Ion Chromatography
Making Standards for Cl-, NO3- and SO42-

1.  Note:  Since we acid wash with HCl, and we will be measuring Cl-, it is imperative that all dishware be rinsed well with Milli-Q water.   The biggest problem is usually with pipets, so I would suggest you soak them in a plastic graduated cylinder with DI.   I will have some of these in the lab.  A poor standard curve for Cl- is a sure indicator of poor technique.

 
2. Since NO3- is usually present at much lower concentrations than Cl- or SO42-, we will make up a stock containing less NO3- in order to minimize the number of standards.  Cl- and SO42- are often present at 1-10 ppm in surface waters, whereas NO3- is almost always less than 1 ppm (down to a few ppb).  This is because NO3- is a limiting nutrient in forests and is scavenged by vegetation.  An exception is snow and snowmelt, with the NO3- coming largely from automobile derived pollution (.NO is oxidized to HNO3). Our primary stock will be (1,000/200/1000) ppm of  Cl- /NO3-/SO42- , respectively.

    

3. Dry chemicals of NaCl, NaNO3 and Na2SO4 are kept in the oven (at 105oC) or at room temperature in the dessicator next to it in room 014.  Take these down to the LCRI prep room to weigh out your chemicals for the single stock solution in a single 1L volumetric flask.  Weights are as follows:  

Mixed into a single 1L volumetric, these will make a (1,000/200/1000) ppm primary stock of  Cl- /NO3-/SO42

 
4. Acid wash and rinse Class A glassware including 500 ml and 100 ml volumetric flasks, as well as a range of appropriate volumetric pipets.  All glassware should be rinsed at least 5 times in small amounts of deionized (milli-q) water.  Be particularly careful of residual acid-wash contamination.

 
5. Dilutions for Standards are calculated using the formula C1.V1 = C2.V2, where C1 is the volume of a stock solution, V1 is the volume of stock transferred with a volumetric pipet to a volumetric flask of size V2, and C2 is the final concentration of the standard.   Make sure that the units of C1 and C2, and those of V1 and V2 correspond, respectively.  

 

6.  C1  
  (Cl- /NO3-/SO42)
(mg/L)		 V1
pipet (ml)

C2  
(Cl- /NO3-/SO42)
final concentration (mg/L)

 V2
vol. flask
1,000/200/1000
(primary stock)		 10 20/4/20
  (secondary stock)		 500
1,000/200/1000 5 10/2/10 500
20/4/20 25 5/1/5 100
20/4/20 10 2/0.4/2 100
20/4/20 5 1/0.2/1 100
20/4/20 2 0.4/0.08/0.4 100
20/4/20 1 0.2/0.04/0.2 100



IC Gradient  (Solution A  - Water: Solution B -   40 mM NaOH)

 

 

 

 

 

 

 

 

 

 

 

 

 

The gradient starts off with pure water until the sample is loaded onto the front of the column.  It then slowly starts mixing in 40 mM NaOH, which elutes the monovalents (F-,Cl- and NO3-) with maximum spread, but does not elute CO32-.  When NO3- is out, we rapidly increase the NaOH up to 80%, to elute the CO32-, but to retain the SO42-.  This runs until the end when we go back to pure water to get ready for the next run.  You can see the baseline rise and then fall from roughly 0.5 to 2.5 mS as we go through this gradient.  The ultimate purpose of a gradient is to spread out the analytes which are weakly retained and tend to bunch together, and to compress or more rapidly elute those which are strongly retained by bringing in the strong eluent.  Gradients always go from a weak eluent to a strong eluent.

 

 

 

Link to Powerpoint files