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This is a limit test for chlorides.

1.0g of NaOH sample was dissolved in 5ml of deionized water, acidified with 4ml of dilute nitric acid and diluted to 15ml with deionized water. (Test tube A)
10ml of 50ppm Standard NaCl solution was prepared. 5ml of water was added, follwed by 1ml of dilute nitric acid. (Test tube B)
1ml of AgNO3 was added to each of the test tubes and compared for opalescence.

Test tube A (with NaOH sample) gave a dirty brown ppt.
Test tube A (standard NaCl solution) gave a cloudy ppt characteristic of the AgCl formed.

Does this mean my sample contains chlorides?

Thanks.

Posted by candy on Tuesday, September 21, 2010 at 4:56pm.


I obtained a lot of dirty brown ppt the moment the mixture was poured into AgNO3 solution. Does this mean my NaOH sample contains a lot of chlorides? Could other silver halides be present?

However for my control solution of 50ppm sodium chloride solution,a cloudy white ppt was obtained after standing for a while.

Please advise how should I conduct the test?

Posted by candy on Tuesday, September 21, 2010 at 4:05pm.

This is a test for chlorides in NaOH sample.
1.0g of NaOH was dissolved in 5ml of distilled water, acidified with 4ml of dilute nitric acid and diluted to 15ml with distilled water. This mixture was poured into a test tube containing 1ml of AgNO3 solution.
However, I obtained a dirty brown ppt. Do you have any idea what the problem might be?
Thanks.

Chemistry - help - bobpursley, Tuesday, September 21, 2010 at 4:12pm
Yes. You did this in light, the silver chloride is photosenstive, it darkens when exposed to light. Brown is not unusual (dark bluish, with slight yellow).



Chemistry - more advice needed thanks - bobpursley, Tuesday, September 21, 2010 at 5:23pm
I would have to examine your methods, but sulfides (silver sulfides) will be dark.

2 answers

I am wondering if you diluted the NaOH with Nitric acid.

Here is my thinking:
1gNaOH/40=.025moles
Now, if you used .004liters of Nitric acid, what was its concentration?

.004*X=.025
x=6.25M
Ok that is dilute. Normally, one would use a standardized solution of 6M, so it wouldn't quite acidify the 1gram NaOH.

So, if OH ions were left, and you added Silver ions, some AgOH would be formed. AgOH is a thick, dark TAN precipate. Some would call this brown. I don't think you acified the solution: did you check the pH?
Doesn't my 4ml HNO3 acidify the solution?
What should I do then? Add another 1ml of dilute Nitric acid?