AWT Technical Reference Manual: Chapter 1 Questions

If 1 ml of a sodium hydroxide solution is required to raise the pH of a water sample from 7 to 8, how many ml will be required to raise the same sample from pH 8 to 9? How many from pH 9 to 1 0? Explain your answer

10 ml from pH 8 to 9, and 100 ml from pH 9 to 10. pH is a logarithmic function. Each unit increase in pH requires tenfold more base.

Explain what is meant by the term "inverse solubility"? Why is this important in water treatment? List four substances common in boiler and cooling water systems that are inversely soluble in water.

A substance that is inversely soluble becomes less soluble as the temperature increases. This is important because it causes scale to form preferentially on hot heat transfer surfaces. Also, for this reason it is possible to remove dissolved gases from water by boiling the water. Four inversely _soluble substances common in water treatment are: calcium carbonate, calcium phosphate, oxygen and carbon dioxide.

Would you expect the pH of a solution of ammonium chloride (NH4CI) to be acidic (less than 7), neutral, or basic (greater than 7)? How about a solution of trisodium phosphate?

Ammonium chloride solutions are acidic because ammonium hydroxide is a weak base and hydrochloric acid is a strong acid. Similarly, solutions of trisodium phosphate are basic because phosphoric acid is a weak acid and sodium hydroxide is a strong base.

The ionization constant of pure water at 100°C is 10·12• Calculate the pH of pure water at 100°C. Explain why the pH of pure water is not always 7.

At 100°C, [H+][OH1 = 10·12• Therefore, [H+] = 10-s and the pH = 6. Ionization and equilibrium constants increase with increasing temperature. To avoid confusion, pH data are normally reported at 25°C, using temperature-compensating pH probes. Colorimetric pH measurements are not temperature-compensated, so the solution must be cooled to about 25°C before the pH is measured.

Explain the difference between polarization and passivation as they relate to the corrosion of steel in water.

Both processes cover cathodic areas on the surface with films that interfere with the oxygen reduction reaction. Because corrosion rates are controlled by the rate of the cathodic reaction, these processes reduce corrosion rates. Polarization is a dynamic process. Tl1e films are generally layers of hydroxide ions, or in some cases hydrogen gas. These films can be changed or swept away by changing system conditions such as flow velocity, temperature, etc. Passivation is a similar process, except that the protective films are more permanent in nature. The protective film formed by tolyltriazole on copper, and the aluminum oxide film that forms on aluminum in water, are examples of passive films.

Explain the basic corrosion process on steel in cooling water.

Corrosion is an electrochemical process. It requires a complete electrical circuit, including both the metal and the water. Iron dissolves at points on the metal surface called anodes, and dissolved oxygen in the water is reduced at other points called cathodes. Electrical neutrality must be preserved, so that only equal numbers of ionic charges can react (not necessarily equal numbers of atoms or equal ppm of iron and oxygen).

One of the objectives of water treatment is to keep heat transfer surfaces clean. Explain why the cost of operating a building chiller machine, or a steaming boiler, increases as deposits form in the tubes.

Deposits insulate the tube surfaces and restrict heat transfer. Deposits that form in heat transfer tubes act as insulation. This means that less heat is transferred, and the process becomes less efficient. More fuel must be burned to maintain steam production, and chillers must run longer to maintain the chilled water temperature. Eventually, boiler tubes may fail from overheating, and the chiller machine may be unable to produce chilled water at the required temperature.

The pH of a 10-3 N solution of hydrochloric acid is 3, but the pH of a 10-3 N solution of citric acid is over 5. Why the difference?

Hydrochloric acid is a strong acid that is completely ionized in solution. Citric acid is a weak acid that is only partially ionized.

Is the LSI a good indicator of the corrosivity of a circulating cooling water? Explain your answer.

LSI values should never be used to determine the corrosivity of a circulating cooling water. The LSI calculation was developed for municipal water supplies with no heat transfer. In this calculation, corrosion control is assumed to depend upon formation of a protective calcium carbonate film. This does not apply to cooling water systems.

