I need some assistance. I am truly missing something very fundamental. Background: I have a sodium sulphide stock solution with the conc. of 0.0429M. I prepare five standards through serial dilution (10,000; 5,000; 1,000; 500; 100uM). Buffer is added in a 1:1 with the standard - the standards are used to calibrate a ion-selective electrode. Once the electrode is calibrated I use it to determine the concentration of sulphide in sediment whereby each sample is prepared with buffer in a 1:1 manner (i.e. 30ml sediment + 30ml buffer). None of this is the issue... So, here is my problem. I want to spike sediment containing 0uM sulphide with 5,000uM using my stock solution (0.0429M). Here is my attempt at the calculation: X-mL = ((desired spike conc.)(vol. of sediment))/(conc. of sodium sulphide stock sol'n) X-mL = ((5,000uM)(30mL))/(0.0429M) = 3.50ml When I spike my sediment sample (30mL) with the buffer (30mL) with 3.50mL of the sodium sulphide stock solution the concentration is double (10,000uM) than what it should be - 5,000uM. If I add 1.75ml of the stock solution to 30mL sediment + 30mL buffer I get the desired answer - 5,000uM. Should I add 3.50mL of buffer when I add 3.50mL of stock to the 30ml sediment + 30ml buffer or is my calculation incorrect? Any assistance would be greatly appreciated.