reason for differences between pH meter & indicators (2 Viewers)

[stargaze]

Anyone have an scientific (exam-worthy) explanation of why there can often be such a big difference in the pH reading (or range) you get from using indicators compared to using a pH meter?

Obviously a pH meter is more accurate, and indicators show just colours which ppl can not see correctly etc, but is there an actual explanation?

thanks~

 

[Jumbo Cactuar]

pH meters => improper calibration, instrument drift, lack of temperature compensation, improper sampling, or a dirty probe.

Some indicators have thermochromic properties, though not terribly significant errors here. Remember that most indicators are weak buffers under equilibrium between protonated and ionic forms (hence the colour change).

How about the difference between end point and equivelence point.

pH meters measure H+ activity not concentration. For low concentrations the difference isn't huge. I will leave this to you to investigate because I know I'll get something wrong. Involves Nernst eqaution. I don't think this is within the scope of HSC though .

 

[Jumbo Cactuar]

Following on from my last post..

pH probe errors:

agitation
temperature
matrix effects
ionic strength
response time
conditioning
rinsing

read for further explaination

Also I read that a recent discovery traced that pH probes measure Na+ transport rather than H+. I don't know if I believe that, but if anyone knows any more I'd be interested!

 

[stargaze]

thanks Jumbo_Cactuar

I'm not even sure if this type of thing is in the syllabus but I have a prac test which has a part of 'measuring the pH of an acid using a pH meter and indicator/s"

so i'm guessing in the conclusion i'll need something on why there was a difference in the readings

thanks again~

 

[Jumbo Cactuar]

NO WORRIES

 

[Jumbo Cactuar]

I encourage people to go off syllabus from time to time because not only do you get a deeper understanding of the topic, but it makes other elements come more intuitively.

 

[xiao1985]

I encourage people to go off syllabus from time to time because not only do you get a deeper understanding of the topic, but it makes other elements come more intuitively.

i heartly endorse this comment ... =)