Chlorination of Water Part 2

In this formula calcium hypochlorite has been written CaOCl_{2}, but this substance actually contains one atom of oxygen less than the true hypochlorite, which has the const.i.tutional formula ClO-Ca-OCl. This difference led some of the earlier chemists to regard CaOCl_{2} as a mixture of equal molecules of calcium chloride and calcium hypochlorite (CaCl_{2} + Ca(OCl)_{2} = 2CaOCl_{2}), but it has been definitely established that no calcium chloride exists in the free state in dry commercial bleach.

Since the very earliest days when the process of bleaching was investigated it was considered to be a process of oxidation and it is not surprising that Lavoisier and his pupils, who had noted the strong decolourising action of the gas discovered previously by Scheele, should regard it as a compound that contained oxygen. They were confirmed in this view by the fact that an aqueous solution of the gas slowly evolved oxygen when placed in bright sunlight, and lost its bleaching properties. Watt disproved this and showed that the evolution of oxygen was due to the action of the chlorine on water.

Cl_{2} + H_{2}O = 2HCl + O.

The bleaching action was not due to the chlorine "per se" but to the nascent oxygen produced in the presence of moisture. Later, when bleach and other chlorine compounds came into use as deodourisers, their action was attributed to the oxygen produced and when their germicidal properties became known it was natural to a.s.sume that the destruction of bacteria was due to the same cause. Some of the earlier experimental work supported this view. Fischer and Proskauer[1] found that humidity played an important part in chlorine disinfection, probably because it favoured oxidation. In air saturated with moisture micro-organisms were killed by 0.3 per cent of chlorine in three hours but when the air was dry practically no action occurred. They concluded that chlorine was not directly toxic. Warouzoff, Winogradoff, and Kolessnikoff[2] were unable to confirm the results of Fischer and Proskauer and found that a mixture of chlorine gas and air killed teta.n.u.s spores in one minute.

The nascent oxygen hypothesis was clearly and succinctly expressed by Prof. Leal during the hearing of the Boonton, N. J., case and the following abstracts have been taken from his evidence:

"... That on the addition of bleach to water the loosely formed combination forming the bleach splits up into chloride of calcium and hypochlorite of calcium. The chloride of calcium being inert, the hypochlorite acted upon by the carbonic acid in the water either free or half bound, splits up into carbonate of calcium and hypochlorous acid.

The hypochlorous acid in the presence of oxidisable matter gives off its oxygen; hydrochloric acid being left. The hydrochloric acid then drives off the weaker carbonic acid and unites with the calcium forming chloride of calcium.

"That the process was wholly an oxidising one, the work being done entirely by the oxygen set free from the hypochlorous acids in the presence of oxidizable matter....

"We have used during our investigations, the term 'potential oxygen' as expressing its factor of power. When set free, it is really nascent or atomic oxygen and is, in its most active state, entirely different from the oxygen normally in water...."

The reactions suggested are expressed in the following equations:

(i). 2CaOCl_{2} = CaCl_{2} + Ca(OCl)_{2}

(ii). Ca(OCl)_{2} + CO_{2} + H_{2}O = CaCO_{3} + 2HClO

(iii). 2HClO = 2HCl + O_{2}

(iv). CaCO_{3} + 2HCl = CaCl_{2} + CO_{2} + H_{2}O.

Phelps, during the hearing of this case, suggested that hypochlorites were directly toxic to micro-organisms but this view was not supported by any definite evidence and the nascent oxygen hypothesis met with almost universal acceptance. Investigations made by the author in 1915, 1916 and 1917 have produced data which cannot be adequately explained by the nascent oxygen hypothesis.[3]

The disinfecting action of bleach can be most conveniently considered by regarding it as a heterogeneous mixture of the reactants and resultants of the reaction

CaO + H_{2}O + Cl_{2} <=> CaOCl_{2} + H_{2}O

which is in equilibrium for the temperature and pressure obtaining during the process of manufacture. Under suitable physical conditions the chlorine content can be increased to 40-42 per cent but such a product is not so stable as those represented by the a.n.a.lyses on page 14 and which contain approximately 20 per cent of excess hydrate of lime.

The stability of bleach depends upon this excess of base (Griffen and Hedallen[4]) and although magnesia can be partially subst.i.tuted for this excess of lime, a minimum of 5 per cent of free hydrate of lime is required to ensure stability.

On dissolving bleach in water the first action is the decomposition of calcium oxychloride into an equal number of molecules of calcium hypochlorite and calcium chloride.

