I prepared a chromium stock solution with a concentration of 1000 ppm. I used chromium metal powder with 99% purity, taking 1 g of the metal powder and dissolving it in 3 mL of HCl. Later, I added 1000 mL of distilled water to prepare the 1000 ppm solution. However, when I checked the initial concentration of chromium in the stock using AAS, the result was still negative. I encountered a similar issue with zinc stock preparation. I would appreciate your explanation for these problems.
Thank you.
The preparation method is entirely reasonable. However, unless the concentration of the calibration standards is accurate and they are in the same matrix, e,g., acid and its concentration, you are unlikely to match the results.
Hello, if results of measurement is negative value it can cam from blank solution that you use for setting baseline of equipment. Prepare new blank solution and make sure that you do not have detectable amount of analyte inside.
The issue with the preparation of the chromium and zinc stock solutions could be due to incomplete dissolution of the metal powder in HCl or incorrect dilution of the stock solution. One possible method to prepare a 1000 ppm chromium stock solution is to dissolve 2.7476 g of K2Cr2O7 in water and make up the volume to 1 L, which will contain 1 g Cr/L or 1000 mg/L
Dear Koffi:
When you say the result is "negative", do you mean that you are getting a negative number for the "concentration", OR a negative "error" (i.e. lower concentration than the expected 1000, for example, 900 ppm)?
You describe clearly how you prepared the stock solution of 1000 ppm Cr, which seems to be fine as long as you reached complete dissolution. However, your measurement details are not clear. When you do the AA measurement, obviously you will have to dilute the above solution to bring the concentration down to the linear range of the calibration curve of the AA. For example, 100X, 1000X dilution would give you 10, or 1 ppm of Cr in the measured solution. If your AA calibration shows a linear range and your expected concentration in the diluted solution is within that range, you should be able to get accurate numbers.
Best wishes,
Nimal
If you use metal elements to react with acids, you should ensure that they can react completely in order to calculate the solution concentration. If you are unsure, using metal salts for solution preparation would be more accurate.
Nimal De Silva Chen Jingwen Abdelhak Maghchiche Karen A. Darbinyan Thank you all for your responses
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