Sodium thiosulfate solutions standardization

N sodium thiosulfate solution, standardized (see Support Protocol)... 

The hberated iodine is measured spectrometricaHy or titrated with Standard sodium thiosulfate solution (I2 +28203 — 2 1 VS Og following acidification with sulfuric acid buffers are sometimes employed. The method requires measurement of the total gas volume used in the procedure. The presence of other oxidants, such as H2O2 and NO, can interfere with the analysis. The analysis is also technique-sensitive, since it can be affected by a number of variables, including temperature, time, pH, iodide concentration, sampling techniques, etc (140). A detailed procedure is given in Reference 141. 

The hberated iodine is titrated with standard sodium thiosulfate solution. In the thiosulfate method, selenous acid is treated with an excess of standard sodium thiosulfate solution ... 

Absorb in an impinger containing a standardized solution of iodine and potassium iodide titrate with standard sodium thiosulfate solution. Iodometric titration No data NR EPA 1978... 

Soliman and Belal investigated argentimetric (67,68) and mercurimetric (69) methods. Hydralazine precipitates silver from ammoniacal silver nitrate solution. The silver is dissolved with hot nitric acid and titrated with ammonium thiocyanate solution. Alternatively, mercury is precipitated from alkaline potassium mercuric iodide solution. The precipitated mercury is dissolved by adding excess standard iodine solution. The excess iodine is back-titrated with sodium thiosulfate solution after acidifying with acetic acid. 

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu2+(aq), adjusting the pH to a weakly acidic level, allowing the Cu2+ to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure ... 

The peroxy group is detemiined by treatment with sodium iodide. The liberated iodine is then titrated with standard sodium thiosulfate solution. The reaction is... 

Sodium thiosulfate (Na2S203) standard solution (see recipe)... 

The determination of peroxide value (PV) by an iodometric titration is described in Basic Protocol 2. Iodine is liberated by hydroperoxides in the oil in the presence of excess iodide in a stoichiometric ratio. The amount of iodine present is determined by titration with a standard sodium thiosulfate solution using a starch indicator, thereby reflecting how much peroxide is present in the oil or lipid extract. 

If the expected peroxide content in the sample is low, a standardized -0.05 N or 0.01 N sodium thiosulfate solution should be used for the titration. 

The blank titration must not exceed 0.1 ml sodium thiosulfate solution. If a 0.1-ml volume is exceeded, fresh reagents should be prepared and the new thiosulfate solution standardized. If the problem persists, the purity of the chemicals employed should be examined. 

Spectrophotometric measurements of CD and CT values in oils or lipid extracts requires 45 min depending on the number of samples to be analyzed. The classical PV determination requires a bit of ti me for preparation of reagents and standardization of the sodium thiosulfate solution. The analysis itself is not very time consuming ( 30 min), but in all, about 3 to 3.5 hr should be anticipated. The FOX method has an assay time of < 10 min however, with sample preparation and time to construct the calibration curve, 2 to 3 hr should be expected. 

A 2-ml. aliquot of the solution, withdrawn by means of a safety pipet, is added to 20 ml. of water containing 2 g. of potassium iodide, and the liberated iodine is titrated with good shaking with standard sodium thiosulfate solution. 

The excess iodine (unreacted surplus iodine) is back titrated with standard sodium thiosulfate solution using starch indicator. Phenylarsine oxide may be used as a titrant instead of sodium thiosulfate. 

Sodium Chlorite. An accurately weighed sample of about 2.0 g. of sodium chlorite is dissolved in 1 1. of water a 25-ml. aliquot serves as the sample for analysis. Two milliliters of 50 % potassium iodide and 10 ml. of acetic acid are added to the aliquot, and the ensuing reaction is allowed to proceed in the dark for 5 minutes. The liberated iodine is then titrated with 0.1 A standard sodium thiosulfate solution, using starch as an indicator. The equations for the reactions are written below.1... 

Ozone Production Rate. The ozone rate was determined by passing the gas stream from the ozone generator into an aqueous 10% potassium iodide solution and titrating the iodine that was liberated with standard sodium thiosulfate solution. 

The purity is determined by titration with standard sodium thiosulfate solution. An accurately weighed sample of about 200 mg. is dissolved in water, and a solution of approximately 1 g. of potassium iodide in 10 ml. of water is added. The solution is acidified with 10 ml. of 10% sulfuric acid and titrated with 0.1 A thiosulfate to the starch end point. 

Sodium Thiosulfate Solution (0.100 N) Dissolve 24.820 g of sodium thiosulfate in approximately 75 25 mL of freshly boiled water, and add 0.10 0.01 g of sodium carbonate. Quantitatively transfer the mixture to a 1-L volumetric flask, and dilute to volume with water. Allow the solution to stand for a minimum of 4 days before standardizing. Store the solution in an amber bottle. 

To standardize the solution, perform the following in triplicate Pipet 25.0 mL of 0.1000 N Potassium Iodate Solution into a wide-mouthed Erlenmeyer flask. Add 2.00 0.01 g of potassium iodide, and shake the flask to dissolve the potassium iodide crystals. Pipet 5.0 mL of concentrated hydrochloric acid into the flask, and titrate the free iodine with Sodium Thiosulfate Solution to a light yellow color. Add a few drops of Starch Solution, and continue the titration until 1 drop produces a colorless solution. Determine the Sodium Thiosulfate Solution normality using the following formula ... 

For each filtrate, use the first 20 to 30 mL to rinse a pipet, and discard the rinse portions. Use clean beakers to collect the remaining filtrates. Mix each filtrate by swirling the beaker, and pipet 50.0 mL of each filtrate into one of three clean 250-mL Erlenmeyer flasks. Titrate each filtrate with standardized Sodium Thiosulfate Solution until a pale yellow color develops. Add 2 mL of Starch Solution, and continue the titration with standardized Sodium Thiosulfate Solution until 1 drop produces a colorless solution. Record the volume (5) of standardized Sodium Thiosulfate Solution used. 

Use of this table presumes the ability of the analyst to duplicate exactly the conditions under which the data were developed. The risk of error can be avoided by careful duplicate standardization with known quantities of pure dextrose (five samples, ranging from 10 to 70 mg). A plot of Titer Difference versus mg of dextrose is slightly curvilinear, passing through the origin. If use of a standardization curve is adopted, the thiosulfate solution need not be standardized. Some additional increase in accuracy results from use of a 0.065 N sodium thiosulfate solution, which increases the blank titer to about 44 to 45 mL. 

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