Analysis of Chloride

Analysis of Chloride in Varying Water Samples

Geoff Simpson

Department of Chemistry and Biochemistry, McMurry University, Abilene, Tx 79697

The purpose of this experiment is to determine the chloride (Cl^-) ion

concentrations in different samples of water. The types of water used in this experiment

are tap water, Lake Kirby water, and reused water. Some of the sources of the chloride ion in these water sources come from the weathering of rocks, the evaporation of a sea in this area millions of years ago that left chloride behind, and the salt from food and products that are present in the waste water. The tap water came from Lake Ft. Phantom and Lake O.H. Ivie. Lake O.H. Ivie has a higher salt content than that of Lake Ft. Phantom. The Lake Kirby water is used to be a storage lake for reused water, but now is mostly run off from rain. Reused water is waste water that has been cleaned and purified to that of regular tap water so that it can be reused.

Chloride, a major inorganic anion in water and waste water can cause a salty taste when the water has a high concentration of Cl-. This high concentration can cause damage to metallic pipes, structures, as well as growing plants (1). To test for the chloride ion concentration, a titration of Silver nitrate solution will

be performed. Equation 1 describes this procedure.

Equation 1: Ag⁺ + Cl^- à AgCl.

There were two methods used in this experiment to determine the chloride concentration in the different water samples, the Fajans and the Mohr methods. The Fajans and the Mohr methods differ in that they use different indicators, and are performed a little differently. The Mohr method uses the indicator potassium chromate, while the Fajans method uses the indicator dichlorofluorescein, which is an adsorption indicator An adsorption indicator is a negatively charged dye that is attracted to a positively charged precipitate surface. This is produced after the equivalence point where all the chloride ions have reacted to form a precipitate (2). This experiment will determine and illustrate which method is more accurate and effective in determining the amount of chloride in the different samples of water. The author used the Fajans method to analyze tap water.

Experimental

Apparatus and Materials: Primary grade sodium chloride (Thorn Smith- Royal Oak, Michigan) and silver nitrate (Sargent Welch Scientific Co.-Skokie, Illinois) were used in the titrations. The indicator in this experiment is dichlorofluorescein. A calibrated 50 mL previously calibrated buret was also used in the titrations.

Procedure: Silver nitrate solution (approximately 0.01 M) was prepared by weighing out between 1.27g and 1.7g of AgNO₃ and adding it to 300 mL of deionized water in a 500 mL beaker. The solution was then transferred to a 1-L brown bottle and deionized water was added to the neck. Once dissolved, the AgNO₃ plus water solution was transferred into a brown bottle because the AgNO₃ solution is sensitive to the light. The bottle was filled the rest of the way up with deionized water. The AgNO₃ solution was standardized against an HCl solution. Between 0.013 g and 0.0175 g of NaCl was accurately weighed. The NaCl was then added to 50 mL deionized water in a 250 mL Erlenmeyer flask. For the Fajans method, the indicator dichlorofluorescein was added to 50 the sample, which was then titrated by the AgNO₃. Dextrin was also used. The observed endpoint went from a yellow/green color to a pink endpoint. 50 mL of tap water was accutately measured using a pipet, and then was titrated with AgNO₃ to determine the amount of chloride in the tap water.

Results and Discussion

Table 1 illustrates the results for the standardization of silver nitrate.

Table 1. Standardization of AgNO₃

Trial #	Mass of NaCl used (g)	Volume of AgNO₃ titrated	Molarity of AgNO₃
1	0.0148	22.95	0.0113
2	0.0143	25.78	0.0095
3	0.0165	23.75	0.0119
4	0.0132	24.61	0.0092
Average M AgNO3-	0.0105 M
Standard Deviation-	0.0013

Table 2 illustrates the results of titrating tap water drawn from the faucet in the McMurry chemistry lab.

Table 2. Tap Water Titrations with Silver Nitrate

Trial #	Volume AgNO₃, mL	Amount of Chloride in tap water (ppm)
1	41.23	307
2	39.62	295
3	40.96	305
Average Chloride Amount-	302
Standard Deviation-	6.4
Relative Standard Deviation	2.12 %

The relative uncertainty was calculated to be 302 +/- 39. This number is large because of the large relative standard deviation of the Molarity of the titrant which was calculated to be 12.8 % rsd. The relative standard deviation for the volume titrated was 2.1 % rsd, while the % rsd of the pipet used in calculation was 0.10. Therefore, the number of the rsd for the molarity was obviously larger, and therefore contributed to the large value calculated for the relative uncertainty. To reduce the uncertainty, an improvement on the technique of standardization of the titrant would be necessary.

