Chemical Forums

Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: susanna on June 11, 2015, 08:12:34 AM

Title: Spectrophotometer uv/vis Problem
Post by: susanna on June 11, 2015, 08:12:34 AM

hey People, I'm trying to determine Tungsten in uv/vis but I'm having Problem by the calibration curve! I have 3 Standards ( 100µG 500µG 1000µG ) and after reseting the absorbance with destilated water it shows me that the absorbance from Standard 2 (500µG) is bigger than Standard 3 (1000µG). Anyone has some idea of what am I possible doing wrong?

Thanks

Title: Re: Spectrophotometer uv/vis Problem
Post by: Hunter2 on June 11, 2015, 08:42:05 AM

Probably something wrong with the preparation of the standard. Pipette error.

Title: Re: Spectrophotometer uv/vis Problem
Post by: marquis on June 11, 2015, 11:11:54 AM

Can you give a few more details?

Usually, for something like tungsten, you need some chelating agents. 
Other wise, there is little absorption in the uv/vis.

 Unless you use something like atomic absorption (AA).  Then you get
plenty of absorption in the uv/vis.

Good luck.

Title: Re: Spectrophotometer uv/vis Problem
Post by: Arkcon on June 11, 2015, 12:21:09 PM

susanna:, you actual numbers, might help us pin down your problem.  But like Hunter2: said, this is most likely a sample prep error.

Title: Re: Spectrophotometer uv/vis Problem
Post by: susanna on June 12, 2015, 05:19:02 AM

The chelating Agent that I'm using is thioglycolic acid.The absorbance given was:
100µG: 0,002
500µG: 0,015
1000µG: 0,008
so I've thought that maybe during the preparation the Person wrote "500µG" in the 1000µG Flask, so for the calibration curve I've considered that:
destilated H2O: 0,000
100µG: 0,002
500µG: 0,008
1000µG: 0,015

But now if I measure the standard it Shows me that the 100µG standard has 277,01µG Tungsten
Could be also a Problem from the Spectrofotometer?

Title: Re: Spectrophotometer uv/vis Problem
Post by: Arkcon on June 12, 2015, 08:43:12 AM

Making the substitution you suggest in the second case, you get a fairly decent line.  What result do you get for your unknown, and how are you computing the concentration.  We can't exclude the possibility that you're outside of the proper range for your instrument, so that is something to consider if you suspect your instrument.

Title: Re: Spectrophotometer uv/vis Problem
Post by: FireflySci on July 21, 2015, 12:49:56 PM

When was the last time your spectrophotometer was calibrated?  How many hours are on the lamps?  I guess a good question to determine the specs for your machine would be to ask what the make and model is so we can have a better idea of the absorbance range of your spectrophotometer.