Following is a typical analysis of a cooling tower water: Parameter, As, mg/L, CaCO3 conversion factor Tot. alkalinity, CaCO3, 125, 1.0 Calcium, Ca, 50, 2.5 Magnesium, Mg, 15, 4.1 Sodium, Na, 45, 2.2 Chloride, Cl, 52, 1.4 Sulfate, SO4, 125, 1.0 Silica, SiO2, 45, 1.7 Using the common denominator concept in section 3.4.3, check this analysis for analytical errors, by calculating the cation/anion ratio. Use the calcium carbonate conversion factors shown above. Why is this calculation necessary? Why not just add the cations and anions to see if they match? Use the concepts of equivalent weight, electrical neutrality and common denominators in your answer. As an extra challenge. use the data in Table 3.1 to calculate the calcium carbonate conversion factors shown above.

Multiply mg/L x conversion factor to get calcium carbonate equivalents, and add the cations and anions separately (alkalinity is an anion). The cation/anion ratio is 1.04. The analysis is correct. Equal concentrations of different substances, in mg/L, contain different numbers of ions because they have different molecular weights and the ions have different charges. To. compare the analytical data, the data must be converted to an equivalent ·(common denominator) basis so that electrical neutrality is preserved in the solution. To convert each analysis to a calcium carbonate equivalent, divide by the equivalent weight of the ion and multiply by the equivalent weight of calcium carbonate.

Calculate the valence of bromine and nitrogen in the following compounds. In each case, is bromine or nitrogen a cation or an anion? Sodium bromide; NaBr Ammonium chloride; NH4CI Bromine chloride; BrCI Nitrogen gas; N2 Hypobromous acid; HOBr Sodium nitrite; NaNO2 Sodium bromate; Na8r03 Nitric acid; HNO3

NaBr, Br= -1, Anion BrCI, Br= +1, Cation HOBr, Br= +1, Complex anion NaBrO3, Br= +5, Complex anion NH4CI, N = -3, Complex cation N2, N = 0, Neither - nonionized gas NaNO2, N =+3, Complex anion HNO3, N =+5, Complex anion

Describe the importance of system surveys as the basis for proposals in new accounts.

No two commercial buildings or industrial plants are alike, even though they may have been built as sister facilities or may use the same makeup water supply. Design and operating differences may require very different treatment programs in apparently similar facilities. A thorough survey, as described in section 9.0, provides the critical design, operating and cost information needed to prepare proposals that are matched to the facility, technically sound and cost-effective. If possible, provide examples of good and poor system surveys from your own experience.

Compare the solubilities of oxygen and carbon dioxide in Figures 1-7 and 1-8. Why is carbon dioxide so much more soluble than oxygen in water? Do both of these gases follow Henry's Law of solubility?

Oxygen does not react with water. Carbon dioxide reacts with water to form carbonic acid, which in turn can be neutralized by bases to form carbonates and bicarbonates. Nevertheless, both gases follow Henry's Law.

Why are oxygen scavengers an important part of most boiler water treatment programs?

Reduction of dissolved oxygen to hydroxide ions is the cathodic corrosion reaction under both cooling and boiler water conditions. Oxygen scavengers remove the last traces of dissolved oxygen from boiler feedwater, thus protecting the boiler tubes and drums from oxygen attack.

Using the concepts in sections 3.1, 3.2 and 3.3, write chemical formulas for the following compounds commonly encountered in water treatment: Sodium sulfite (sulfur valence +4) Calcium hydroxide Disodium hydrogen phosphate Hydrogen sulfide

Sodium sulfite (sulfur valence +4): Na2SO3 Calcium hydroxide: Ca(OH)2 Disodium hydrogen phosphate: Na2HPO4 Hydrogen sulfide: H2S

The data processing floors in a large commercial bank building require year-round cooling for heat removal. Some portions of one floor are to be left vacant and unheated during a winter season. However, the cooling system that services this floor cannot be isolated and must be allowed to circulate through the unheated area. To prevent freezing of the cooling coils, the chief engineer suggests adding ethylen~ glycol (antifreeze) to the cooling water, but the building architects very strongly recommend against doing this. Why? What would the effect be on building operations if this change were made? Can you think of any other ways to prevent the unheated cooling coils from freezing?