2CaOCl_{2} = Ca(OCl)_{2} + CaCl_{2}.

In dilute solution these salts are dissociated and hydrolysis tends to occur in accordance with the equations

2Ca(OCl)_{2} + 4H_{2}O <=> 2Ca(OH)_{2} + HOCl + HCl and

CaCl_{2} + 2H_{2}O <=> Ca(OH)_{2} + 2HCl.

Calcium hydrate and hydrochloric acid are both practically completely dissociated, i.e. there is a large and equal quant.i.ty of H^{.} and OH', and the product is much greater than K_{_w_} (ionic product of water), and hence there is a combination of these ions, leaving the solution neutral and no undissociated acid or base exists. This statement is only approximately correct as hydrochloric acid is slightly more dissociated than calcium hydroxide (ratio 9:8) and the solution is consequently slightly acid, i.e. the H^{.} concentration is greater than 1 10^{-7}.

Hypochlorous acid is only very slightly dissociated, especially in the presence of the OCl' ion due to the dissociation of the Ca(OCl)_{2}, as compared with Ca(OH)_{2} and hydrolysis of the Ca(OCl)_{2} proceeds with increased dilution. The action is best represented by the equation

2Ca(OCl)_{2} + 2H_{2}O <=> CaCl_{2} + Ca(OH)_{2} + 2HOCl

The hydrolytic constant of hypochlorous acid has apparently not been determined but as the acid is weaker than carbonic acid, which has a hydrolytic constant of 1 10^{-4}, the value is probably between 1 10^{-3} and 1 10^{-4}. From the formula _x_^{2}/(1 - _x_)_v_ = _k__{_wv_} in which 1 mole of pure Ca(OCl)_{2} is dissolved in _v_ litres, _x_ is the fraction hydrolysed, and _k__{_wv_} is the hydrolytic constant, complete hydrolysis occurs (_x_ = 1) when _v_ is not greater than 1 10^{4} litres. This is equivalent to a concentration of not less than 7.1 p.p.m. of available chlorine. Solutions of pure hypochlorites are alkaline in reaction because of the excess of hydroxyl ions (minimum concentration 1 10^{-4}). In solutions of bleach the hydrolytic action is r.e.t.a.r.ded by the OH' due to the free base, and accelerated by the excess of H^{.} caused by the dissociation and partial hydrolysis of CaCl_{2}; the final result is determined by the relative proportions and the effect of the free base usually preponderates. The addition of any substance that reduces the OH'

concentration enables hydrolysis to proceed to completion and affords a rational explanation of the fact that solutions of bleach, on distillation with such weak acids as boric acid, yield a solution of hypochlorous acid. It also explains why the addition of an acid is necessary in Bunsen's method (_vide_ p. 79) of a.n.a.lysing hypochlorite solutions. It has been stated that when hydrochloric acid is employed the increase in the oxidising power is due to the action of the acid upon calcium chloride but this never occurs under ordinary conditions; weak acids such as carbonic or acetic will give practically the same result as hydrochloric acid in solutions of bleach of the strength used in water treatment. The slightly higher result obtained with strong acids is due to the decomposition of chlorates.

The effect of dilution alone is shown by the data given below. A 2 per cent bleach solution, containing very little excess base, was diluted with distilled water and the various dilutions t.i.trated with thiosulphate after the addition of pota.s.sium iodide. In one series the solutions were t.i.trated directly, and after acidification in the other.

The results[A] were as follows:

HYDROLYSIS OF BLEACH SOLUTION

-----------------------------------+----------------------- Strength of Solution. Grams Bleach Direct t.i.tration 100 Per 100 c.cms. --------------------.

Acid t.i.tration -----------------------------------+----------------------- 2.0 30.8 0.2 34.3 0.1 41.8 0.02 67.5 0.002 100.0 -----------------------------------+-----------------------

[A] Corrected for the alkali produced by HClO + 2KI = KCl + KOH + I_{2}.

Although every precaution was taken to exclude carbonic acid, a portion of the hydrolysis was probably due to this acid, which would remove calcium hydrate from the sphere of action and consequently alter the equilibrium. The above figures are only applicable to the particular sample used; other samples containing different excesses of base would yield different hydrolytic values. The results are in agreement with the hypothesis presented and confirm the theoretical deduction that very dilute bleach solutions are completely hydrolysed if no salts are present that will dissociate and increase the OH' concentration.