Table 3 illustrates the averages of the class for the different methods and the amount of chloride found in the water samples, as well as the standard deviation.

Table 3- Water Sample Averages and Standard Deviation of Chloride

Method	Tap Water RSD	Kirby Water RSD	Reuse Water RSD
Fajans	302.3 / 6.4 2.12	210.1 / 1.3 0.62	343.7 / 5.2 1.51
Fajans	254.9 / 0.41 0.16	271 / 16 5.90	305.6 / 0.51 0.17
Mohr	301.1 / 4.1 1.36
Mohr	306.4 / 7.4 2.42	192.5 / 2.4 1.25	368.2 / 3.9 1.06
Mohr	267.6 / 3.5 1.31	131 / 3 2.29	168.3 / 2.8 1.66

The averages for the amount of chloride present in the water samples were found using data from Table 3. The average amount of chloride found in tap water was 287. The average found for Lake Kirby was 201, and the average for reuse water was 296. The largest amount of chloride present in the water samples was in reuse water, followed by tap water, while the least amount of chloride present was in Kirby water. Reuse water can have high levels of chloride present because it is not as imperative to control the chloride levels as it is to control the levels in tap water. This is the reason that reuse water contains more chloride. Kirby water is mostly rain runoff, and is therefore not treated with chlorine.

In Table 4, the average amounts of chloride in the different samples of water are illustrated according to the City of Abilene Environmental Lab.

Table 4. Amount of Chloride Found in Water for City of Abilene (3)

Type of Water Sample	Amount of Chloride
Tap Water	223
Reuse Water	350-426
Lake Kirby Water	390

In this experiment, the tap amount of chloride found in a sample of tap water was significantly higher than that of the City of Abilene. This experiment calculated that there was 302.3 ppm of chloride in the tap water, while the city calculated an average of 223 ppm.

The relative standard deviation was calculated in order to compare two different titrations. Neither method can be compared due to the fact that the results are staggered, and therefore cannot be accurately used for comparison. This illustrates that every student viewed their endpoint differently, and therefore the calculations were off. This led to the relative standard deviations being staggered.

In Table 5, the separate ppm’s are diagrammed along with their averages and standard deviations of the two different methods in order to calculate a t-test of the comparison of means.

Table 5. Comparison of Fajans and Mohr Methods of Tap Water

Method	Amount of Chloride (ppm)	X - Average	Sx- Standard Deviation	RSD
Fajans	307	302	6.4	2.12
	295
	305
Mohr	263	268	3.5	.013
	268
	272
	268

A t-test was performed using the Mohr and the Fajans methods to determine whether there is a significant difference between the two methods. Spooled was calculated to be 4.86, and the t-test yielded the calculated value of t to be 9.16. At the 95% confidence level, with a degrees of freedom of 5, the Value of Student’s t table reported 2.571. By comparing the means of the two methods, the t-test illustrates that there is a significant difference between the Fajans and the Mohr methods.

To compare this authors data to what the city got, a confidence interval t-test was run. The value of the T-test was 302.3 ppm +/- 15.90. To compare this to the cities value at the 95 % confidence level (233 ppm), it is obvious that the value of the cities sample is different than the calculated T-test for this experiment. A reason for this difference could be that the last time the amount of chloride in tap water for the City of Abilene was measured was in July 2004. Therefore, the amount of time since the last measurement could play a factor, as the level of chloride present in tap water could have significantly changed over this period of time.

Conclusion:

Through calculation, the average amount of chloride present in tap, reuse, and Lake Kirby water samples were able to be found. The experiment resulted in a concentration of 302 ppm of chloride is present in the tap water sample. Through analysis of a comparison of means t-test, the Mohr and Fajans methods are able to be compared, andit can be gathered that the two methods are different. Comparison to the cities values to a personal confidence interval t-test, it is illustrate that at the 95% confidence level that there is a significant difference between the two.

References:

Greenberg, A.E.; Clesceri, L.S.; Eaton, A.d.; ed. Standard Methods for the Examination of Water and Waste Water, 18^th ed. American Public Health Association: Washington D.C., 1992. 4-48 – 4-49.

Harris, D.C. Exploring Chemical Analysis, 2^nd ed.; W.H. Freeman and Company: New York, 2001.

Michaud, Mike. Director of City of Abilene Environmental Lab, Abilene, Tx. Personal Communication, 2004.