# Phosphate Determination

1. Nov 11, 2007

### srose9625

I am doing a project in the lab to determine the phosphate concentration of a water sample. There are so many steps involved. Can someone point me in the right direction. I know that I will be making a reagent using 2.5g ammoniun molybdate in 30ml distilled water.Then mixing 0.125g ammonium metavandate in 30ml water, heating it, cooling it, then adding 33ml HCL. Then I will mix them and dilute to 100ml with water. What is this reagent for, when will I be needing it?
Then I have to prepare my standard phosphate sample using Anhydrous KH2PO4. How do I do this? And what is standard being used for?
Then I have to prepare my samples. How is this done? why am I having to prepare my samples? They are on liquid form already, hence, water is my sample. Then the procedure talks about sample digestion or acidifying the sample. Why is this being done, do I have to do digestion for a tap water sample?

I AM COMPLETELY LOST ON THIS WHOLE PROJECT. I HAVE SPENT THE LAST WEEKS READING EVERY WEBSITE ON THIS LAB. thEY ARE ALL CONFUSING SINCE THEY ARE ALL DOING DIFFERENT THINGS AND USING DIFFERENT TECHNIQUES. I NEED SOME HELP, PLEASE!

Here is the link to what I have to do:
http://quiz2.chem.arizona.edu/genchemlabs/104bsyllabus/ResearchProject/Phosphate/phosphate.pdf

Last edited: Nov 12, 2007
2. Nov 12, 2007

### chemisttree

This will produce a yellow-colored phosphovanadomolybdate complex when exposed to orthophosphate in your analyte and is your phosphate indicator.

Dry it at 110C overnight or for several days in an oven.

Your test (and your grade). DO IT! You have to know how intense the color will be as a function of phosphate concentration, don't you?

This test is for orthophosphate. There may be other types of phosphate present (like colloidal calcium phosphate or polyphosphate, etc...). Are you following a method?

The linked procedure is very clear. You should understand it before you go off looking at other procedures on the Web.

Last edited: Nov 12, 2007
3. Nov 12, 2007

### srose9625

4. Nov 12, 2007

### srose9625

I do not think it is very clear. It jumps around from one topic to another and then repeats some steps over again.
Thanks anyway.

5. Nov 13, 2007

### chemisttree

It's OK that you are a bit intimidated by the method, it's a fairly long one. Try to understand the method in general terms and ignore all the sample prep details at first. A detailed understanding will come later.

Basically, you have an unknown sample that contains some phosphate in one or more forms. Orthophosphate ($$PO_4^{-3}$$) is the species of phosphate you will determine in this test (the analyte). You are asked to determine total phosphate which includes orthophosphate and other types as well. Those other types may be iron phosphate, calcium phosphate, other metallic phosphates, polyphosphates and organic phosphates. These are all water-soluble or colloidal enough to be suspended in water and resist filtration. You will perform a step to convert ALL forms of phosphate into orthophosphate. That step is very important and is called "sample digestion" and, yes, you need to perform it on your water samples as well as other types of samples (like soil, food, detergent, biological, etc...). After you have digested the sample, you will add an indicator which forms a colored complex with orthophosphate (kind of like testing pool water for pH and alkalinity). The intensity of the colored complex corresponds to the amount of orthophosphate in your sample. This means that you will have to both measure your sample and a series of standard solutions for you to tell how the intensity of color corresponds to the concentration.

That's it... the rest is all details.

By the way, this type of analysis is common throughout analytical chemistry. Usually a sample must be digested and have any interferences removed or lowered before analysis. The analysis is often spectroscopic, as in this test, and a series of standards is required to correlate a spectroscopic reading with a concentration of an analyte.