* She was ascending the stairs (Right)

* She was ascending up the stairs (Wrong)

* Redundant No.: 8

The words "Each" and "Every" are synonymous and therefore they shouldn't be used together. Let us see some examples:

* The Tamil literary work "Thirukkural" discusses each and every facet of human life (Wrong)

* The Tamil literary work "Thirukkural" discusses every facet of human life (Right)

* The Tamil literary work "Thirukkural" discusses each facet of human life (Right)