Standard hard water

It is suggested that the standard soft water should contain 34.2mg/kg calculated as calcium carbonate (WHO standard water), not 20.0mg/ kg (CIPAC standard water) and that the preparation of WHO standard hard water and WHO standard soft water should be published as WHO test methods (see Annex 2 in this report). 

A suspension of known concentration of deltamethrin in standard hard water is prepared by adding an entire water-dispersible tablet to standard hard water, poured into a 250-ml graduated cylinder, maintained at a constant temperature, and allowed to remain undisturbed for 30 minutes. The top nine-tenths are drawn off and the content of deltamethrin in the bottom one-tenth is determined, so allowing determination of the active ingredient mass still in suspension after 30 minutes. 

WHO standard hard water (WHO method WHO/M/29 see Annex 2, section II). 

The persistent foam at the top of 100ml of suspension prepared in WHO standard hard water (WHO method WHO/M/29 see Annex 2, section II) with 5ml of emulsion, oil in water, shall not exceed 10ml when tested by CIPAC method MT 47 (2). 

Fill the beaker with 1000 ml of standard hard water at 30°C + 2°C. The stirrer should be centrally located in the beaker and positioned in such a way that the lower edges of the stirrer blades are 60mm above the base of the beaker. The pitch of the stirrer blades and the direction of rotation are such that the propeller pushes the water upwards. Add the tablet to the water without stirring. Wait for 1 minute. Switch on the stirrer with the speed set to about 300 rev/min... 

Dilute one part of WHO standard hard water prepared as described in WHO method WHO/M/29, the hardness of which is 342mg/l, with nine parts of distilled water to give a hardness of 34.2 mg/1. 

The need for cell cultures for tests with viruses requires the incorporation of controls over and above those necessary for working with bacteria. In addition to cell culture controls to demonstrate lack of contamination and vims controls to demonstrate a functioning assay system, the level of input vims and the loss in virus infectivity upon the drying of the inoculum on the carrier need to be measured. In some cases, these measurements are only made once and the data used with a series of tests. However, it is recommended that, for proper accuracy, such measurements should be included with every test due to inherent variations in cell cultures and viruses. When there is a need to separate virus kill from simple mechanical removal of the test vims during the test, it is recommended that a control also be included to determine the mechanical removal of the test virus standard hard water should be used for this purpose in place of the test topical product. For any claim of topical antisepsis, reduction in the vims titer on treatment with the test product must be substantially higher than that obtained with the standard hard water treatment alone. 

One possible compromise here would be to develop an index based on the performance of a neutral agent that only mechanically removes viruses from hands. In the past, nonmedicated soaps have often been used for this purpose. However, soaps on the market differ widely, especially in properties such as pH and detergent action. Tap water also differs at different geographical locations. Therefore, a simple, safe, and readily available solution such as standard hard water (e.g., with 200 ppm hardness) could be used to establish the reference point for this index. Product efficacy claims could then be allowed at a certain differential above the mechanical virus removal with hard water. Manufacturers could publish the performance index of their formulation on the label to aid in product selection. To prevent minor differences in indices being used as a sales feature, a simple product classitication scheme could be developed. 

Into a 250-ml beaker having an internal diameter of 6—6.5 cm and 100-ml calibration mark and containing 75-80 ml of standard hard water, 5 ml of EC is added, using a Mohr-type pipette, while stirring using a glass rod,... 

The standard hard water recommended by WHO, and used in labs around the world, is made from a combination of calcium and magnesium salts in a set ratio. The hardness of this water is 342 ppm, when determined as calcium carbonate. The recipe for preparing this water is to dissolve 0.304 grams anhydrous calcium chloride and 0.139 grams magnesium chloride hexahydrate in distilled water and dilute to one liter. A typical evaluation program will use 342 ppm water as the standard, with other waters ranging from 34 ppm to 1000 ppm hardness added as demanded by end-use requirements. 

Soap Titration The fact that sufficient soap solution has to be added to precipitate all the hardness causing ions from a hard water sample before lather is formed is utilized in the determination of hardness. A. suitable aliquot of the hard water sample is taken in a stoppered flask and after successive additions of a soap solution (previously standardised by titration against standard hard water) from a burette, the mixture is vigorously shaken. The stage where the lather formed persists for 2-3 minutes is taken as the end-point of the titration. The hardness of the water sample is equivalent to the volume of the soap solution used minus lather factor (volume of soap solution used to produce lather with a volume of distilled water equal to the aliquot of the hard water sample). 

Pipet out 10 ml of standard hard water into a conical flask. Add to it 40 ml distilled water with a measuring cylinder, 2 ml of buffer solution and 2 drops of Eriochrome black T indicator. A wine red colour appears. Titrate against EDTA solution, taken in the burette, to a colour change from wine red to pure blue. Record the volume of EDTA used as A ml. Take three coitcordant readings. 

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