Hi,there was an error with the procedure. 1.0g NaOH sample was dissolved in 5ml of water and acidified with 4ml of conc. nitric acid,not dilute nitric acid. This was then made up to 15ml with water.After acidifying with conc. nitric acid instead of dilute nitric acid, no such ppt was observed.

Can you explain why?

Posted by candy on Wednesday, September 22, 2010 at 5:31pm.


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.



Chemistry - pls help - bobpursley, Wednesday, September 22, 2010 at 6:06pm
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?

Chemistry - pls help - candy, Wednesday, September 22, 2010 at 6:21pm
Doesn't my 4ml HNO3 acidify the solution?
What should I do then? Add another 1ml of dilute Nitric acid?

Based on the information provided, it seems that there may have been an error in the procedure for testing for chlorides in the NaOH sample. Here is an explanation of the potential issue and how to address it:

1. In the original procedure, 1.0g of NaOH sample was dissolved in 5ml of water. However, instead of using dilute nitric acid as specified, 4ml of concentrated nitric acid was added to the solution. This may have caused a different reaction or interfered with the formation of the expected precipitate.

2. The solution was then made up to 15ml with water. It is unclear whether this was done after adding the nitric acid or before. If it was done before, it's possible that the concentration of nitric acid in the solution was higher than intended, which could have affected the reaction and observation of the precipitate.

3. When acidifying with conc. nitric acid instead of dilute nitric acid, no precipitate was observed. This could be due to the concentration or other properties of the concentrated nitric acid affecting the reaction, or it could be that the concentration of chlorides in the NaOH sample was below the detectable limit using this method.

To address this issue, consider the following suggestions:

1. Confirm the proper concentration and type of nitric acid to be used in the procedure. Dilute nitric acid is usually recommended for this test, so if concentrated nitric acid was used, it might be necessary to dilute it further or use a different acid.

2. Check the pH of the solution after adding the nitric acid. If the pH is still alkaline, it may suggest that the acidification was not sufficient. In this case, you may need to add more dilute nitric acid to ensure complete acidification.

3. Consider retesting the NaOH sample using the correct procedure with dilute nitric acid and following the specific steps outlined in the original procedure. This will help ensure accurate and reliable results.

Always remember to follow proper safety precautions when handling chemicals and consult with a knowledgeable instructor or expert in the field of chemistry for guidance and clarification on specific experiments and procedures.

In the given procedure, a 1.0g NaOH sample was dissolved in 5ml of water and then acidified with 4ml of concentrated nitric acid. This mixture was then made up to 15ml with water.

The reason for not observing a precipitate after acidifying with concentrated nitric acid instead of dilute nitric acid could be that the concentrated acid inactivated the nitrate ions required for the formation of the silver chloride precipitate. Dilute nitric acid is typically used in this procedure because it allows the reaction to occur without excess acid interfering.

To confirm the presence of chlorides in the sample, a standard solution of 50ppm NaCl was prepared by adding 10ml of the NaCl solution in a test tube, followed by the addition of 5ml of water and 1ml of dilute nitric acid. When 1ml of AgNO3 solution was added to each test tube, Test tube A (with NaOH sample) gave a dirty brown precipitate, while Test tube B (with standard NaCl solution) gave a cloudy precipitate characteristic of silver chloride (AgCl) formation.

Based on these results, it can be concluded that the sample contains chloride ions. Additionally, the presence of other silver halides, such as silver bromide (AgBr) or silver iodide (AgI), can be ruled out since the precipitate obtained in the NaCl solution control was characteristic of silver chloride.

To conduct the test correctly, it is recommended to dissolve the NaOH sample in 5ml of distilled water, acidify with 4ml of dilute nitric acid, and then dilute the mixture to 15ml with distilled water. This diluted mixture should be poured into a test tube containing 1ml of AgNO3 solution.