A Ready Method for Preparing a Silica Turbidity Standard.

Although the candle or electric light turbidimeter has largely replaced comparison with silica standards as a laboratory method of determining the turbidity of water, these instruments, equipped with the usual short tube, are of use only where the turbidity is above 100, while the platinum wire method is applicable only in field work. Even there its use is limited to waters of neither very high nor very low turbidity, besides which there is not infrequently great difficulty in securing the necessary conditions of light. Direct comparison with silica standards is, therefore, the method which must be resorted to in most cases where the turbidity is below 100. Of all standards used by the water analyst, the silica turbidity standard is the most difficult to prepare. Even after the very tedious preparation of the diatomaceous silica itself, the powder so obtained is not always of the required degree of fineness, thus necessitating standardization by the platinum wire method or by use of the candle turbidimeter, instead of simply adding a gram per liter as originally recommended, to secure a standard of 1,000 parts turbidity per million.