The specific heat (heat capacity) of an optimum ethylene glycol/water mixture is about two-thirds that of water. If the cooling system were filled with a glycol/water mixture, it would remove only two-thirds as much heat from the building and might not be able to provide sufficient cooling to the data processing center. The best way to solve this problem is to install the necessary valves and piping so that the cooling system in the unused sections can be isolated and drained. Alternatively, bypass piping can be installed around each fan coil so that the coils can be drained. Sometimes the fan coils are isolated and filled with ethylene glycol solution, but this creates microbiological fouling problems when the system is returned to operation

A steel pipe nipple is screwed into a brass valve in a cooling water system. What problems might occur in this system, and why? How could these problems be prevented?

This is a case of galvanic corrosion. Steel (iron) is more active in the galvanic series than copper (Table 1-11). Steel becomes the anode and brass the cathode. Steel will corrode more rapidly than it would if not in contact with brass (copper), and the brass will be protected. The threaded steel nipple will corrode rapidly to failure. · It is difficult to prevent galvanic corrosion with chemical treatment. Either use the same metal for both fittings and pipe, or use dielectric (nonconducting) fittings between dissimilar metals, to break the circuit and stop the corrosion current from flowing.

Two different circulating cooling waters are found to have the same total (M) alkalinity and calcium hardness, but different P alkalinity values. For each water, calculate the carbonate, bicarbonate and hydroxide values, and the LSI value. What problems would you expect to find in these cooling systems? Should they be treated the same way? Explain why it is important to know more than just pH and hardness when planning a cooling water chemical treatment program. For both waters, assume a temperature of 100°F and a conductivity of 500 microsiemens/cm. Water A: pH = 8.4 Ca as CaCO3 = 250 M=250 P= 10 Water B: pH = 8.9 Ca as CaCO3 = 250 M =250 P = 130

Water A Carbonate = 20 Bicarbonate = 230 Hydroxide = 0 LSI. = +1.6 Water B Carbonate = 240 Bicarbonate = 0 Hydroxide = 10 LSI = +2.1 Even though both water have the same total alkalinity and calcium hardness values, water B will have a much greater tendency to form calcium carbonate scale. This can be seen from the LSI values, but more clearly and dramatically from the difference in carbonate levels in the two waters. Different chemical treatment programs will be required for these two systems.

Using your own words, define the following analytical chemistry terms: • Molecular weight and equivalent or combining weight. • Molarity and normality. Why are these concepts important in water treatment?

• Molecular weight: The sum of the atomic weights of the elements in a compound. • Equivalent or combining weight: The weight of a compound that will react with an equivalent weight of another compound. This is usually the molecular weight divided by the valence of the reactive element in the compound. • Mole and molarity: A mole is one molecular weight of a compound expressed in grams. A one molar solution is one mole of a compound dissolved in one liter of water. • Equivalent and normality: An equivalent is one equivalent weight of a substance expressed in grams. A one normal solution is one equivalent weight of a compound, in grams, dissolved in one liter of water. These concepts are needed in calculating the results of analytical tests, the amounts of acid, base and other chemicals needed in chemical treatment, etc.

List the four essential sources of information required for a complete facility survey, and explain why each is important. Use examples in your answer where possible

• Systems and equipment: Provides detailed information about the facility that can affect the design and control of the water treatment program. Examples include metallurgy, water flow rates, critical or throttled heat exchangers, heat loads, operating cycles, etc. • Operating data: Facility water treatment logs and vendor se1Vice reports, equipment inspections, etc. show how well, or poorly, the facility has been operated, and establishes the current condition of pipes, heat exchangers, cooling towers, boilers, etc. • Historical data: Old water treatment logs, se1Vice reports and maintenance records show what problems the facility has faced in the past. • Personnel inte1Views: Facility personnel can be a valuable source of information about past and present status of the water systems. lnte1Views help in identifying the cost and performance issues that are primarily important in the facility, and in identifying critical heat exchangers.