Hydrolysis is reduced by caustic alkalies and alkaline carbonates, and increased by acids and acid carbonates that reduce the OH'

concentration.

The effect of chlorides is anomalous and no adequate explanation for their action can be given at present. The addition of small quant.i.ties of sodium chloride (0.1 per cent) increases the hydrolysis of bleach solutions but much larger quant.i.ties tend to the opposite direction.

The effect of these substances upon the velocity of the germicidal action of bleach solutions is in the same direction as the hydrolysing effect.[4] Sodium chloride in quant.i.ties up to 10 parts per million has a very limited effect but larger quant.i.ties (90 p.p.m.) increase the velocity of the reaction. Sodium chloride, in the absence of hypochlorites, was found to have no influence upon the viability of _B.

coli_ in water.

In quant.i.ties up to approximately 5 p.p.m., sodium hydroxide has but little influence; 5-10 p.p.m. reduce the velocity to a marked degree, but when the quant.i.ty of caustic is still further increased the germicidal action of the alkali commences to be appreciable and may nullify the r.e.t.a.r.ding action on the hypochlorite. Normal carbonates tend to reduce the velocity of the germicidal action and bicarbonates to increase it.

Sulphuric acid, even in very small quant.i.ties (5 p.p.m.), has a marked accelerating effect and the total effect produced is much greater than can be accounted for by the germicidal activity of the acid alone. Weak acids such as carbonic acid and acetic acid are also effective accelerators. In one experiment a 0.01 per cent solution of bleach was found to be 40 per cent hydrolysed. By pa.s.sing carbonic acid gas this was increased to 95 per cent and the velocity of the germicidal action of this solution was found to be approximately 100 per cent greater than that of the uncarbonated one. Norton and Hsu[5] have shown that the germicidal activity of some disinfectants is a function of the hydrogen ion concentration, but this factor is insufficient to account for the effect of acids on bleach solutions.

The effect of sodium chloride on the bacteriological results, like that on the hydrolytic constant, is anomalous. Similar effects have been observed on the addition of this salt to phenol and other disinfectants.

The _raison d'etre_ of the increased activity is obscure but it is possible that the salt renders the organisms more susceptible to the action of the germicide.

Ammonia, though decreasing the hydrogen ion concentration of bleach and other hypochlorite solutions, markedly increases the velocity of the reaction; chlorinated derivatives of ammonia (chloramines), which have a specific germicidal action, are formed. These will be discussed at length in Chapter IX, p. 115.

Rideal[6] has shown that the addition of ammonia to sodium hypochlorite destroys the bleaching activity in acid solution. This has been found by the author to be also true for calcium hypochlorite (bleach). If the bleaching effect is due to oxidation, the oxidising power of hypochlorites must be considered to be destroyed by the addition of ammonia. The property of oxidising organic matter in water is also destroyed; this is well ill.u.s.trated in Table II which shows the rate of absorption of chlorine and chloramine by the Ottawa River water. The water used in this experiment contained 40 p.p.m. of colour and absorbed 9.5 p.p.m. of oxygen (30 mins. at 100 C.).

TABLE II.[A]

------------------+------------------------------------------------- ABSORPTION OF AVAILABLE CHLORINE AT 63 F.

Time of Contact +----------------------+-------------------------- Minutes. Chlorine as Bleach. Chlorine as Chloramine.

------------------+----------------------+-------------------------- Nil. 10.00 9.98 5 6.50 9.98 10 5.91 9.90 20 5.18 9.90 40 4.47 9.84 60 3.90 9.84 80 3.65 9.84 20 hours .... 9.68 ------------------+----------------------+--------------------------

[A] Results are parts per million.

From a consideration of these and other experiments made by the author in January, 1916, it became apparent that the nascent oxygen hypothesis entirely failed to explain the results obtained, and that they must be attributed to a direct toxic action of the chlorine or chloramine.

Dakin et al.[7] arrived at a similar conclusion from a consideration of the results obtained during the use of hypochlorite solutions in the treatment of wounds by Carrel's method of irrigation. They attributed the marked beneficial action to the formation of chloramines _in situ_ by the action of hypochlorous acid upon amino acids and proteid bodies.

Compound chloramines (chlorinated amin.o.benzoic acids) were prepared in the laboratory and found to give excellent results in reducing wound infection. Later, other compounds were prepared for the purpose of sterilising small quant.i.ties of water for the use of mobile troops (see p. 128).