Why when testing a sample for chlorine residual do I get the same results for 5 ml sample of water as I do with a 10 ml sample, and a 15 ml sample of water using a reagent pack that asks for a 10 ml. The reagent container does not give the weight of the reagent in the packet (it is powder), but I think it might weigh 0.1 grams. I am just using a Hach DR 900 to test the free chlorine residual.

My thinking is if the reagent packet is asking for 10 ml sample of water shouldn't my results differ when using different sample sizes. For the three different volumes of samples, I got a free chlorine residual of 1.00 mg/L for all three sample sizes.

Sorry, the packet is a DPD free chlorine reagent for 10 ml sample. The packet contains Sodium Phosphate, Dibasic, EDTA Disodium Salt, DPD Salt, and Carboxylate Salt.

  • $\begingroup$ First of all welcome! Please take note of the changes made to the formatting of your original question. Also, before this question can reasonably be answered, you will need to provide units (grams, gallons, pounds, etc.) for every number given in your question, and a description of how much of what is in your reagent pack. Also, where you say you get "the same results" you should say what the results are. I think that once your question is thoroughly and properly expressed, there is a good chance that someone will be able to help you find a good explanation for your problem. Good luck! $\endgroup$
    – airhuff
    Commented Jan 21, 2017 at 21:20

1 Answer 1


This is sort of an extended comment, not necessarily an answer.

Overall, yes I'd expect different numbers at least. How they would vary is a matter of some conjecture.

(1) The Hach DR 900 is a colorimeter. I'm not sure how large of a sample you need to fill active volume of the colorimeter cell.

(2) The ratio of water volume relative to the amount of solid reagent is changing. So the chemical equilibrium on which the calibration curve depends might shift. If you dilute 5 and 10 ml samples to 15 ml with "pure" water and take 10 ml of each of the three samples (2 dilutions plus "full strength"), then you be comparing better.

(3) Even given (2), the results seems odd. Normal drinking water should have between 0.2 and 2.0 mg/L.

  • Is pH of the water ok?
  • Does water have some unusual buffer capacity?
  • Reagent pack old?
  • Do you have some standards which you can test?

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